linux-kernel - Re: [PATCH 13/13] sched: Account for the number of preferred tasks running on a node when selecting a preferred node

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Message-ID: <20130704135415.GR1875@suse.de>
Date:	Thu, 4 Jul 2013 14:54:15 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	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 13/13] sched: Account for the number of preferred tasks
 running on a node when selecting a preferred node

On Thu, Jul 04, 2013 at 06:37:19PM +0530, Srikar Dronamraju wrote:
> >  static void task_numa_placement(struct task_struct *p)
> >  {
> >  	int seq, nid, max_nid = 0;
> > @@ -897,7 +924,7 @@ static void task_numa_placement(struct task_struct *p)
> > 
> >  		/* Find maximum private faults */
> >  		faults = p->numa_faults[task_faults_idx(nid, 1)];
> > -		if (faults > max_faults) {
> > +		if (faults > max_faults && !sched_numa_overloaded(nid)) {
> 
> Should we take the other approach of setting the preferred nid but not 
> moving the task to the node?
> 

Why would that be better?

> So if some task moves out of the preferred node, then we should still be
> able to move this task there. 
> 

I think if we were to do that then I'd revisit the "task swap" logic from
autonuma (numacore had something similar) and search for pairs of tasks
that both benefit from a swap. I prototyped something basic alont this
lines but it was premature. It's a more directed approach but one that
should be done only when the private/shared and load logic is solidified.

> However your current approach has an advantage that it atleast runs on
> second preferred choice if not the first.
> 

That was the intention.

> Also should sched_numa_overloaded() also consider pinned tasks?
> 

I don't think sched_numa_overloaded() needs to as such, least I don't see how
it would do it sensibly right now. However, you still make an important point
in that find_idlest_cpu_node should take it into account. How about this?

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 9247345..387f28d 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -863,9 +863,13 @@ find_idlest_cpu_node(int this_cpu, int nid)
 		load = weighted_cpuload(i);
 
 		if (load < min_load) {
-			/* Do not preempt a task running on a preferred node */
+			/*
+			 * Do not preempt a task running on a preferred node or
+			 * tasks are are pinned to their current CPU
+			 */
 			struct task_struct *p = cpu_rq(i)->curr;
-			if (p->numa_preferred_nid != nid) {
+			if (p->numa_preferred_nid != nid &&
+			    cpumask_weight(tsk_cpus_allowed(p)) > 1) {
 				min_load = load;
 				idlest_cpu = i;
 			}
--
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