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Date:   Thu, 2 Jul 2020 09:23:24 -0400
From:   Joel Fernandes <joel@joelfernandes.org>
To:     Tim Chen <tim.c.chen@linux.intel.com>
Cc:     Vineeth Remanan Pillai <vpillai@digitalocean.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        tglx@linutronix.de, pjt@google.com, torvalds@linux-foundation.org,
        linux-kernel@vger.kernel.org, subhra.mazumdar@oracle.com,
        mingo@kernel.org, fweisbec@gmail.com, keescook@chromium.org,
        kerrnel@google.com, Phil Auld <pauld@redhat.com>,
        Aaron Lu <aaron.lwe@gmail.com>,
        Aubrey Li <aubrey.intel@gmail.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>, vineethrp@gmail.com,
        Chen Yu <yu.c.chen@intel.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Aaron Lu <aaron.lu@linux.alibaba.com>, paulmck@kernel.org
Subject: Re: [RFC PATCH 06/16] sched: Add core wide task selection and
 scheduling.
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On Thu, Jul 02, 2020 at 08:57:57AM -0400, Joel Fernandes wrote:
[...] 
> > > unconstrained pick, then rq->core_pick is set. The next time task selection
> > > logic runs when stopper needs to switch to idle, the current CPU is not in
> > > the smt_mask. This causes the previous ->core_pick to be picked again which
> > > happens to be the unconstrained task! so the stopper keeps getting selected
> > > forever.
> > > 
> > > That and there are a few more safe guards and checks around checking/setting
> > > rq->core_pick. To test it, I ran rcutorture and made it tag all torture
> > > threads. Then ran it in hotplug mode (hotplugging every 200ms) and it hit the
> > > issue. Now it runs for an hour or so without issue. (Torture testing debug
> > > changes: https://bit.ly/38htfqK ).
> > > 
> > > Various fixes were tried causing varying degrees of crashes.  Finally I found
> > > that it is easiest to just add current CPU to the smt_mask's copy always.
> > > This is so that task selection logic always runs on the current CPU which
> > > called schedule().
> > 
> > 
> > It looks good to me. 
> 
> Thank you for your review! Could I add your Reviewed-by tag to the patch?

Julien and Vineeth, here is by coresched tree updated with this patch for
when you are sending the next series:
git://git.kernel.org/pub/scm/linux/kernel/git/jfern/linux.git (branch coresched)

There are some trivial fixups to the debug patch, due to this commit. So
pulling from the above branch may save you some time.

thanks,

 - Joel