Re: vchiq: Performance regression since 5.18-rc1

Stefan Wahren
2022-05-22T15:12:49Z

Hi Paul,

Am 22.05.22 um 01:46 schrieb 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/

I tried your patch and Rik's patches but in both cases vchiq_test runs 7
minutes instead of ~ 1 second.

Best regards

>
> There is work ongoing to produce something better, but ongoing slowly.
> Especially my part of that work.
>
> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> From paulmck@kernel.org Mon Feb 14 11:05:49 2022
> Date: Mon, 14 Feb 2022 11:05:49 -0800
> From: "Paul E. McKenney" <paulmck@kernel.org>
> To: clm@fb.com
> Cc: riel@surriel.com, viro@zeniv.linux.org.uk, linux-kernel@vger.kernel.org,
> linux-fsdevel@vger.kernel.org, kernel-team@fb.com
> Subject: [PATCH RFC fs/namespace] Make kern_unmount() use
> synchronize_rcu_expedited()
> Message-ID: <20220214190549.GA2815154@paulmck-ThinkPad-P17-Gen-1>
> Reply-To: paulmck@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@zeniv.linux.org.uk>
> Cc: <linux-fsdevel@vger.kernel.org>
> Cc: <linux-kernel@vger.kernel.org>
> Not-yet-signed-off-by: Paul E. McKenney <paulmck@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);
> }
> }
>