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![Mel Gorman](/assets/img/avatar_default.png)
Commit c0ff7453bb
("cpuset,mm: fix no node to alloc memory when
changing cpuset's mems") wins a super prize for the largest number of
memory barriers entered into fast paths for one commit.
[get|put]_mems_allowed is incredibly heavy with pairs of full memory
barriers inserted into a number of hot paths. This was detected while
investigating at large page allocator slowdown introduced some time
after 2.6.32. The largest portion of this overhead was shown by
oprofile to be at an mfence introduced by this commit into the page
allocator hot path.
For extra style points, the commit introduced the use of yield() in an
implementation of what looks like a spinning mutex.
This patch replaces the full memory barriers on both read and write
sides with a sequence counter with just read barriers on the fast path
side. This is much cheaper on some architectures, including x86. The
main bulk of the patch is the retry logic if the nodemask changes in a
manner that can cause a false failure.
While updating the nodemask, a check is made to see if a false failure
is a risk. If it is, the sequence number gets bumped and parallel
allocators will briefly stall while the nodemask update takes place.
In a page fault test microbenchmark, oprofile samples from
__alloc_pages_nodemask went from 4.53% of all samples to 1.15%. The
actual results were
3.3.0-rc3 3.3.0-rc3
rc3-vanilla nobarrier-v2r1
Clients 1 UserTime 0.07 ( 0.00%) 0.08 (-14.19%)
Clients 2 UserTime 0.07 ( 0.00%) 0.07 ( 2.72%)
Clients 4 UserTime 0.08 ( 0.00%) 0.07 ( 3.29%)
Clients 1 SysTime 0.70 ( 0.00%) 0.65 ( 6.65%)
Clients 2 SysTime 0.85 ( 0.00%) 0.82 ( 3.65%)
Clients 4 SysTime 1.41 ( 0.00%) 1.41 ( 0.32%)
Clients 1 WallTime 0.77 ( 0.00%) 0.74 ( 4.19%)
Clients 2 WallTime 0.47 ( 0.00%) 0.45 ( 3.73%)
Clients 4 WallTime 0.38 ( 0.00%) 0.37 ( 1.58%)
Clients 1 Flt/sec/cpu 497620.28 ( 0.00%) 520294.53 ( 4.56%)
Clients 2 Flt/sec/cpu 414639.05 ( 0.00%) 429882.01 ( 3.68%)
Clients 4 Flt/sec/cpu 257959.16 ( 0.00%) 258761.48 ( 0.31%)
Clients 1 Flt/sec 495161.39 ( 0.00%) 517292.87 ( 4.47%)
Clients 2 Flt/sec 820325.95 ( 0.00%) 850289.77 ( 3.65%)
Clients 4 Flt/sec 1020068.93 ( 0.00%) 1022674.06 ( 0.26%)
MMTests Statistics: duration
Sys Time Running Test (seconds) 135.68 132.17
User+Sys Time Running Test (seconds) 164.2 160.13
Total Elapsed Time (seconds) 123.46 120.87
The overall improvement is small but the System CPU time is much
improved and roughly in correlation to what oprofile reported (these
performance figures are without profiling so skew is expected). The
actual number of page faults is noticeably improved.
For benchmarks like kernel builds, the overall benefit is marginal but
the system CPU time is slightly reduced.
To test the actual bug the commit fixed I opened two terminals. The
first ran within a cpuset and continually ran a small program that
faulted 100M of anonymous data. In a second window, the nodemask of the
cpuset was continually randomised in a loop.
Without the commit, the program would fail every so often (usually
within 10 seconds) and obviously with the commit everything worked fine.
With this patch applied, it also worked fine so the fix should be
functionally equivalent.
Signed-off-by: Mel Gorman <mgorman@suse.de>
Cc: Miao Xie <miaox@cn.fujitsu.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Christoph Lameter <cl@linux.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
240 lines
5.7 KiB
C
240 lines
5.7 KiB
C
#ifndef _LINUX_CPUSET_H
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#define _LINUX_CPUSET_H
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/*
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* cpuset interface
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*
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* Copyright (C) 2003 BULL SA
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* Copyright (C) 2004-2006 Silicon Graphics, Inc.
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*
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*/
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#include <linux/sched.h>
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#include <linux/cpumask.h>
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#include <linux/nodemask.h>
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#include <linux/cgroup.h>
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#include <linux/mm.h>
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#ifdef CONFIG_CPUSETS
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extern int number_of_cpusets; /* How many cpusets are defined in system? */
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extern int cpuset_init(void);
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extern void cpuset_init_smp(void);
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extern void cpuset_update_active_cpus(void);
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extern void cpuset_cpus_allowed(struct task_struct *p, struct cpumask *mask);
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extern int cpuset_cpus_allowed_fallback(struct task_struct *p);
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extern nodemask_t cpuset_mems_allowed(struct task_struct *p);
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#define cpuset_current_mems_allowed (current->mems_allowed)
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void cpuset_init_current_mems_allowed(void);
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int cpuset_nodemask_valid_mems_allowed(nodemask_t *nodemask);
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extern int __cpuset_node_allowed_softwall(int node, gfp_t gfp_mask);
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extern int __cpuset_node_allowed_hardwall(int node, gfp_t gfp_mask);
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static inline int cpuset_node_allowed_softwall(int node, gfp_t gfp_mask)
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{
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return number_of_cpusets <= 1 ||
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__cpuset_node_allowed_softwall(node, gfp_mask);
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}
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static inline int cpuset_node_allowed_hardwall(int node, gfp_t gfp_mask)
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{
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return number_of_cpusets <= 1 ||
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__cpuset_node_allowed_hardwall(node, gfp_mask);
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}
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static inline int cpuset_zone_allowed_softwall(struct zone *z, gfp_t gfp_mask)
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{
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return cpuset_node_allowed_softwall(zone_to_nid(z), gfp_mask);
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}
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static inline int cpuset_zone_allowed_hardwall(struct zone *z, gfp_t gfp_mask)
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{
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return cpuset_node_allowed_hardwall(zone_to_nid(z), gfp_mask);
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}
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extern int cpuset_mems_allowed_intersects(const struct task_struct *tsk1,
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const struct task_struct *tsk2);
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#define cpuset_memory_pressure_bump() \
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do { \
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if (cpuset_memory_pressure_enabled) \
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__cpuset_memory_pressure_bump(); \
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} while (0)
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extern int cpuset_memory_pressure_enabled;
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extern void __cpuset_memory_pressure_bump(void);
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extern const struct file_operations proc_cpuset_operations;
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struct seq_file;
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extern void cpuset_task_status_allowed(struct seq_file *m,
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struct task_struct *task);
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extern int cpuset_mem_spread_node(void);
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extern int cpuset_slab_spread_node(void);
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static inline int cpuset_do_page_mem_spread(void)
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{
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return current->flags & PF_SPREAD_PAGE;
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}
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static inline int cpuset_do_slab_mem_spread(void)
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{
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return current->flags & PF_SPREAD_SLAB;
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}
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extern int current_cpuset_is_being_rebound(void);
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extern void rebuild_sched_domains(void);
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extern void cpuset_print_task_mems_allowed(struct task_struct *p);
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/*
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* get_mems_allowed is required when making decisions involving mems_allowed
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* such as during page allocation. mems_allowed can be updated in parallel
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* and depending on the new value an operation can fail potentially causing
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* process failure. A retry loop with get_mems_allowed and put_mems_allowed
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* prevents these artificial failures.
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*/
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static inline unsigned int get_mems_allowed(void)
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{
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return read_seqcount_begin(¤t->mems_allowed_seq);
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}
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/*
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* If this returns false, the operation that took place after get_mems_allowed
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* may have failed. It is up to the caller to retry the operation if
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* appropriate.
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*/
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static inline bool put_mems_allowed(unsigned int seq)
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{
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return !read_seqcount_retry(¤t->mems_allowed_seq, seq);
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}
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static inline void set_mems_allowed(nodemask_t nodemask)
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{
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task_lock(current);
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write_seqcount_begin(¤t->mems_allowed_seq);
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current->mems_allowed = nodemask;
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write_seqcount_end(¤t->mems_allowed_seq);
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task_unlock(current);
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}
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#else /* !CONFIG_CPUSETS */
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static inline int cpuset_init(void) { return 0; }
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static inline void cpuset_init_smp(void) {}
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static inline void cpuset_update_active_cpus(void)
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{
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partition_sched_domains(1, NULL, NULL);
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}
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static inline void cpuset_cpus_allowed(struct task_struct *p,
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struct cpumask *mask)
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{
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cpumask_copy(mask, cpu_possible_mask);
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}
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static inline int cpuset_cpus_allowed_fallback(struct task_struct *p)
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{
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do_set_cpus_allowed(p, cpu_possible_mask);
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return cpumask_any(cpu_active_mask);
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}
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static inline nodemask_t cpuset_mems_allowed(struct task_struct *p)
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{
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return node_possible_map;
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}
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#define cpuset_current_mems_allowed (node_states[N_HIGH_MEMORY])
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static inline void cpuset_init_current_mems_allowed(void) {}
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static inline int cpuset_nodemask_valid_mems_allowed(nodemask_t *nodemask)
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{
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return 1;
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}
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static inline int cpuset_node_allowed_softwall(int node, gfp_t gfp_mask)
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{
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return 1;
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}
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static inline int cpuset_node_allowed_hardwall(int node, gfp_t gfp_mask)
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{
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return 1;
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}
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static inline int cpuset_zone_allowed_softwall(struct zone *z, gfp_t gfp_mask)
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{
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return 1;
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}
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static inline int cpuset_zone_allowed_hardwall(struct zone *z, gfp_t gfp_mask)
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{
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return 1;
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}
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static inline int cpuset_mems_allowed_intersects(const struct task_struct *tsk1,
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const struct task_struct *tsk2)
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{
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return 1;
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}
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static inline void cpuset_memory_pressure_bump(void) {}
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static inline void cpuset_task_status_allowed(struct seq_file *m,
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struct task_struct *task)
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{
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}
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static inline int cpuset_mem_spread_node(void)
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{
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return 0;
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}
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static inline int cpuset_slab_spread_node(void)
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{
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return 0;
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}
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static inline int cpuset_do_page_mem_spread(void)
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{
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return 0;
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}
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static inline int cpuset_do_slab_mem_spread(void)
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{
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return 0;
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}
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static inline int current_cpuset_is_being_rebound(void)
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{
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return 0;
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}
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static inline void rebuild_sched_domains(void)
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{
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partition_sched_domains(1, NULL, NULL);
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}
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static inline void cpuset_print_task_mems_allowed(struct task_struct *p)
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{
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}
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static inline void set_mems_allowed(nodemask_t nodemask)
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{
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}
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static inline unsigned int get_mems_allowed(void)
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{
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return 0;
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}
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static inline bool put_mems_allowed(unsigned int seq)
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{
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return true;
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}
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#endif /* !CONFIG_CPUSETS */
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#endif /* _LINUX_CPUSET_H */
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