linux-kernel - Re: [PATCH 5/8] sched: Favour moving tasks towards the preferred node

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Message-ID: <20130628102455.GE28407@twins.programming.kicks-ass.net>
Date:	Fri, 28 Jun 2013 12:24:55 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	Mel Gorman <mgorman@...e.de>, Ingo Molnar <mingo@...nel.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/8] sched: Favour moving tasks towards the preferred node

On Fri, Jun 28, 2013 at 03:37:23PM +0530, Srikar Dronamraju wrote:
> > > > +
> > > > +
> > > >  /*
> > > >   * can_migrate_task - may task p from runqueue rq be migrated to this_cpu?
> > > >   */
> > > > @@ -3945,10 +3977,14 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
> > > >  
> > > >  	/*
> > > >  	 * Aggressive migration if:
> > > > -	 * 1) task is cache cold, or
> > > > -	 * 2) too many balance attempts have failed.
> > > > +	 * 1) destination numa is preferred
> > > > +	 * 2) task is cache cold, or
> > > > +	 * 3) too many balance attempts have failed.
> > > >  	 */
> > > >  
> > > > +	if (migrate_improves_locality(p, env))
> > > > +		return 1;
> > > 
> > > Shouldnt this be under tsk_cache_hot check?
> > > 
> > > If the task is cache hot, then we would have to update the corresponding  schedstat
> > > metrics.
> > 
> > No; you want migrate_degrades_locality() to be like task_hot(). You want
> > to _always_ migrate tasks towards better locality irrespective of local
> > cache hotness.
> > 
> 
> Yes, I understand that numa should have more priority over cache.
> But the schedstats will not be updated about whether the task was hot or
> cold.
> 
> So lets say the task was cache hot but numa wants it to move, then we
> should certainly move it but we should update the schedstats to mention that we
> moved a cache hot task.
> 
> Something akin to this.
> 
> 	tsk_cache_hot = task_hot(p, env->src_rq->clock_task, env->sd);
> 	if (tsk_cache_hot) {
> 		if (migrate_improves_locality(p, env) || 
> 		 	(env->sd->nr_balance_failed > env->sd->cache_nice_tries)) {
> #ifdef CONFIG_SCHEDSTATS
> 			schedstat_inc(env->sd, lb_hot_gained[env->idle]);
> 			schedstat_inc(p, se.statistics.nr_forced_migrations);
> #endif
> 			return 1;
> 		}
> 		schedstat_inc(p, se.statistics.nr_failed_migrations_hot);
> 		return 0;
> 	}
> 	return 1;

Ah right.. ok that might make sense.
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