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Date:   Mon, 22 Mar 2021 08:48:48 +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>,
        Antonio Gomez Iglesias <antonio.gomez.iglesias@intel.com>,
        graf@amazon.com, konrad.wilk@oracle.com, dfaggioli@suse.com,
        rostedt@goodmis.org, benbjiang@tencent.com,
        Alexandre Chartre <alexandre.chartre@oracle.com>,
        James.Bottomley@hansenpartnership.com, OWeisse@umich.edu,
        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
Message-ID: <YFhL4CMPB+Pfo965@hirez.programming.kicks-ass.net>
References: <20210319203253.3352417-1-joel@joelfernandes.org>
 <20210319203253.3352417-2-joel@joelfernandes.org>
 <20210320153457.GX4746@worktop.programming.kicks-ass.net>
 <28e13609-c526-c6ee-22a3-898652aed5e6@linux.intel.com>
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On Sun, Mar 21, 2021 at 09:34:00PM +0800, Li, Aubrey wrote:
> Hi Peter,
> 
> On 2021/3/20 23:34, Peter Zijlstra wrote:
> > On Fri, Mar 19, 2021 at 04:32:48PM -0400, Joel Fernandes (Google) wrote:
> >> @@ -7530,8 +7543,9 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
> >>  	 * We do not migrate tasks that are:
> >>  	 * 1) throttled_lb_pair, or
> >>  	 * 2) cannot be migrated to this CPU due to cpus_ptr, or
> >> -	 * 3) running (obviously), or
> >> -	 * 4) are cache-hot on their current CPU.
> >> +	 * 3) task's cookie does not match with this CPU's core cookie
> >> +	 * 4) running (obviously), or
> >> +	 * 5) are cache-hot on their current CPU.
> >>  	 */
> >>  	if (throttled_lb_pair(task_group(p), env->src_cpu, env->dst_cpu))
> >>  		return 0;
> >> @@ -7566,6 +7580,13 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
> >>  		return 0;
> >>  	}
> >>  
> >> +	/*
> >> +	 * Don't migrate task if the task's cookie does not match
> >> +	 * with the destination CPU's core cookie.
> >> +	 */
> >> +	if (!sched_core_cookie_match(cpu_rq(env->dst_cpu), p))
> >> +		return 0;
> >> +
> >>  	/* Record that we found atleast one task that could run on dst_cpu */
> >>  	env->flags &= ~LBF_ALL_PINNED;
> >>  
> > 
> > This one is too strong.. persistent imbalance should be able to override
> > it.
> > 
> 
> IIRC, this change can avoid the following scenario:
> 
> One sysbench cpu thread(cookieA) and sysbench mysql thread(cookieB) running
> on the two siblings of core_1, the other sysbench cpu thread(cookieA) and
> sysbench mysql thread(cookieB) running on the two siblings of core2, which
> causes 50% force idle.
> 
> This is not an imbalance case.

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.