From: Mel Gorman <mgorman@suse.de>
To: Yicong Yang <yangyicong@hisilicon.com>
Cc: mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com,
vincent.guittot@linaro.org, linux-kernel@vger.kernel.org,
dietmar.eggemann@arm.com, rostedt@goodmis.org,
bsegall@google.com, bristot@redhat.com,
song.bao.hua@hisilicon.com, prime.zeng@huawei.com,
linuxarm@huawei.com, 21cnbao@gmail.com
Subject: Re: [PATCH] sched/fair: Clear target from cpus to scan in select_idle_cpu
Date: Thu, 25 Nov 2021 11:17:31 +0000 [thread overview]
Message-ID: <20211125111731.GE3301@suse.de> (raw)
In-Reply-To: <20211124085401.14411-1-yangyicong@hisilicon.com>
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@hisilicon.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
next prev parent reply other threads:[~2021-11-25 11:19 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-11-24 8:54 [PATCH] sched/fair: Clear target from cpus to scan in select_idle_cpu Yicong Yang
2021-11-25 11:17 ` Mel Gorman [this message]
2021-11-25 12:46 ` Yicong Yang
2021-11-26 9:38 ` Yicong Yang
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