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From: Mel Gorman <mgorman@techsingularity.net>
To: "Song Bao Hua (Barry Song)" <song.bao.hua@hisilicon.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Valentin Schneider <valentin.schneider@arm.com>,
	Aubrey Li <aubrey.li@linux.intel.com>,
	yangyicong <yangyicong@huawei.com>
Subject: Re: [PATCH 8/9] sched/fair: select idle cpu from idle cpumask for task wakeup
Date: Wed, 4 Aug 2021 11:26:13 +0100	[thread overview]
Message-ID: <20210804102613.GC6464@techsingularity.net> (raw)
In-Reply-To: <9dde989df06b483790cc24dc7670a919@hisilicon.com>

On Mon, Aug 02, 2021 at 10:41:13AM +0000, Song Bao Hua (Barry Song) wrote:
> Hi Mel, Aubrey,
> A similar thing Yicong and me has discussed is having a mask or a count for
> idle cores. And then we can only scan idle cores in this mask in
> select_idle_cpu().
> 

Either approach would require a lot of updates.

> A obvious problem is that has_idle_cores is a bool, it can seriously lag
> from the real status. I mean, after system enters the status without idle
> cores, has_idle_cores could be still true.
> 

True.

> Right now, we are setting has_idle_cores to true while cpu enters idle
> and its smt sibling is also idle. But we are setting has_idle_cores to
> false only after we scan all cores in a llc.
> 
> So we have thought for a while to provide an idle core mask. But never
> really made a workable patch.
> 
> Mel's patch7/9 limits the number of cores which will be scanned in
> select_idle_cpu(), it might somehow alleviate the problem we redundantly
> scan all cores while we actually have no idle core even has_idle_cores
> is true.
> 

I prototyped a patch that tracked the location of a known idle core and
use it as a starting hint for a search. It's cleared if the core is
selected for use. Unfortunately, it was not a universal win so was
dropped for the moment but either way, improving the accurate of
has_idle_cores without being too expensive would be niuce.

> However, if we can get idle core mask, it could be also good to
> select_idle_core()? Maybe after that, we don't have to enforce
> proportional scan limits while scanning for an idle core?
> 

To remove the limits entirely, it would have to be very accurate and
it's hard to see how that can be done without having a heavily updated
shared cache line.

-- 
Mel Gorman
SUSE Labs

  reply	other threads:[~2021-08-04 10:26 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 [this message]
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
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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