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Subject: Re: [PATCH 8/9] sched/fair: select idle cpu from idle cpumask for
 task wakeup
To:     Barry Song <21cnbao@gmail.com>
Cc:     linux-kernel@vger.kernel.org, mgorman@techsingularity.net,
        mingo@kernel.org, peterz@infradead.org, song.bao.hua@hisilicon.com,
        valentin.schneider@arm.com, vincent.guittot@linaro.org,
        yangyicong@huawei.com
References: <125eb98a-241b-078f-1844-b0521425ed1e@linux.intel.com>
 <20210917041539.7862-1-21cnbao@gmail.com>
From:   Aubrey Li <aubrey.li@linux.intel.com>
Message-ID: <865702ea-b1c1-e39d-94ea-d55bf03b86db@linux.intel.com>
Date:   Fri, 17 Sep 2021 17:11:11 +0800
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On 9/17/21 12:15 PM, Barry Song wrote:
>> @@ -4965,6 +4965,7 @@ void scheduler_tick(void)
>>
>>  #ifdef CONFIG_SMP
>> 	rq->idle_balance = idle_cpu(cpu);
>> +	update_idle_cpumask(cpu, rq->idle_balance);
>>  	trigger_load_balance(rq);
>>  #endif
>> }
> 
> might be stupid, a question bothering yicong and me is that why don't we
> choose to update_idle_cpumask() while idle task exits and switches to a
> normal task?

I implemented that way and we discussed before(RFC v1 ?), updating a cpumask
at every enter/exit idle is more expensive than we expected, though it's
per LLC domain, Vincent saw a significant regression IIRC. You can also
take a look at nohz.idle_cpus_mask as a reference.

> for example, before tick comes, cpu has exited from idle, but we are only
> able to update it in tick. this makes idle_cpus_span inaccurate, thus we
> will scan cpu which isn't actually idle in select_idle_sibling.
> is it because of the huge update overhead?
> 

Yes, we'll have false positive but we don't miss true positive. So things
won't be worse than the current way.

Thanks,
-Aubrey