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From: Peter Zijlstra <peterz@infradead.org>
To: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Andy Lutomirski <luto@kernel.org>, Ingo Molnar <mingo@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: Abysmal scheduler performance in Linus' tree?
Date: Wed, 6 Sep 2017 12:51:31 +0200
Message-ID: <20170906105131.gqjmaextmn3u6tj2@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20170906104420.ic5lbpacpyyz53w5@hirez.programming.kicks-ass.net>

On Wed, Sep 06, 2017 at 12:44:20PM +0200, Peter Zijlstra wrote:
> > > +       /* if this cache has capacity, come here */
> > > +       if (this_stats.has_capacity && this_stats.nr_running < prev_stats.nr_running+1)
> > > +               return true;
> > 
> > This is still not working as intended, it should be 
> > 
> > 	if (this_stats.has_capacity && this_stats.nr_running+1 < prev_stats.nr_running)
> > 		return true;
> > 
> > to fix the regression.
> 
> Argh, you're quite right. Let me do a patch for that.

---
Subject: sched/fair: Fix wake_affine_llc() balance rules
From: Peter Zijlstra <peterz@infradead.org>
Date: Wed Sep  6 12:45:45 CEST 2017

Chris reported that the SMT balance rules got the +1 on the wrong
side, resulting in a bias towards the current LLC; which the
load-balancer would then try and undo.

Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Fixes: 90001d67be2f ("sched/fair: Fix wake_affine() for !NUMA_BALANCING")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/sched/fair.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5435,7 +5435,7 @@ wake_affine_llc(struct sched_domain *sd,
 		return false;
 
 	/* if this cache has capacity, come here */
-	if (this_stats.has_capacity && this_stats.nr_running < prev_stats.nr_running+1)
+	if (this_stats.has_capacity && this_stats.nr_running+1 < prev_stats.nr_running)
 		return true;
 
 	/*

  reply index

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-09-06  5:13 Andy Lutomirski
2017-09-06  8:25 ` Peter Zijlstra
2017-09-06  8:59   ` Andy Lutomirski
2017-09-06  9:03     ` Peter Zijlstra
2017-09-06 16:14       ` Peter Zijlstra
2017-09-07  6:15         ` Mike Galbraith
2017-09-07  7:31           ` Ingo Molnar
2017-09-07  9:13           ` [PATCH] sched/cpuset/pm: Fix cpuset vs suspend-resume Peter Zijlstra
2017-09-07  9:26             ` Peter Zijlstra
2017-09-07 10:54               ` Rafael J. Wysocki
2017-09-07 20:38               ` Tejun Heo
2017-09-07 10:33             ` [tip:sched/urgent] sched/cpuset/pm: Fix cpuset vs. suspend-resume bugs tip-bot for Peter Zijlstra
2017-09-06  9:15 ` Abysmal scheduler performance in Linus' tree? Chris Wilson
2017-09-06  9:24   ` Peter Zijlstra
     [not found]     ` <150469312649.28581.17626550155735691534@mail.alporthouse.com>
2017-09-06 10:44       ` Peter Zijlstra
2017-09-06 10:51         ` Peter Zijlstra [this message]
2017-09-07  8:16           ` [tip:sched/urgent] sched/fair: Fix wake_affine_llc() balancing rules tip-bot for Peter Zijlstra

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