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From: Aubrey Li <>
To: Mel Gorman <>,
	"Song Bao Hua (Barry Song)" <>
Cc: LKML <>,
	Ingo Molnar <>,
	Peter Zijlstra <>,
	Vincent Guittot <>,
	Valentin Schneider <>,
	yangyicong <>
Subject: Re: [PATCH 8/9] sched/fair: select idle cpu from idle cpumask for task wakeup
Date: Thu, 5 Aug 2021 08:23:15 +0800	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On 8/4/21 6:26 PM, Mel Gorman wrote:
> 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.

The idle core information is already in idle cpu mask, do we have a quick
and cheap way to extract out?

>> 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.

Bad idea, is it helpful to make a sparse cpu mask for this? :p


  reply	other threads:[~2021-08-04 15:16 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 [this message]
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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