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Date:   Mon, 5 Sep 2022 20:02:34 +0800
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Subject: Re: [PATCH] ipc/msg.c: mitigate the lock contention with percpu
 counter
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To:     Shakeel Butt <shakeelb@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>, vasily.averin@linux.dev,
        Dennis Zhou <dennis@kernel.org>, Tejun Heo <tj@kernel.org>,
        Christoph Lameter <cl@linux.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Alexey Gladkov <legion@kernel.org>,
        Manfred Spraul <manfred@colorfullife.com>,
        alexander.mikhalitsyn@virtuozzo.com, Linux MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        "Chen, Tim C" <tim.c.chen@intel.com>,
        Feng Tang <feng.tang@intel.com>,
        Huang Ying <ying.huang@intel.com>, tianyou.li@intel.com,
        wangyang.guo@intel.com, jiebin.sun@intel.com
References: <20220902152243.479592-1-jiebin.sun@intel.com>
 <CALvZod44uUFnwfF4StC24t+d1s_XE10hkmSCgb04FjtTATo6xQ@mail.gmail.com>
From:   "Sun, Jiebin" <jiebin.sun@intel.com>
In-Reply-To: <CALvZod44uUFnwfF4StC24t+d1s_XE10hkmSCgb04FjtTATo6xQ@mail.gmail.com>
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On 9/3/2022 12:27 AM, Shakeel Butt wrote:
> On Fri, Sep 2, 2022 at 12:04 AM Jiebin Sun <jiebin.sun@intel.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@intel.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.