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

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Message-ID: <20181024112153.GD23537@techsingularity.net>
Date:   Wed, 24 Oct 2018 12:21:53 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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 04:11:24PM +0530, Srikar Dronamraju wrote:
> * Peter Zijlstra <peterz@...radead.org> [2018-10-24 12:15:08]:
> 
> > 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.
> 
> Agree, but thats something thats been exposed long time back.
> Do we have an option to remove that?  Hopefully nobody is using it.
> 

I occasionally see bugs asking questions about interference from the kernel
when isolcpus are in use. The last one was related to a timer interrupt
every HZ (not a mainline kernel) but still, it's some evidence that it
has users :(

-- 
Mel Gorman
SUSE Labs