linux-kernel - Re: [PATCH v2] sched/core: Don't mix isolcpus and housekeeping CPUs

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Message-ID: <20181024101508.GP3109@worktop.c.hoisthospitality.com>
Date:   Wed, 24 Oct 2018 12:15:08 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:     Mel Gorman <mgorman@...hsingularity.net>,
        Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Rik van Riel <riel@...riel.com>,
        Yi Wang <wang.yi59@....com.cn>, zhong.weidong@....com.cn,
        Yi Liu <liu.yi24@....com.cn>,
        Frederic Weisbecker <frederic@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2] sched/core: Don't mix isolcpus and housekeeping CPUs

On Wed, Oct 24, 2018 at 03:16:46PM +0530, Srikar Dronamraju wrote:
> * Mel Gorman <mgorman@...hsingularity.net> [2018-10-24 09:56:36]:
> 
> > On Wed, Oct 24, 2018 at 08:32:49AM +0530, Srikar Dronamraju wrote:
> > It would certainly be a bit odd because the
> > application is asking for some protection but no guarantees are given
> > and the application is not made aware via an error code that there is a
> > problem. Asking the application to parse dmesg hoping to find the right
> > error message is going to be fragile.
> 
> Its a actually a good question.
> What should we be doing if a mix of isolcpus and housekeeping (aka
> non-isolcpus) is given in the mask.
> 
> Right now as you pointed, there is no easy way for the application to know
> which are the non-isolcpus to set its affinity. cpusets effective_cpus and
> cpus_allowed both will contain isolcpus too.

The easy option is to not use isolcpus :-) It is a horrifically bad
interface.