linux/ipc
Waiman Long d60c4d01a9 ipc/mqueue: use get_tree_nodev() in mqueue_get_tree()
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>
2022-05-09 18:29:21 -07:00
..
compat.c License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
ipc_sysctl.c ipc/ipc_sysctl.c: remove fallback for !CONFIG_PROC_SYSCTL 2021-11-09 10:02:53 -08:00
Makefile License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
mq_sysctl.c sysctl: pass kernel pointers to ->proc_handler 2020-04-27 02:07:40 -04:00
mqueue.c ipc/mqueue: use get_tree_nodev() in mqueue_get_tree() 2022-05-09 18:29:21 -07:00
msg.c memcg: enable accounting of ipc resources 2021-09-03 09:58:12 -07:00
msgutil.c ipc: Use generic ns_common::count 2020-08-19 14:13:52 +02:00
namespace.c memcg: enable accounting for new namesapces and struct nsproxy 2021-09-03 09:58:12 -07:00
sem.c ipc: update semtimedop() to use hrtimer 2022-05-09 18:29:20 -07:00
shm.c shm: extend forced shm destroy to support objects from several IPC nses 2021-11-20 10:35:54 -08:00
syscall.c y2038: remove CONFIG_64BIT_TIME 2019-11-15 14:38:27 +01:00
util.c proc: remove PDE_DATA() completely 2022-01-22 08:33:37 +02:00
util.h ipc/util.c: use binary search for max_idx 2021-07-01 11:06:07 -07:00