vchiq: Performance regression since 5.18-rc1

[Paul E. McKenney]

On Sun, May 22, 2022 at 01:22:00AM +0200, Stefan Wahren wrote:
> Hi,
> 
> while testing the staging/vc04_services/interface/vchiq_arm driver with my
> Raspberry Pi 3 B+ (multi_v7_defconfig) i noticed a huge performance
> regression since [ff042f4a9b050895a42cae893cc01fa2ca81b95c] mm:
> lru_cache_disable: replace work queue synchronization with synchronize_rcu
> 
> Usually i run "vchiq_test -f 1" to see the driver is still working [1].
> 
> Before commit:
> 
> real    0m1,500s
> user    0m0,068s
> sys    0m0,846s
> 
> After commit:
> 
> real    7m11,449s
> user    0m2,049s
> sys    0m0,023s
> 
> Best regards
> 
> [1] - https://github.com/raspberrypi/userland

Please feel free to try the patch shown below.  Or the pair of patches
from Rik here:

https://lore.kernel.org/lkml/20220218183114.2867528-2-riel@surriel.com/
https://lore.kernel.org/lkml/20220218183114.2867528-3-riel@surriel.com/

There is work ongoing to produce something better, but ongoing slowly.
Especially my part of that work.

							Thanx, Paul

------------------------------------------------------------------------

>From paulmck at kernel.org Mon Feb 14 11:05:49 2022
Date: Mon, 14 Feb 2022 11:05:49 -0800
From: "Paul E. McKenney" <paulmck at kernel.org>
To: clm at fb.com
Cc: riel at surriel.com, viro at zeniv.linux.org.uk, linux-kernel at vger.kernel.org,
	linux-fsdevel at vger.kernel.org, kernel-team at fb.com
Subject: [PATCH RFC fs/namespace] Make kern_unmount() use
 synchronize_rcu_expedited()
Message-ID: <20220214190549.GA2815154 at paulmck-ThinkPad-P17-Gen-1>
Reply-To: paulmck at kernel.org
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
Status: RO
Content-Length: 1036
Lines: 32

Experimental.  Not for inclusion.  Yet, anyway.

Freeing large numbers of namespaces in quick succession can result in
a bottleneck on the synchronize_rcu() invoked from kern_unmount().
This patch applies the synchronize_rcu_expedited() hammer to allow
further testing and fault isolation.

Hey, at least there was no need to change the comment!  ;-)

Cc: Alexander Viro <viro at zeniv.linux.org.uk>
Cc: <linux-fsdevel at vger.kernel.org>
Cc: <linux-kernel at vger.kernel.org>
Not-yet-signed-off-by: Paul E. McKenney <paulmck at kernel.org>

---

 namespace.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 40b994a29e90d..79c50ad0ade5b 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -4389,7 +4389,7 @@ void kern_unmount(struct vfsmount *mnt)
 	/* release long term mount so mount point can be released */
 	if (!IS_ERR_OR_NULL(mnt)) {
 		real_mount(mnt)->mnt_ns = NULL;
-		synchronize_rcu();	/* yecchhh... */
+		synchronize_rcu_expedited();	/* yecchhh... */
 		mntput(mnt);
 	}
 }