linux-kernel - Re: [PATCH] ipc/msg.c: mitigate the lock contention with percpu counter

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Date:   Mon, 5 Sep 2022 20:02:34 +0800
From:   "Sun, Jiebin" <jiebin.sun@...el.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, vasily.averin@...ux.dev,
        Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Alexey Gladkov <legion@...nel.org>,
        Manfred Spraul <manfred@...orfullife.com>,
        alexander.mikhalitsyn@...tuozzo.com, Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Chen, Tim C" <tim.c.chen@...el.com>,
        Feng Tang <feng.tang@...el.com>,
        Huang Ying <ying.huang@...el.com>, tianyou.li@...el.com,
        wangyang.guo@...el.com, jiebin.sun@...el.com
Subject: Re: [PATCH] ipc/msg.c: mitigate the lock contention with percpu
 counter


On 9/3/2022 12:27 AM, Shakeel Butt wrote:
> On Fri, Sep 2, 2022 at 12:04 AM Jiebin Sun <jiebin.sun@...el.com> wrote:
>> The msg_bytes and msg_hdrs atomic counters are frequently
>> updated when IPC msg queue is in heavy use, causing heavy
>> cache bounce and overhead. Change them to percpu_counters
>> greatly improve the performance. Since there is one unique
>> ipc namespace, additional memory cost is minimal. Reading
>> of the count done in msgctl call, which is infrequent. So
>> the need to sum up the counts in each CPU is infrequent.
>>
>> Apply the patch and test the pts/stress-ng-1.4.0
>> -- system v message passing (160 threads).
>>
>> Score gain: 3.38x
>>
>> CPU: ICX 8380 x 2 sockets
>> Core number: 40 x 2 physical cores
>> Benchmark: pts/stress-ng-1.4.0
>> -- system v message passing (160 threads)
>>
>> Signed-off-by: Jiebin Sun <jiebin.sun@...el.com>
> [...]
>> +void percpu_counter_add_local(struct percpu_counter *fbc, s64 amount)
>> +{
>> +       this_cpu_add(*fbc->counters, amount);
>> +}
>> +EXPORT_SYMBOL(percpu_counter_add_local);
> Why not percpu_counter_add()? This may drift the fbc->count more than
> batch*nr_cpus. I am assuming that is not the issue for you as you
> always do an expensive sum in the slow path. As Andrew asked, this
> should be a separate patch.

Yes. It will always do sum in msgctl_info. So there is no need to
do global updating in percpu_counter_add when the percpu counter
reaches the batch size. We add percpu_counter_add_local in this
case. The sum in slow path is infrequent. So the additional cost
is much less compared to the atomic updating in do_msgsnd and
do_msgrcv every time. I have separate the original patch into two
patches.

Thanks.