linux-kernel - Re: [PATCH] sched: Fix numabalancing to work with isolated cpus

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Message-ID: <20170405125743.GB7258@dhcp22.suse.cz>
Date:   Wed, 5 Apr 2017 14:57:43 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH] sched: Fix numabalancing to work with isolated cpus

On Tue 04-04-17 22:57:28, Srikar Dronamraju wrote:
[...]
> For example:
> perf bench numa mem --no-data_rand_walk -p 4 -t $THREADS -G 0 -P 3072 -T 0 -l 50 -c -s 1000
> would call sched_setaffinity that resets the cpus_allowed mask.
> 
> Cpus_allowed_list:	0-55,57-63,65-71,73-79,81-87,89-175
> Cpus_allowed_list:	0,8,16,24,32,40,48,56,64,72,80,88,96,104,112,120,128,136,144,152,160,168
> Cpus_allowed_list:	0,8,16,24,32,40,48,56,64,72,80,88,96,104,112,120,128,136,144,152,160,168
> Cpus_allowed_list:	0,8,16,24,32,40,48,56,64,72,80,88,96,104,112,120,128,136,144,152,160,168
> Cpus_allowed_list:	0,8,16,24,32,40,48,56,64,72,80,88,96,104,112,120,128,136,144,152,160,168
> 
> The isolated cpus are part of the cpus allowed list. In the above case,
> numabalancing ends up scheduling some of these tasks on isolated cpus.

Why is this bad? If the task is allowed to run on isolated CPUs then why
shouldn't its numa balancing be allowed the same? The changelog
describes what but doesn't explain _why_ this change is needed/useful.
-- 
Michal Hocko
SUSE Labs