commit 8318d6a636 upstream.
Since we have set the WQ_NAME_LEN to 32, decrease the name of
events_freezable_power_efficient so that it does not trip the name length
warning when the workqueue is created.
Signed-off-by: Audra Mitchell <audra@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[ Upstream commit 15930da42f ]
For wq_update_node_max_active(), @off_cpu of -1 indicates that no CPU is
going down. The function was incorrectly calling cpumask_test_cpu() with -1
CPU leading to oopses like the following on some archs:
Unable to handle kernel paging request at virtual address ffff0002100296e0
..
pc : wq_update_node_max_active+0x50/0x1fc
lr : wq_update_node_max_active+0x1f0/0x1fc
...
Call trace:
wq_update_node_max_active+0x50/0x1fc
apply_wqattrs_commit+0xf0/0x114
apply_workqueue_attrs_locked+0x58/0xa0
alloc_workqueue+0x5ac/0x774
workqueue_init_early+0x460/0x540
start_kernel+0x258/0x684
__primary_switched+0xb8/0xc0
Code: 9100a273 35000d01 53067f00 d0016dc1 (f8607a60)
---[ end trace 0000000000000000 ]---
Kernel panic - not syncing: Attempted to kill the idle task!
---[ end Kernel panic - not syncing: Attempted to kill the idle task! ]---
Fix it.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reported-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Link: http://lkml.kernel.org/r/91eacde0-df99-4d5c-a980-91046f66e612@samsung.com
Fixes: 5797b1c189 ("workqueue: Implement system-wide nr_active enforcement for unbound workqueues")
Signed-off-by: Sasha Levin <sashal@kernel.org>
[ Upstream commit 5797b1c189 ]
A pool_workqueue (pwq) represents the connection between a workqueue and a
worker_pool. One of the roles that a pwq plays is enforcement of the
max_active concurrency limit. Before 636b927eba ("workqueue: Make unbound
workqueues to use per-cpu pool_workqueues"), there was one pwq per each CPU
for per-cpu workqueues and per each NUMA node for unbound workqueues, which
was a natural result of per-cpu workqueues being served by per-cpu pools and
unbound by per-NUMA pools.
In terms of max_active enforcement, this was, while not perfect, workable.
For per-cpu workqueues, it was fine. For unbound, it wasn't great in that
NUMA machines would get max_active that's multiplied by the number of nodes
but didn't cause huge problems because NUMA machines are relatively rare and
the node count is usually pretty low.
However, cache layouts are more complex now and sharing a worker pool across
a whole node didn't really work well for unbound workqueues. Thus, a series
of commits culminating on 8639ecebc9 ("workqueue: Make unbound workqueues
to use per-cpu pool_workqueues") implemented more flexible affinity
mechanism for unbound workqueues which enables using e.g. last-level-cache
aligned pools. In the process, 636b927eba ("workqueue: Make unbound
workqueues to use per-cpu pool_workqueues") made unbound workqueues use
per-cpu pwqs like per-cpu workqueues.
While the change was necessary to enable more flexible affinity scopes, this
came with the side effect of blowing up the effective max_active for unbound
workqueues. Before, the effective max_active for unbound workqueues was
multiplied by the number of nodes. After, by the number of CPUs.
636b927eba ("workqueue: Make unbound workqueues to use per-cpu
pool_workqueues") claims that this should generally be okay. It is okay for
users which self-regulates concurrency level which are the vast majority;
however, there are enough use cases which actually depend on max_active to
prevent the level of concurrency from going bonkers including several IO
handling workqueues that can issue a work item for each in-flight IO. With
targeted benchmarks, the misbehavior can easily be exposed as reported in
http://lkml.kernel.org/r/dbu6wiwu3sdhmhikb2w6lns7b27gbobfavhjj57kwi2quafgwl@htjcc5oikcr3.
Unfortunately, there is no way to express what these use cases need using
per-cpu max_active. A CPU may issue most of in-flight IOs, so we don't want
to set max_active too low but as soon as we increase max_active a bit, we
can end up with unreasonable number of in-flight work items when many CPUs
issue IOs at the same time. ie. The acceptable lowest max_active is higher
than the acceptable highest max_active.
Ideally, max_active for an unbound workqueue should be system-wide so that
the users can regulate the total level of concurrency regardless of node and
cache layout. The reasons workqueue hasn't implemented that yet are:
- One max_active enforcement decouples from pool boundaires, chaining
execution after a work item finishes requires inter-pool operations which
would require lock dancing, which is nasty.
- Sharing a single nr_active count across the whole system can be pretty
expensive on NUMA machines.
- Per-pwq enforcement had been more or less okay while we were using
per-node pools.
It looks like we no longer can avoid decoupling max_active enforcement from
pool boundaries. This patch implements system-wide nr_active mechanism with
the following design characteristics:
- To avoid sharing a single counter across multiple nodes, the configured
max_active is split across nodes according to the proportion of each
workqueue's online effective CPUs per node. e.g. A node with twice more
online effective CPUs will get twice higher portion of max_active.
- Workqueue used to be able to process a chain of interdependent work items
which is as long as max_active. We can't do this anymore as max_active is
distributed across the nodes. Instead, a new parameter min_active is
introduced which determines the minimum level of concurrency within a node
regardless of how max_active distribution comes out to be.
It is set to the smaller of max_active and WQ_DFL_MIN_ACTIVE which is 8.
This can lead to higher effective max_weight than configured and also
deadlocks if a workqueue was depending on being able to handle chains of
interdependent work items that are longer than 8.
I believe these should be fine given that the number of CPUs in each NUMA
node is usually higher than 8 and work item chain longer than 8 is pretty
unlikely. However, if these assumptions turn out to be wrong, we'll need
to add an interface to adjust min_active.
- Each unbound wq has an array of struct wq_node_nr_active which tracks
per-node nr_active. When its pwq wants to run a work item, it has to
obtain the matching node's nr_active. If over the node's max_active, the
pwq is queued on wq_node_nr_active->pending_pwqs. As work items finish,
the completion path round-robins the pending pwqs activating the first
inactive work item of each, which involves some pool lock dancing and
kicking other pools. It's not the simplest code but doesn't look too bad.
v4: - wq_adjust_max_active() updated to invoke wq_update_node_max_active().
- wq_adjust_max_active() is now protected by wq->mutex instead of
wq_pool_mutex.
v3: - wq_node_max_active() used to calculate per-node max_active on the fly
based on system-wide CPU online states. Lai pointed out that this can
lead to skewed distributions for workqueues with restricted cpumasks.
Update the max_active distribution to use per-workqueue effective
online CPU counts instead of system-wide and cache the calculation
results in node_nr_active->max.
v2: - wq->min/max_active now uses WRITE/READ_ONCE() as suggested by Lai.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Naohiro Aota <Naohiro.Aota@wdc.com>
Link: http://lkml.kernel.org/r/dbu6wiwu3sdhmhikb2w6lns7b27gbobfavhjj57kwi2quafgwl@htjcc5oikcr3
Fixes: 636b927eba ("workqueue: Make unbound workqueues to use per-cpu pool_workqueues")
Reviewed-by: Lai Jiangshan <jiangshanlai@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
[ Upstream commit 91ccc6e723 ]
Currently, for both percpu and unbound workqueues, max_active applies
per-cpu, which is a recent change for unbound workqueues. The change for
unbound workqueues was a significant departure from the previous behavior of
per-node application. It made some use cases create undesirable number of
concurrent work items and left no good way of fixing them. To address the
problem, workqueue is implementing a NUMA node segmented global nr_active
mechanism, which will be explained further in the next patch.
As a preparation, this patch introduces struct wq_node_nr_active. It's a
data structured allocated for each workqueue and NUMA node pair and
currently only tracks the workqueue's number of active work items on the
node. This is split out from the next patch to make it easier to understand
and review.
Note that there is an extra wq_node_nr_active allocated for the invalid node
nr_node_ids which is used to track nr_active for pools which don't have NUMA
node associated such as the default fallback system-wide pool.
This doesn't cause any behavior changes visible to userland yet. The next
patch will expand to implement the control mechanism on top.
v4: - Fixed out-of-bound access when freeing per-cpu workqueues.
v3: - Use flexible array for wq->node_nr_active as suggested by Lai.
v2: - wq->max_active now uses WRITE/READ_ONCE() as suggested by Lai.
- Lai pointed out that pwq_tryinc_nr_active() incorrectly dropped
pwq->max_active check. Restored. As the next patch replaces the
max_active enforcement mechanism, this doesn't change the end result.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <jiangshanlai@gmail.com>
Stable-dep-of: 5797b1c189 ("workqueue: Implement system-wide nr_active enforcement for unbound workqueues")
Signed-off-by: Sasha Levin <sashal@kernel.org>
[ Upstream commit 9f66cff212 ]
wq->cpu_pwq is RCU protected but wq->dfl_pwq isn't. This is okay because
currently wq->dfl_pwq is used only accessed to install it into wq->cpu_pwq
which doesn't require RCU access. However, we want to be able to access
wq->dfl_pwq under RCU in the future to access its __pod_cpumask and the code
can be made easier to read by making the two pwq fields behave in the same
way.
- Make wq->dfl_pwq RCU protected.
- Add unbound_pwq_slot() and unbound_pwq() which can access both ->dfl_pwq
and ->cpu_pwq. The former returns the double pointer that can be used
access and update the pwqs. The latter performs locking check and
dereferences the double pointer.
- pwq accesses and updates are converted to use unbound_pwq[_slot]().
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <jiangshanlai@gmail.com>
Stable-dep-of: 5797b1c189 ("workqueue: Implement system-wide nr_active enforcement for unbound workqueues")
Signed-off-by: Sasha Levin <sashal@kernel.org>
[ Upstream commit c5404d4e6d ]
wq_adjust_max_active() needs to activate work items after max_active is
increased. Previously, it did that by visiting each pwq once activating all
that could be activated. While this makes sense with per-pwq nr_active,
nr_active will be shared across multiple pwqs for unbound wqs. Then, we'd
want to round-robin through pwqs to be fairer.
In preparation, this patch makes wq_adjust_max_active() round-robin pwqs
while activating. While the activation ordering changes, this shouldn't
cause user-noticeable behavior changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <jiangshanlai@gmail.com>
Stable-dep-of: 5797b1c189 ("workqueue: Implement system-wide nr_active enforcement for unbound workqueues")
Signed-off-by: Sasha Levin <sashal@kernel.org>
[ Upstream commit 1c270b79ce ]
__queue_work(), pwq_dec_nr_in_flight() and wq_adjust_max_active() were
open-coding nr_active handling, which is fine given that the operations are
trivial. However, the planned unbound nr_active update will make them more
complicated, so let's move them into helpers.
- pwq_tryinc_nr_active() is added. It increments nr_active if under
max_active limit and return a boolean indicating whether inc was
successful. Note that the function is structured to accommodate future
changes. __queue_work() is updated to use the new helper.
- pwq_activate_first_inactive() is updated to use pwq_tryinc_nr_active() and
thus no longer assumes that nr_active is under max_active and returns a
boolean to indicate whether a work item has been activated.
- wq_adjust_max_active() no longer tests directly whether a work item can be
activated. Instead, it's updated to use the return value of
pwq_activate_first_inactive() to tell whether a work item has been
activated.
- nr_active decrement and activating the first inactive work item is
factored into pwq_dec_nr_active().
v3: - WARN_ON_ONCE(!WORK_STRUCT_INACTIVE) added to __pwq_activate_work() as
now we're calling the function unconditionally from
pwq_activate_first_inactive().
v2: - wq->max_active now uses WRITE/READ_ONCE() as suggested by Lai.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <jiangshanlai@gmail.com>
Stable-dep-of: 5797b1c189 ("workqueue: Implement system-wide nr_active enforcement for unbound workqueues")
Signed-off-by: Sasha Levin <sashal@kernel.org>
[ Upstream commit 4c6380305d ]
To prepare for unbound nr_active handling improvements, move work activation
part of pwq_activate_inactive_work() into __pwq_activate_work() and add
pwq_activate_work() which tests WORK_STRUCT_INACTIVE and updates nr_active.
pwq_activate_first_inactive() and try_to_grab_pending() are updated to use
pwq_activate_work(). The latter conversion is functionally identical. For
the former, this conversion adds an unnecessary WORK_STRUCT_INACTIVE
testing. This is temporary and will be removed by the next patch.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <jiangshanlai@gmail.com>
Stable-dep-of: 5797b1c189 ("workqueue: Implement system-wide nr_active enforcement for unbound workqueues")
Signed-off-by: Sasha Levin <sashal@kernel.org>
[ Upstream commit a045a272d8 ]
max_active is a workqueue-wide setting and the configured value is stored in
wq->saved_max_active; however, the effective value was stored in
pwq->max_active. While this is harmless, it makes max_active update process
more complicated and gets in the way of the planned max_active semantic
updates for unbound workqueues.
This patches moves pwq->max_active to wq->max_active. This simplifies the
code and makes freezing and noop max_active updates cheaper too. No
user-visible behavior change is intended.
As wq->max_active is updated while holding wq mutex but read without any
locking, it now uses WRITE/READ_ONCE(). A new locking locking rule WO is
added for it.
v2: wq->max_active now uses WRITE/READ_ONCE() as suggested by Lai.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <jiangshanlai@gmail.com>
Stable-dep-of: 5797b1c189 ("workqueue: Implement system-wide nr_active enforcement for unbound workqueues")
Signed-off-by: Sasha Levin <sashal@kernel.org>
[ Upstream commit 31c8900728 ]
Currently we limit the size of the workqueue name to 24 characters due to
commit ecf6881ff3 ("workqueue: make workqueue->name[] fixed len")
Increase the size to 32 characters and print a warning in the event
the requested name is larger than the limit of 32 characters.
Signed-off-by: Audra Mitchell <audra@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Stable-dep-of: 5797b1c189 ("workqueue: Implement system-wide nr_active enforcement for unbound workqueues")
Signed-off-by: Sasha Levin <sashal@kernel.org>
This reverts commit ca10d851b9.
The commit allowed workqueue_apply_unbound_cpumask() to clear __WQ_ORDERED
on now removed implicitly ordered workqueues. This was incorrect in that
system-wide config change shouldn't break ordering properties of all
workqueues. The reason why apply_workqueue_attrs() path was allowed to do so
was because it was targeting the specific workqueue - either the workqueue
had WQ_SYSFS set or the workqueue user specifically tried to change
max_active, both of which indicate that the workqueue doesn't need to be
ordered.
The implicitly ordered workqueue promotion was removed by the previous
commit 3bc1e711c2 ("workqueue: Don't implicitly make UNBOUND workqueues w/
@max_active==1 ordered"). However, it didn't update this path and broke
build. Let's revert the commit which was incorrect in the first place which
also fixes build.
Signed-off-by: Tejun Heo <tj@kernel.org>
Fixes: 3bc1e711c2 ("workqueue: Don't implicitly make UNBOUND workqueues w/ @max_active==1 ordered")
Fixes: ca10d851b9 ("workqueue: Override implicit ordered attribute in workqueue_apply_unbound_cpumask()")
Cc: stable@vger.kernel.org # v6.6+
Signed-off-by: Tejun Heo <tj@kernel.org>
cgroup/for-6.8 is carrying two workqueue changes to allow cpuset to restrict
the CPUs used by unbound workqueues. Unfortunately, this conflicts with a
new bug fix in wq/for-6.7-fixes. The conflict is contextual but can be a bit
confusing to resolve. Pull the fix branch to resolve the conflict.
Signed-off-by: Tejun Heo <tj@kernel.org>
During boot, depending on how the housekeeping and workqueue.unbound_cpus
masks are set, wq_unbound_cpumask can end up empty. Since 8639ecebc9
("workqueue: Implement non-strict affinity scope for unbound workqueues"),
this may end up feeding -1 as a CPU number into scheduler leading to oopses.
BUG: unable to handle page fault for address: ffffffff8305e9c0
#PF: supervisor read access in kernel mode
#PF: error_code(0x0000) - not-present page
...
Call Trace:
<TASK>
select_idle_sibling+0x79/0xaf0
select_task_rq_fair+0x1cb/0x7b0
try_to_wake_up+0x29c/0x5c0
wake_up_process+0x19/0x20
kick_pool+0x5e/0xb0
__queue_work+0x119/0x430
queue_work_on+0x29/0x30
...
An empty wq_unbound_cpumask is a clear misconfiguration and already
disallowed once system is booted up. Let's warn on and ignore
unbound_cpumask restrictions which lead to no unbound cpus. While at it,
also remove now unncessary empty check on wq_unbound_cpumask in
wq_select_unbound_cpu().
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-and-Tested-by: Yong He <alexyonghe@tencent.com>
Link: http://lkml.kernel.org/r/20231120121623.119780-1-alexyonghe@tencent.com
Fixes: 8639ecebc9 ("workqueue: Implement non-strict affinity scope for unbound workqueues")
Cc: stable@vger.kernel.org # v6.6+
Reviewed-by: Waiman Long <longman@redhat.com>
Commit fe28f631fa ("workqueue: Add workqueue_unbound_exclude_cpumask()
to exclude CPUs from wq_unbound_cpumask") makes
workqueue_set_unbound_cpumask() static as it is not used elsewhere in
the kernel. However, this triggers a kernel test robot warning about
'workqueue_set_unbound_cpumask' defined but not used when CONFIG_SYS
isn't defined. It happens that workqueue_set_unbound_cpumask() is only
called when CONFIG_SYS is defined.
Move workqueue_set_unbound_cpumask() and its helpers inside the
CONFIG_SYSFS compilation block to avoid the warning. There is no
functional change.
Fixes: fe28f631fa ("workqueue: Add workqueue_unbound_exclude_cpumask() to exclude CPUs from wq_unbound_cpumask")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202311130831.uh0AoCd1-lkp@intel.com/
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
When the "isolcpus" boot command line option is used to add a set
of isolated CPUs, those CPUs will be excluded automatically from
wq_unbound_cpumask to avoid running work functions from unbound
workqueues.
Recently cpuset has been extended to allow the creation of partitions
of isolated CPUs dynamically. To make it closer to the "isolcpus"
in functionality, the CPUs in those isolated cpuset partitions should be
excluded from wq_unbound_cpumask as well. This can be done currently by
explicitly writing to the workqueue's cpumask sysfs file after creating
the isolated partitions. However, this process can be error prone.
Ideally, the cpuset code should be allowed to request the workqueue code
to exclude those isolated CPUs from wq_unbound_cpumask so that this
operation can be done automatically and the isolated CPUs will be returned
back to wq_unbound_cpumask after the destructions of the isolated
cpuset partitions.
This patch adds a new workqueue_unbound_exclude_cpumask() function to
enable that. This new function will exclude the specified isolated
CPUs from wq_unbound_cpumask. To be able to restore those isolated
CPUs back after the destruction of isolated cpuset partitions, a new
wq_requested_unbound_cpumask is added to store the user provided unbound
cpumask either from the boot command line options or from writing to
the cpumask sysfs file. This new cpumask provides the basis for CPU
exclusion.
To enable users to understand how the wq_unbound_cpumask is being
modified internally, this patch also exposes the newly introduced
wq_requested_unbound_cpumask as well as a wq_isolated_cpumask to
store the cpumask to be excluded from wq_unbound_cpumask as read-only
sysfs files.
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
there's little I can say which isn't in the individual changelogs.
The lengthier patch series are
- "kdump: use generic functions to simplify crashkernel reservation in
arch", from Baoquan He. This is mainly cleanups and consolidation of
the "crashkernel=" kernel parameter handling.
- After much discussion, David Laight's "minmax: Relax type checks in
min() and max()" is here. Hopefully reduces some typecasting and the
use of min_t() and max_t().
- A group of patches from Oleg Nesterov which clean up and slightly fix
our handling of reads from /proc/PID/task/... and which remove
task_struct.therad_group.
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Merge tag 'mm-nonmm-stable-2023-11-02-14-08' of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
Pull non-MM updates from Andrew Morton:
"As usual, lots of singleton and doubleton patches all over the tree
and there's little I can say which isn't in the individual changelogs.
The lengthier patch series are
- 'kdump: use generic functions to simplify crashkernel reservation
in arch', from Baoquan He. This is mainly cleanups and
consolidation of the 'crashkernel=' kernel parameter handling
- After much discussion, David Laight's 'minmax: Relax type checks in
min() and max()' is here. Hopefully reduces some typecasting and
the use of min_t() and max_t()
- A group of patches from Oleg Nesterov which clean up and slightly
fix our handling of reads from /proc/PID/task/... and which remove
task_struct.thread_group"
* tag 'mm-nonmm-stable-2023-11-02-14-08' of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm: (64 commits)
scripts/gdb/vmalloc: disable on no-MMU
scripts/gdb: fix usage of MOD_TEXT not defined when CONFIG_MODULES=n
.mailmap: add address mapping for Tomeu Vizoso
mailmap: update email address for Claudiu Beznea
tools/testing/selftests/mm/run_vmtests.sh: lower the ptrace permissions
.mailmap: map Benjamin Poirier's address
scripts/gdb: add lx_current support for riscv
ocfs2: fix a spelling typo in comment
proc: test ProtectionKey in proc-empty-vm test
proc: fix proc-empty-vm test with vsyscall
fs/proc/base.c: remove unneeded semicolon
do_io_accounting: use sig->stats_lock
do_io_accounting: use __for_each_thread()
ocfs2: replace BUG_ON() at ocfs2_num_free_extents() with ocfs2_error()
ocfs2: fix a typo in a comment
scripts/show_delta: add __main__ judgement before main code
treewide: mark stuff as __ro_after_init
fs: ocfs2: check status values
proc: test /proc/${pid}/statm
compiler.h: move __is_constexpr() to compiler.h
...
__read_mostly predates __ro_after_init. Many variables which are marked
__read_mostly should have been __ro_after_init from day 1.
Also, mark some stuff as "const" and "__init" while I'm at it.
[akpm@linux-foundation.org: revert sysctl_nr_open_min, sysctl_nr_open_max changes due to arm warning]
[akpm@linux-foundation.org: coding-style cleanups]
Link: https://lkml.kernel.org/r/4f6bb9c0-abba-4ee4-a7aa-89265e886817@p183
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
All callers of work_on_cpu() share the same lock class key for all the
functions queued. As a result the workqueue related locking scenario for
a function A may be spuriously accounted as an inversion against the
locking scenario of function B such as in the following model:
long A(void *arg)
{
mutex_lock(&mutex);
mutex_unlock(&mutex);
}
long B(void *arg)
{
}
void launchA(void)
{
work_on_cpu(0, A, NULL);
}
void launchB(void)
{
mutex_lock(&mutex);
work_on_cpu(1, B, NULL);
mutex_unlock(&mutex);
}
launchA and launchB running concurrently have no chance to deadlock.
However the above can be reported by lockdep as a possible locking
inversion because the works containing A() and B() are treated as
belonging to the same locking class.
The following shows an existing example of such a spurious lockdep splat:
======================================================
WARNING: possible circular locking dependency detected
6.6.0-rc1-00065-g934ebd6e5359 #35409 Not tainted
------------------------------------------------------
kworker/0:1/9 is trying to acquire lock:
ffffffff9bc72f30 (cpu_hotplug_lock){++++}-{0:0}, at: _cpu_down+0x57/0x2b0
but task is already holding lock:
ffff9e3bc0057e60 ((work_completion)(&wfc.work)){+.+.}-{0:0}, at: process_scheduled_works+0x216/0x500
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #2 ((work_completion)(&wfc.work)){+.+.}-{0:0}:
__flush_work+0x83/0x4e0
work_on_cpu+0x97/0xc0
rcu_nocb_cpu_offload+0x62/0xb0
rcu_nocb_toggle+0xd0/0x1d0
kthread+0xe6/0x120
ret_from_fork+0x2f/0x40
ret_from_fork_asm+0x1b/0x30
-> #1 (rcu_state.barrier_mutex){+.+.}-{3:3}:
__mutex_lock+0x81/0xc80
rcu_nocb_cpu_deoffload+0x38/0xb0
rcu_nocb_toggle+0x144/0x1d0
kthread+0xe6/0x120
ret_from_fork+0x2f/0x40
ret_from_fork_asm+0x1b/0x30
-> #0 (cpu_hotplug_lock){++++}-{0:0}:
__lock_acquire+0x1538/0x2500
lock_acquire+0xbf/0x2a0
percpu_down_write+0x31/0x200
_cpu_down+0x57/0x2b0
__cpu_down_maps_locked+0x10/0x20
work_for_cpu_fn+0x15/0x20
process_scheduled_works+0x2a7/0x500
worker_thread+0x173/0x330
kthread+0xe6/0x120
ret_from_fork+0x2f/0x40
ret_from_fork_asm+0x1b/0x30
other info that might help us debug this:
Chain exists of:
cpu_hotplug_lock --> rcu_state.barrier_mutex --> (work_completion)(&wfc.work)
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
lock((work_completion)(&wfc.work));
lock(rcu_state.barrier_mutex);
lock((work_completion)(&wfc.work));
lock(cpu_hotplug_lock);
*** DEADLOCK ***
2 locks held by kworker/0:1/9:
#0: ffff900481068b38 ((wq_completion)events){+.+.}-{0:0}, at: process_scheduled_works+0x212/0x500
#1: ffff9e3bc0057e60 ((work_completion)(&wfc.work)){+.+.}-{0:0}, at: process_scheduled_works+0x216/0x500
stack backtrace:
CPU: 0 PID: 9 Comm: kworker/0:1 Not tainted 6.6.0-rc1-00065-g934ebd6e5359 #35409
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.12.0-59-gc9ba5276e321-prebuilt.qemu.org 04/01/2014
Workqueue: events work_for_cpu_fn
Call Trace:
rcu-torture: rcu_torture_read_exit: Start of episode
<TASK>
dump_stack_lvl+0x4a/0x80
check_noncircular+0x132/0x150
__lock_acquire+0x1538/0x2500
lock_acquire+0xbf/0x2a0
? _cpu_down+0x57/0x2b0
percpu_down_write+0x31/0x200
? _cpu_down+0x57/0x2b0
_cpu_down+0x57/0x2b0
__cpu_down_maps_locked+0x10/0x20
work_for_cpu_fn+0x15/0x20
process_scheduled_works+0x2a7/0x500
worker_thread+0x173/0x330
? __pfx_worker_thread+0x10/0x10
kthread+0xe6/0x120
? __pfx_kthread+0x10/0x10
ret_from_fork+0x2f/0x40
? __pfx_kthread+0x10/0x10
ret_from_fork_asm+0x1b/0x30
</TASK
Fix this with providing one lock class key per work_on_cpu() caller.
Reported-and-tested-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
Compiling with W=1 emitted the following warning
(Compiler: gcc (x86-64, ver. 13.2.1, .config: result of make allyesconfig,
"Treat warnings as errors" turned off):
kernel/workqueue.c:2188:54: warning: ‘%d’ directive output may be
truncated writing between 1 and 10 bytes into a region of size
between 5 and 14 [-Wformat-truncation=]
kernel/workqueue.c:2188:50: note: directive argument in the range
[0, 2147483647]
kernel/workqueue.c:2188:17: note: ‘snprintf’ output between 4 and 23 bytes
into a destination of size 16
setting "id_buf" to size 23 will silence the warning, since GCC
determines snprintf's output to be max. 23 bytes in line 2188.
Please let me know if there are any mistakes in my patch!
Signed-off-by: Lucy Mielke <lucymielke@icloud.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Commit 5c0338c687 ("workqueue: restore WQ_UNBOUND/max_active==1
to be ordered") enabled implicit ordered attribute to be added to
WQ_UNBOUND workqueues with max_active of 1. This prevented the changing
of attributes to these workqueues leading to fix commit 0a94efb5ac
("workqueue: implicit ordered attribute should be overridable").
However, workqueue_apply_unbound_cpumask() was not updated at that time.
So sysfs changes to wq_unbound_cpumask has no effect on WQ_UNBOUND
workqueues with implicit ordered attribute. Since not all WQ_UNBOUND
workqueues are visible on sysfs, we are not able to make all the
necessary cpumask changes even if we iterates all the workqueue cpumasks
in sysfs and changing them one by one.
Fix this problem by applying the corresponding change made
to apply_workqueue_attrs_locked() in the fix commit to
workqueue_apply_unbound_cpumask().
Fixes: 5c0338c687 ("workqueue: restore WQ_UNBOUND/max_active==1 to be ordered")
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Currently, the kfree() be used for pwq objects allocated with
kmem_cache_alloc() in alloc_and_link_pwqs(), this isn't wrong.
but usually, use "trace_kmem_cache_alloc/trace_kmem_cache_free"
to track memory allocation and free. this commit therefore use
kmem_cache_free() instead of kfree() in alloc_and_link_pwqs()
and also consistent with release of the pwq in rcu_free_pwq().
Signed-off-by: Zqiang <qiang.zhang1211@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Currently, if the wq_cpu_intensive_thresh_us is set to specific
value, will cause the wq_cpu_intensive_thresh_init() early exit
and missed creation of pwq_release_worker. this commit therefore
create the pwq_release_worker in advance before checking the
wq_cpu_intensive_thresh_us.
Signed-off-by: Zqiang <qiang.zhang1211@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Fixes: 967b494e2f ("workqueue: Use a kthread_worker to release pool_workqueues")
First commit 2930155b2e ("workqueue: Initialize unbound CPU pods later in
the boot") added the initialization of wq_update_pod_attrs_buf to
workqueue_init_early(), and then latter on, commit 84193c0710
("workqueue: Generalize unbound CPU pods") added it as well. This appeared
in a kmemleak run where the second allocation made the first allocation
leak.
Fixes: 84193c0710 ("workqueue: Generalize unbound CPU pods")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Tejun Heo <tj@kernel.org>
Each CPU-specific and unbound kworker kthread conforms to a particular
naming scheme. However, this does not extend to the rescuer kworker.
At present, a rescuer kworker is simply named according to its
workqueue's name. This can be cryptic.
This patch modifies a rescuer to follow the kworker naming scheme.
The "R" is indicative of a rescuer and after "-" is its workqueue's
name e.g. "kworker/R-ext4-rsv-conver".
tj: Use "R" instead of "r" as the prefix to make it more distinctive and
consistent with how highpri pools are marked.
Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
While workqueue.default_affinity_scope is writable, it only affects
workqueues which are created afterwards and isn't very useful. Instead,
let's introduce explicit "default" scope and update the effective scope
dynamically when workqueue.default_affinity_scope is changed.
Signed-off-by: Tejun Heo <tj@kernel.org>
An unbound workqueue can be served by multiple worker_pools to improve
locality. The segmentation is achieved by grouping CPUs into pods. By
default, the cache boundaries according to cpus_share_cache() define the
CPUs are grouped. Let's a workqueue is allowed to run on all CPUs and the
system has two L3 caches. The workqueue would be mapped to two worker_pools
each serving one L3 cache domains.
While this improves locality, because the pod boundaries are strict, it
limits the total bandwidth a given issuer can consume. For example, let's
say there is a thread pinned to a CPU issuing enough work items to saturate
the whole machine. With the machine segmented into two pods, no matter how
many work items it issues, it can only use half of the CPUs on the system.
While this limitation has existed for a very long time, it wasn't very
pronounced because the affinity grouping used to be always by NUMA nodes.
With cache boundaries as the default and support for even finer grained
scopes (smt and cpu), it is now an a lot more pressing problem.
This patch implements non-strict affinity scope where the pod boundaries
aren't enforced strictly. Going back to the previous example, the workqueue
would still be mapped to two worker_pools; however, the affinity enforcement
would be soft. The workers in both pools would have their cpus_allowed set
to the whole machine thus allowing the scheduler to migrate them anywhere on
the machine. However, whenever an idle worker is woken up, the workqueue
code asks the scheduler to bring back the task within the pod if the worker
is outside. ie. work items start executing within its affinity scope but can
be migrated outside as the scheduler sees fit. This removes the hard cap on
utilization while maintaining the benefits of affinity scopes.
After the earlier ->__pod_cpumask changes, the implementation is pretty
simple. When non-strict which is the new default:
* pool_allowed_cpus() returns @pool->attrs->cpumask instead of
->__pod_cpumask so that the workers are allowed to run on any CPU that
the associated workqueues allow.
* If the idle worker task's ->wake_cpu is outside the pod, kick_pool() sets
the field to a CPU within the pod.
This would be the first use of task_struct->wake_cpu outside scheduler
proper, so it isn't clear whether this would be acceptable. However, other
methods of migrating tasks are significantly more expensive and are likely
prohibitively so if we want to do this on every work item. This needs
discussion with scheduler folks.
There is also a race window where setting ->wake_cpu wouldn't be effective
as the target task is still on CPU. However, the window is pretty small and
this being a best-effort optimization, it doesn't seem to warrant more
complexity at the moment.
While the non-strict cache affinity scopes seem to be the best option, the
performance picture interacts with the affinity scope and is a bit
complicated to fully discuss in this patch, so the behavior is made easily
selectable through wqattrs and sysfs and the next patch will add
documentation to discuss performance implications.
v2: pool->attrs->affn_strict is set to true for per-cpu worker_pools.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
workqueue_attrs has two uses:
* to specify the required unouned workqueue properties by users
* to match worker_pool's properties to workqueues by core code
For example, if the user wants to restrict a workqueue to run only CPUs 0
and 2, and the two CPUs are on different affinity scopes, the workqueue's
attrs->cpumask would contains CPUs 0 and 2, and the workqueue would be
associated with two worker_pools, one with attrs->cpumask containing just
CPU 0 and the other CPU 2.
Workqueue wants to support non-strict affinity scopes where work items are
started in their matching affinity scopes but the scheduler is free to
migrate them outside the starting scopes, which can enable utilizing the
whole machine while maintaining most of the locality benefits from affinity
scopes.
To enable that, worker_pools need to distinguish the strict affinity that it
has to follow (because that's the restriction coming from the user) and the
soft affinity that it wants to apply when dispatching work items. Note that
two worker_pools with different soft dispatching requirements have to be
separate; otherwise, for example, we'd be ping-ponging worker threads across
NUMA boundaries constantly.
This patch adds workqueue_attrs->__pod_cpumask. The new field is double
underscored as it's only used internally to distinguish worker_pools. A
worker_pool's ->cpumask is now always the same as the online subset of
allowed CPUs of the associated workqueues, and ->__pod_cpumask is the pod's
subset of that ->cpumask. Going back to the example above, both worker_pools
would have ->cpumask containing both CPUs 0 and 2 but one's ->__pod_cpumask
would contain 0 while the other's 2.
* pool_allowed_cpus() is added. It returns the worker_pool's strict cpumask
that the pool's workers must stay within. This is currently always
->__pod_cpumask as all boundaries are still strict.
* As a workqueue_attrs can now track both the associated workqueues' cpumask
and its per-pod subset, wq_calc_pod_cpumask() no longer needs an external
out-argument. Drop @cpumask and instead store the result in
->__pod_cpumask.
* The above also simplifies apply_wqattrs_prepare() as the same
workqueue_attrs can be used to create all pods associated with a
workqueue. tmp_attrs is dropped.
* wq_update_pod() is updated to use wqattrs_equal() to test whether a pwq
update is needed instead of only comparing ->cpumask so that
->__pod_cpumask is compared too. It can directly compare ->__pod_cpumaks
but the code is easier to understand and more robust this way.
The only user-visible behavior change is that two workqueues with different
cpumasks no longer can share worker_pools even when their pod subsets
coincide. Going back to the example, let's say there's another workqueue
with cpumask 0, 2, 3, where 2 and 3 are in the same pod. It would be mapped
to two worker_pools - one with CPU 0, the other with 2 and 3. The former has
the same cpumask as the first pod of the earlier example and would have
shared the same worker_pool but that's no longer the case after this patch.
The worker_pools would have the same ->__pod_cpumask but their ->cpumask's
wouldn't match.
While this is necessary to support non-strict affinity scopes, there can be
further optimizations to maintain sharing among strict affinity scopes.
However, non-strict affinity scopes are going to be preferable for most use
cases and we don't see very diverse mixture of unbound workqueue cpumasks
anyway, so the additional overhead doesn't seem to justify the extra
complexity.
v2: - wq_update_pod() was incorrectly comparing target_attrs->__pod_cpumask
to pool->attrs->cpumask instead of its ->__pod_cpumask. Fix it by
using wqattrs_equal() for comparison instead.
- Per-cpu worker pools weren't initializing ->__pod_cpumask which caused
a subtle problem later on. Set it to cpumask_of(cpu) like ->cpumask.
Signed-off-by: Tejun Heo <tj@kernel.org>
Checking need_more_worker() and calling wake_up_worker() is a repeated
pattern. Let's add kick_pool(), which checks need_more_worker() and
open-code wake_up_worker(), and replace wake_up_worker() uses. The following
conversions aren't one-to-one:
* __queue_work() was using __need_more_work() because it knows that
pool->worklist isn't empty. Switching to kick_pool() adds an extra
list_empty() test.
* create_worker() always needs to wake up the newly minted worker whether
there's more work to do or not to avoid triggering hung task check on the
new task. Keep the current wake_up_process() and still add kick_pool().
This may lead to an extra wakeup which isn't harmful.
* pwq_adjust_max_active() was explicitly checking whether it needs to wake
up a worker or not to avoid spurious wakeups. As kick_pool() only wakes up
a worker when necessary, this explicit check is no longer necessary and
dropped.
* unbind_workers() now calls kick_pool() instead of wake_up_worker() adding
a need_more_worker() test. This avoids spurious wakeups and shouldn't
break anything.
wake_up_worker() is dropped as kick_pool() replaces all its users. After
this patch, all paths that wakes up a non-rescuer worker to initiate work
item execution use kick_pool(). This will enable future changes to improve
locality.
Signed-off-by: Tejun Heo <tj@kernel.org>
The two work execution paths in worker_thread() and rescuer_thread() use
move_linked_works() to claim work items from @pool->worklist. Once claimed,
process_schedule_works() is called which invokes process_one_work() on each
work item. process_one_work() then uses find_worker_executing_work() to
detect and handle collisions - situations where the work item to be executed
is still running on another worker.
This works fine, but, to improve work execution locality, we want to
establish work to worker association earlier and know for sure that the
worker is going to excute the work once asssigned, which requires performing
collision handling earlier while trying to assign the work item to the
worker.
This patch introduces assign_work() which assigns a work item to a worker
using move_linked_works() and then performs collision handling. As collision
handling is handled earlier, process_one_work() no longer needs to worry
about them.
After the this patch, collision checks for linked work items are skipped,
which should be fine as they can't be queued multiple times concurrently.
For work items running from rescuers, the timing of collision handling may
change but the invariant that the work items go through collision handling
before starting execution does not.
This patch shouldn't cause noticeable behavior changes, especially given
that worker_thread() behavior remains the same.
Signed-off-by: Tejun Heo <tj@kernel.org>
Add three more affinity scopes - WQ_AFFN_CPU, SMT and CACHE - and make CACHE
the default. The code changes to actually add the additional scopes are
trivial.
Also add module parameter "workqueue.default_affinity_scope" to override the
default scope and "affinity_scope" sysfs file to configure it per workqueue.
wq_dump.py and documentations are updated accordingly.
This enables significant flexibility in configuring how unbound workqueues
behave. If affinity scope is set to "cpu", it'll behave close to a per-cpu
workqueue. On the other hand, "system" removes all locality boundaries.
Many modern machines have multiple L3 caches often while being mostly
uniform in terms of memory access. Thus, workqueue's previous behavior of
spreading work items in each NUMA node had negative performance implications
from unncessarily crossing L3 boundaries between issue and execution.
However, picking a finer grained affinity scope also has a downside in that
an issuer in one group can't utilize CPUs in other groups.
While dependent on the specifics of workload, there's usually a noticeable
penalty in crossing L3 boundaries, so let's default to CACHE. This issue
will be further addressed and documented with examples in future patches.
Signed-off-by: Tejun Heo <tj@kernel.org>
While wq_pod_type[] can now group CPUs in any aribitrary way, WQ_AFFN_NUM
init is hard coded into workqueue_init_topology(). This patch modularizes
the init path by introducing init_pod_type() which takes a callback to
determine whether two CPUs should share a pod as an argument.
init_pod_type() first scans the CPU combinations testing for sharing to
assign consecutive pod IDs and initialize pod_type->cpu_pod[]. Once
->cpu_pod[] is determined, ->pod_cpus[] and ->pod_node[] are initialized
accordingly. WQ_AFFN_NUMA is now initialized by calling init_pod_type() with
cpus_share_numa() which tests whether the CPU belongs to the same NUMA node.
This patch may change the pod ID assigned to each NUMA node but that
shouldn't cause any behavior changes as the NUMA node to use for allocations
are tracked separately in pod_type->pod_node[]. This makes adding new
affinty types pretty easy.
Signed-off-by: Tejun Heo <tj@kernel.org>
While renamed to pod, the code still assumes that the pods are defined by
NUMA boundaries. Let's generalize it:
* workqueue_attrs->affn_scope is added. Each enum represents the type of
boundaries that define the pods. There are currently two scopes -
WQ_AFFN_NUMA and WQ_AFFN_SYSTEM. The former is the same behavior as before
- one pod per NUMA node. The latter defines one global pod across the
whole system.
* struct wq_pod_type is added which describes how pods are configured for
each affnity scope. For each pod, it lists the member CPUs and the
preferred NUMA node for memory allocations. The reverse mapping from CPU
to pod is also available.
* wq_pod_enabled is dropped. Pod is now always enabled. The previously
disabled behavior is now implemented through WQ_AFFN_SYSTEM.
* get_unbound_pool() wants to determine the NUMA node to allocate memory
from for the new pool. The variables are renamed from node to pod but the
logic still assumes they're one and the same. Clearly distinguish them -
walk the WQ_AFFN_NUMA pods to find the matching pod and then use the pod's
NUMA node.
* wq_calc_pod_cpumask() was taking @pod but assumed that it was the NUMA
node. Take @cpu instead and determine the cpumask to use from the pod_type
matching @attrs.
* apply_wqattrs_prepare() is update to return ERR_PTR() on error instead of
NULL so that it can indicate -EINVAL on invalid affinity scopes.
This patch allows CPUs to be grouped into pods however desired per type.
While this patch causes some internal behavior changes, nothing material
should change for workqueue users.
v2: Trigger WARN_ON_ONCE() in wqattrs_pod_type() if affn_scope is
WQ_AFFN_NR_TYPES which indicates that the function is called with a
worker_pool's attrs instead of a workqueue's.
Signed-off-by: Tejun Heo <tj@kernel.org>
workqueue_attrs can be used for both workqueues and worker_pools. However,
some fields, currently only ->ordered, only apply to workqueues and should
be cleared to the default / invalid values.
Currently, an unbound workqueue explicitly clears attrs->ordered in
get_unbound_pool() after copying the source workqueue attrs, while per-cpu
workqueues rely on the fact that zeroing on allocation gives us the desired
default value for pool->attrs->ordered.
This is fragile. Let's add wqattrs_clear_for_pool() which clears
attrs->ordered and is called from both init_worker_pool() and
get_unbound_pool(). This will ease adding more workqueue-only attrs fields.
In get_unbound_pool(), pool->node initialization is moved upwards for
readability. This shouldn't cause any behavior changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
For an unbound pool, multiple cpumasks are involved.
U: The user-specified cpumask (may be filtered with cpu_possible_mask).
A: The actual cpumask filtered by wq_unbound_cpumask. If the filtering
leaves no CPU, wq_unbound_cpumask is used.
P: Per-pod subsets of #A.
wq->attrs stores #U, wq->dfl_pwq->pool->attrs->cpumask #A, and
wq->cpu_pwq[CPU]->pool->attrs->cpumask #P.
wq_update_pod() is called to update per-pod pwq's during CPU hotplug. To
calculate the new #P for each workqueue, it needs to call
wq_calc_pod_cpumask() with @attrs that contains #A. Currently,
wq_update_pod() achieves this by calling wq_calc_pod_cpumask() with
wq->dfl_pwq->pool->attrs.
This is rather fragile because we're calling wq_calc_pod_cpumask() with
@attrs of a worker_pool rather than the workqueue's actual attrs when what
we want to calculate is the workqueue's cpumask on the pod. While this works
fine currently, future changes will add fields which are used differently
between workqueues and worker_pools and this subtlety will bite us.
This patch factors out #U -> #A calculation from apply_wqattrs_prepare()
into wqattrs_actualize_cpumask and updates wq_update_pod() to copy
wq->unbound_attrs and use the new helper to obtain #A freshly instead of
abusing wq->dfl_pwq->pool_attrs.
This shouldn't cause any behavior changes in the current code.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: K Prateek Nayak <kprateek.nayak@amd.com>
Reference: http://lkml.kernel.org/r/30625cdd-4d61-594b-8db9-6816b017dde3@amd.com
During boot, to initialize unbound CPU pods, wq_pod_init() was called from
workqueue_init(). This is early enough for NUMA nodes to be set up but
before SMP is brought up and CPU topology information is populated.
Workqueue is in the process of improving CPU locality for unbound workqueues
and will need access to topology information during pod init. This adds a
new init function workqueue_init_topology() which is called after CPU
topology information is available and replaces wq_pod_init().
As unbound CPU pods are now initialized after workqueues are activated, we
need to revisit the workqueues to apply the pod configuration. Workqueues
which are created before workqueue_init_topology() are set up so that they
always use the default worker pool. After pods are set up in
workqueue_init_topology(), wq_update_pod() is called on all existing
workqueues to update the pool associations accordingly.
Note that wq_update_pod_attrs_buf allocation is moved to
workqueue_init_early(). This isn't necessary right now but enables further
generalization of pod handling in the future.
This patch changes the initialization sequence but the end result should be
the same.
Signed-off-by: Tejun Heo <tj@kernel.org>
wq_pod_init() is called from workqueue_init() and responsible for
initializing unbound CPU pods according to NUMA node. Workqueue is in the
process of improving affinity awareness and wants to use other topology
information to initialize unbound CPU pods; however, unlike NUMA nodes,
other topology information isn't yet available in workqueue_init().
The next patch will introduce a later stage init function for workqueue
which will be responsible for initializing unbound CPU pods. Relocate
wq_pod_init() below workqueue_init() where the new init function is going to
be located so that the diff can show the content differences.
Just a relocation. No functional changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
