linux-kernel - Re: [PATCH v5 0/4] Scan for an idle sibling in a single pass

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Date:   Mon, 1 Feb 2021 13:06:03 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     "Li, Aubrey" <aubrey.li@...ux.intel.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Qais Yousef <qais.yousef@....com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 0/4] Scan for an idle sibling in a single pass

On Mon, Feb 01, 2021 at 09:13:16AM +0800, Li, Aubrey wrote:
> > Peter's series tried to solve three problems at once, this subset addresses
> > one problem.
> > 
> >  kernel/sched/fair.c     | 151 +++++++++++++++++++---------------------
> >  kernel/sched/features.h |   1 -
> >  2 files changed, 70 insertions(+), 82 deletions(-)
> > 
> 
> 4 benchmarks measured on a x86 4s system with 24 cores per socket and
> 2 HTs per core, total 192 CPUs. 
> 
> The load level is [25%, 50%, 75%, 100%].
> 
> - hackbench almost has a universal win.
> - netperf high load has notable changes, as well as tbench 50% load.
> 

Ok, both netperf and tbench are somewhat expected as at those loads are
rapidly idling. Previously I observed that rapidly idling loads can
allow the has_idle_cores test pass for short durations and the double
scanning means there is a greater chance of finding an idle CPU over the
two passes. I think overall it's better to avoid double scanning even if
there are counter examples as it's possible we'll get that back through
better depth selection in the future.

Thanks.

-- 
Mel Gorman
SUSE Labs