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

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Date:   Fri, 2 Sep 2022 09:06:59 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Jiebin Sun <jiebin.sun@...el.com>
Cc:     vasily.averin@...ux.dev, shakeelb@...gle.com, dennis@...nel.org,
        tj@...nel.org, cl@...ux.com, ebiederm@...ssion.com,
        legion@...nel.org, manfred@...orfullife.com,
        alexander.mikhalitsyn@...tuozzo.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, tim.c.chen@...el.com,
        feng.tang@...el.com, 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,  2 Sep 2022 23:22:43 +0800 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

So this test became 3x faster?

> 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)
> 
> ...
>
> @@ -138,6 +139,14 @@ percpu_counter_add(struct percpu_counter *fbc, s64 amount)
>  	preempt_enable();
>  }
>  
> +static inline void
> +percpu_counter_add_local(struct percpu_counter *fbc, s64 amount)
> +{
> +	preempt_disable();
> +	fbc->count += amount;
> +	preempt_enable();
> +}

What's this and why is it added?

It would be best to propose this as a separate preparatory patch. 
Fully changelogged and perhaps even with a code comment explaining why
and when it should be used.

Thanks.