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

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Date:   Fri, 2 Sep 2022 09:27:54 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Jiebin Sun <jiebin.sun@...el.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
Subject: Re: [PATCH] ipc/msg.c: mitigate the lock contention with percpu counter

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.