[Devel] [PATCH RHEL8 COMMIT] ms/VFS: use synchronize_rcu_expedited() in namespace_unlock()

[Konstantin Khorenko]

The commit is pushed to "branch-rh8-4.18.0-240.1.1.vz8.5.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh8-4.18.0-240.1.1.vz8.5.34
------>
commit 38e66b6edf83266ab611b535f8338b02bf69295b
Author: Vasily Averin <vvs at virtuozzo.com>
Date:   Wed Jun 2 19:48:04 2021 +0300

    ms/VFS: use synchronize_rcu_expedited() in namespace_unlock()
    
    The synchronize_rcu() in namespace_unlock() is called every time
    a filesystem is unmounted.  If a great many filesystems are mounted,
    this can cause a noticable slow-down in, for example, system shutdown.
    
    The sequence:
      mkdir -p /tmp/Mtest/{0..5000}
      time for i in /tmp/Mtest/*; do mount -t tmpfs tmpfs $i ; done
      time umount /tmp/Mtest/*
    
    on a 4-cpu VM can report 8 seconds to mount the tmpfs filesystems, and
    100 seconds to unmount them.
    
    Boot the same VM with 1 CPU and it takes 18 seconds to mount the
    tmpfs filesystems, but only 36 to unmount.
    
    If we change the synchronize_rcu() to synchronize_rcu_expedited()
    the umount time on a 4-cpu VM drop to 0.6 seconds
    
    I think this 200-fold speed up is worth the slightly high system
    impact of using synchronize_rcu_expedited().
    
    Acked-by: Paul E. McKenney <paulmck at linux.vnet.ibm.com> (from general rcu perspective)
    Signed-off-by: NeilBrown <neilb at suse.com>
    Signed-off-by: Al Viro <viro at zeniv.linux.org.uk>
    
    https://jira.sw.ru/browse/PSBM-101530
    (cherry-picked from ms commit 22cb7405fada ("VFS: use synchronize_rcu_expedited()
    in namespace_unlock()"))
    
    Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
    
    (cherry picked from vz7 commit 61c6fb066873 ("ms/VFS: use
    synchronize_rcu_expedited() in namespace_unlock()"))
    
    Signed-off-by: Vasily Averin <vvs at virtuozzo.com>
---
 fs/namespace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 8b07ab1bff61..feee87d3c89a 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1402,7 +1402,7 @@ static void namespace_unlock(void)
 	if (likely(hlist_empty(&head)))
 		return;
 
-	synchronize_rcu();
+	synchronize_rcu_expedited();
 
 	group_pin_kill(&head);
 }