Re: vchiq: Performance regression since 5.18-rc1

Stefan Wahren
2022-05-23T07:37:08Z

Hi Paul,

Am 23.05.22 um 06:48 schrieb Paul E. McKenney:
> On Sun, May 22, 2022 at 05:11:36PM +0200, Stefan Wahren wrote:
>> 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.
> That is surprising. Do you boot with rcupdate.rcu_normal=1?
No, not explicit.
> That would
> nullify my patch, but I would expect that Rik's patch would still provide
> increased performance even in that case.
I will retest with a fresh SD card image.
>
> Could you please characterize where the slowdown is occurring?

Unfortunately i don't have a deep insight into driver and vchiq_test
tool. Just a user view.

Do you think an strace would be a good starting point?

@Phil Any advices to analyse this issue?

>
> Thanx, Paul
>
>> 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);
>>> }
>>> }
>>>