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From: Aubrey Li <aubrey.li@linux.intel.com>
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
Subject: Re: [PATCH 8/9] sched/fair: select idle cpu from idle cpumask for task wakeup
Date: Fri, 17 Sep 2021 17:11:11 +0800	[thread overview]
Message-ID: <865702ea-b1c1-e39d-94ea-d55bf03b86db@linux.intel.com> (raw)
In-Reply-To: <20210917041539.7862-1-21cnbao@gmail.com>

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

  reply	other threads:[~2021-09-17  9:12 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-26 10:22 [RFC PATCH 0/9] Modify and/or delete SIS_PROP Mel Gorman
2021-07-26 10:22 ` [PATCH 1/9] sched/fair: Track efficiency of select_idle_sibling Mel Gorman
2021-07-26 10:22 ` [PATCH 2/9] sched/fair: Track efficiency of task recent_used_cpu Mel Gorman
2021-07-26 10:22 ` [PATCH 3/9] sched/fair: Track efficiency of select_idle_core Mel Gorman
2021-07-26 10:22 ` [PATCH 4/9] sched/fair: Use prev instead of new target as recent_used_cpu Mel Gorman
2021-07-26 10:22 ` [PATCH 5/9] sched/fair: Avoid a second scan of target in select_idle_cpu Mel Gorman
2021-07-26 10:22 ` [PATCH 6/9] sched/fair: Make select_idle_cpu() proportional to cores Mel Gorman
2021-07-26 10:22 ` [PATCH 7/9] sched/fair: Enforce proportional scan limits when scanning for an idle core Mel Gorman
2021-08-02 10:52   ` Song Bao Hua (Barry Song)
2021-08-04 10:22     ` Mel Gorman
2021-07-26 10:22 ` [PATCH 8/9] sched/fair: select idle cpu from idle cpumask for task wakeup Mel Gorman
2021-08-02 10:41   ` Song Bao Hua (Barry Song)
2021-08-04 10:26     ` Mel Gorman
2021-08-05  0:23       ` Aubrey Li
2021-09-17  3:44         ` Barry Song
2021-09-17  4:15         ` Barry Song
2021-09-17  9:11           ` Aubrey Li [this message]
2021-09-17 13:35             ` Mel Gorman
2021-07-26 10:22 ` [PATCH 9/9] sched/core: Delete SIS_PROP and rely on the idle cpu mask Mel Gorman

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