linux-kernel - Re: [PATCH] sched/fair: Clear target from cpus to scan in select_idle

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Date:   Thu, 25 Nov 2021 11:17:31 +0000
From:   Mel Gorman <mgorman@...e.de>
To:     Yicong Yang <yangyicong@...ilicon.com>
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, linux-kernel@...r.kernel.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        bristot@...hat.com, song.bao.hua@...ilicon.com,
        prime.zeng@...wei.com, linuxarm@...wei.com, 21cnbao@...il.com
Subject: Re: [PATCH] sched/fair: Clear target from cpus to scan in
 select_idle_cpu

On Wed, Nov 24, 2021 at 04:54:01PM +0800, Yicong Yang wrote:
> Commit 56498cfb045d noticed that "When select_idle_cpu starts scanning for
> an idle CPU, it starts with a target CPU that has already been checked
> by select_idle_sibling. This patch starts with the next CPU instead."
> It only changed the scanning start cpu to target + 1 but still leave
> the target in the scanning cpumask. The target still have a chance to be
> checked in the last turn. Fix this by clear the target from the cpus
> to scan.
> 
> Fixes: 56498cfb045d ("sched/fair: Avoid a second scan of target in select_idle_cpu")
> Signed-off-by: Yicong Yang <yangyicong@...ilicon.com>

Did you check the performance of this? When I tried something like this
in a different context, I found that the cost of clearing the bit was
more expensive than simply using target + 1. For the target to be
rescanned, the whole mask would have to be scanned as no other CPUs are
idle which is the unlikely case. By clearing the bit, a cost is always
incurred even if the first CPU scanned is idle.

-- 
Mel Gorman
SUSE Labs