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

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Date:   Mon, 26 Apr 2021 16:00:32 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     Mel Gorman <mgorman@...hsingularity.net>
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

* 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.

-- 
Thanks and Regards
Srikar Dronamraju