linux-kernel - Re: [PATCH 00/10] sched/fair: wake

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Date:   Mon, 26 Apr 2021 12:35:33 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
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>,
        Rik van Riel <riel@...riel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Valentin Schneider <valentin.schneider@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Gautham R Shenoy <ego@...ux.vnet.ibm.com>,
        Parth Shah <parth@...ux.ibm.com>
Subject: Re: [PATCH 00/10] sched/fair: wake_affine improvements

On Mon, Apr 26, 2021 at 04:00:32PM +0530, Srikar Dronamraju wrote:
> * Mel Gorman <mgorman@...hsingularity.net> [2021-04-23 13:38:55]:
> 
> Hi Mel,
> 
> > On Fri, Apr 23, 2021 at 04:01:29PM +0530, Srikar Dronamraju wrote:
> > > > The series also oopses a *lot* and didn't get through a run of basic
> > > > workloads on x86 on any of three machines. An example oops is
> > > > 
> > > 
> > > Can you pass me your failing config. I am somehow not been seeing this
> > > either on x86 or on Powerpc on multiple systems.
> > 
> > The machines have since moved onto testing something else (Rik's patch
> > for newidle) but the attached config should be close enough.
> > 
> > > Also if possible cat /proc/schedstat and cat
> > > /proc/sys/kernel/sched_domain/cpu0/domain*/name
> > > 
> > 
> > For the vanilla kernel
> > 
> > SMT
> > MC
> > NUMA
> 
> I was able to reproduce the problem and analyze why it would panic in
> cpus_share_cache.
> 
> In my patch(es), we have code snippets like this.
> 
> 	if (tsds->idle_core != -1) {
> 		if (cpumask_test_cpu(tsds->idle_core, p->cpus_ptr))
> 			return tsds->idle_core;
> 		return this_cpu;
> 	}
> 
> Here when we tested the idle_core and cpumask_test_cpu,
> tsds->idle_core may not have been -1; However by the time it returns,
> tsds->idle_core could be -1;
> 
> cpus_share_cpus() then tries to find sd_llc_id for -1 and crashes.
> 
> Its more easier to reproduce this on a machine with more cores in a
> LLC than say a Power10/Power9.  Hence we are hitting this more often
> on x86.
> 
> One way could be to save the idle_core to a local variable, but that
> negates the whole purpose since we may end up choosing a busy CPU.  I
> will find a way to fix this problem.
> 

As there is no locking that protects the variable, it's inherently
race-prone. A READ_ONCE to a local variable may be your only choice

-- 
Mel Gorman
SUSE Labs