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Waiman Long 1b5939fc99 ipc/mqueue: use get_tree_nodev() in mqueue_get_tree() 
[ Upstream commit d60c4d01a9 ]

When running the stress-ng clone benchmark with multiple testing threads,
it was found that there were significant spinlock contention in sget_fc().
The contended spinlock was the sb_lock.  It is under heavy contention
because the following code in the critcal section of sget_fc():

  hlist_for_each_entry(old, &fc->fs_type->fs_supers, s_instances) {
      if (test(old, fc))
          goto share_extant_sb;
  }

After testing with added instrumentation code, it was found that the
benchmark could generate thousands of ipc namespaces with the
corresponding number of entries in the mqueue's fs_supers list where the
namespaces are the key for the search.  This leads to excessive time in
scanning the list for a match.

Looking back at the mqueue calling sequence leading to sget_fc():

  mq_init_ns()
  => mq_create_mount()
  => fc_mount()
  => vfs_get_tree()
  => mqueue_get_tree()
  => get_tree_keyed()
  => vfs_get_super()
  => sget_fc()

Currently, mq_init_ns() is the only mqueue function that will indirectly
call mqueue_get_tree() with a newly allocated ipc namespace as the key for
searching.  As a result, there will never be a match with the exising ipc
namespaces stored in the mqueue's fs_supers list.

So using get_tree_keyed() to do an existing ipc namespace search is just a
waste of time.  Instead, we could use get_tree_nodev() to eliminate the
useless search.  By doing so, we can greatly reduce the sb_lock hold time
and avoid the spinlock contention problem in case a large number of ipc
namespaces are present.

Of course, if the code is modified in the future to allow
mqueue_get_tree() to be called with an existing ipc namespace instead of a
new one, we will have to use get_tree_keyed() in this case.

The following stress-ng clone benchmark command was run on a 2-socket
48-core Intel system:

./stress-ng --clone 32 --verbose --oomable --metrics-brief -t 20

The "bogo ops/s" increased from 5948.45 before patch to 9137.06 after
patch. This is an increase of 54% in performance.

Link: https://lkml.kernel.org/r/20220121172315.19652-1-longman@redhat.com
Fixes: 935c6912b1 ("ipc: Convert mqueue fs to fs_context")
Signed-off-by: Waiman Long <longman@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: David Howells <dhowells@redhat.com>
Cc: Manfred Spraul <manfred@colorfullife.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
2022-06-09 10:26:06 +02:00
arch arm64: dts: marvell: espressobin-ultra: enable front USB3 port 2022-06-09 10:26:06 +02:00
block bfq: Allow current waker to defend against a tentative one 2022-06-09 10:25:56 +02:00
certs certs: Fix build error when CONFIG_MODULE_SIG_KEY is empty 2022-01-23 00:08:44 +09:00
crypto crypto: ecrdsa - Fix incorrect use of vli_cmp 2022-06-06 08:47:53 +02:00
Documentation dt-bindings: soc: qcom: smd-rpm: Fix missing MSM8936 compatible 2022-06-09 10:26:01 +02:00
drivers RDMA/hns: Add the detection for CMDQ status in the device initialization process 2022-06-09 10:26:06 +02:00
fs proc: fix dentry/inode overinstantiating under /proc/${pid}/net 2022-06-09 10:26:06 +02:00
include list: fix a data-race around ep->rdllist 2022-06-09 10:26:05 +02:00
init kthread: Don't allocate kthread_struct for init and umh 2022-06-09 10:25:16 +02:00
ipc ipc/mqueue: use get_tree_nodev() in mqueue_get_tree() 2022-06-09 10:26:06 +02:00
kernel dma-direct: don't over-decrypt memory 2022-06-09 10:25:58 +02:00
lib kunit: fix debugfs code to use enum kunit_status, not bool 2022-06-09 10:25:46 +02:00
LICENSES LICENSES/LGPL-2.1: Add LGPL-2.1-or-later as valid identifiers 2021-12-16 14:33:10 +01:00
mm zsmalloc: fix races between asynchronous zspage free and page migration 2022-06-06 08:47:53 +02:00
net Revert "net/smc: fix listen processing for SMC-Rv2" 2022-06-09 10:25:59 +02:00
samples samples: bpf: Don't fail for a missing VMLINUX_BTF when VMLINUX_H is provided 2022-06-09 10:25:48 +02:00
scripts scripts/faddr2line: Fix overlapping text section failures 2022-06-09 10:25:50 +02:00
security lockdown: also lock down previous kgdb use 2022-05-25 09:59:10 +02:00
sound ASoC: atmel-classd: Remove endianness flag on class d component 2022-06-09 10:26:06 +02:00
tools perf parse-events: Support different format of the topdown event name 2022-06-09 10:25:57 +02:00
usr kbuild: remove include/linux/cyclades.h from header file check 2022-01-27 08:51:08 +01:00
virt KVM: Free new dirty bitmap if creating a new memslot fails 2022-05-25 09:59:03 +02:00
.clang-format genirq/msi: Make interrupt allocation less convoluted 2021-12-16 22:22:20 +01:00
.cocciconfig
.get_maintainer.ignore Opt out of scripts/get_maintainer.pl 2019-05-16 10:53:40 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore .gitignore: ignore only top-level modules.builtin 2021-05-02 00:43:35 +09:00
.mailmap MAINTAINERS: Update Jisheng's email address 2022-03-08 17:30:32 +01:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: replace a Microchip AT91 maintainer 2022-02-09 11:30:01 +01:00
Kbuild kbuild: rename hostprogs-y/always to hostprogs/always-y 2020-02-04 01:53:07 +09:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS net: dsa: realtek-smi: move to subdirectory 2022-04-08 13:58:12 +02:00
Makefile Linux 5.17.13 2022-06-06 08:47:57 +02:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.