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Date:   Mon, 22 Mar 2021 09:57:51 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Li, Aubrey" <aubrey.li@linux.intel.com>
Cc:     "Joel Fernandes (Google)" <joel@joelfernandes.org>,
        Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Vineeth Pillai <viremana@linux.microsoft.com>,
        Aaron Lu <aaron.lwe@gmail.com>,
        Aubrey Li <aubrey.intel@gmail.com>, tglx@linutronix.de,
        linux-kernel@vger.kernel.org, mingo@kernel.org,
        torvalds@linux-foundation.org, fweisbec@gmail.com,
        keescook@chromium.org, Phil Auld <pauld@redhat.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>, vineeth@bitbyteword.org,
        Chen Yu <yu.c.chen@intel.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Agata Gruza <agata.gruza@intel.com>,
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        Alexandre Chartre <alexandre.chartre@oracle.com>,
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        Dhaval Giani <dhaval.giani@oracle.com>, chris.hyser@oracle.com,
        Josh Don <joshdon@google.com>, Hao Luo <haoluo@google.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Aubrey Li <aubrey.li@intel.com>
Subject: Re: [PATCH 1/6] sched: migration changes for core scheduling
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On Mon, Mar 22, 2021 at 04:12:17PM +0800, Li, Aubrey wrote:

> > But suppose there is an imbalance; then this cookie crud can forever
> > stall balance.
> > 
> > Imagine this cpu running a while(1); with a uniqie cookie on, then it
> > will _never_ accept other tasks == BAD.
> > 
> 
> How about putting the following check in sched_core_cookie_match()?
> 
> +	/*
> +	 * Ignore cookie match if there is a big imbalance between the src rq
> +	 * and dst rq.
> +	 */
> +	if ((src_rq->cfs.h_nr_running - rq->cfs.h_nr_running) > 1)
> +		return true;

That's just tinkering... cookie matching should not also look at
imbalance of any kind.

> Do you have any suggestions before we drop it?

Yeah, how about you make it part of task_hot() ? Have task_hot() refuse
migration it the cookie doesn't match.

task_hot() is a hint and will get ignored when appropriate.