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Subject: Re: [RFC PATCH v2] sched/fair: select idle cpu from idle cpumask in
 sched domain
To:     Vincent Guittot <vincent.guittot@linaro.org>
Cc:     Mel Gorman <mgorman@suse.de>, Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Qais Yousef <qais.yousef@arm.com>,
        Jiang Biao <benbjiang@gmail.com>
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From:   "Li, Aubrey" <aubrey.li@linux.intel.com>
Message-ID: <aff0c293-cf4d-9770-cd54-fc0d06662f09@linux.intel.com>
Date:   Fri, 25 Sep 2020 17:21:46 +0800
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Hi Vicent,

On 2020/9/24 21:09, Vincent Guittot wrote:
>>>>
>>>> Would you mind share uperf(netperf load) result on your side? That's the
>>>> workload I have seen the most benefit this patch contributed under heavy
>>>> load level.
>>>
>>> with uperf, i've got the same kind of result as sched pipe
>>> tip/sched/core: Throughput 24.83Mb/s (+/- 0.09%)
>>> with this patch:  Throughput 19.02Mb/s (+/- 0.71%) which is a 23%
>>> regression as for sched pipe
>>>
>> In case this is caused by the logic error in this patch(sorry again), did
>> you see any improvement in patch V2? Though it does not helps for nohz=off
>> case, just want to know if it helps or does not help at all on arm platform.
> 
> With the v2 which rate limit the update of the cpumask (but doesn't
> support sched_idle stask),  I don't see any performance impact:

I agree we should go the way with cpumask update rate limited.

And I think no performance impact for sched-pipe is expected, as this workload
has only 2 threads and the platform has 8 cores, so mostly previous cpu is
returned, and even if select_idle_sibling is called, select_idle_core is hit
and rarely call select_idle_cpu.

But I'm more curious why there is 23% performance penalty? So for this patch, if
you revert this change but keep cpumask updated, is 23% penalty still there?

-       cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
+       cpumask_and(cpus, sds_idle_cpus(sd->shared), p->cpus_ptr);

I just wonder if it's caused by the atomic ops as you have two cache domains with
sd_llc(?). Do you have a x86 machine to make a comparison? It's hard for me to find
an ARM machine but I'll try.

Also, for uperf(task thread num = cpu num) workload, how is it on patch v2? no any
performance impact?

Thanks,
-Aubrey