linux-kernel - Re: 20% performance drop on PostgreSQL 9.2 from kernel 3.5.3 to 3.6-rc5 on AMD chipsets

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Message-ID: <20120925184245.GA13694@liondog.tnic>
Date:	Tue, 25 Sep 2012 20:42:45 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mike Galbraith <efault@....de>, Mel Gorman <mgorman@...e.de>,
	Nikolay Ulyanitsky <lystor@...il.com>,
	linux-kernel@...r.kernel.org,
	Andreas Herrmann <andreas.herrmann3@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Suresh Siddha <suresh.b.siddha@...el.com>
Subject: Re: 20% performance drop on PostgreSQL 9.2 from kernel 3.5.3 to
 3.6-rc5 on AMD chipsets - bisected

On Tue, Sep 25, 2012 at 10:21:28AM -0700, Linus Torvalds wrote:
> On Tue, Sep 25, 2012 at 10:00 AM, Borislav Petkov <bp@...en8.de> wrote:
> >
> > 3.6-rc6+tip/auto-latest-kill select_idle_sibling()
> 
> Is this literally just removing it entirely?

Basically yes:

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6b800a14b990..016ba387c7f2 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2640,6 +2640,8 @@ static int select_idle_sibling(struct task_struct *p, int target)
        struct sched_group *sg;
        int i;
 
+       goto done;
+
        /*
         * If the task is going to be woken-up on this cpu and if it is
         * already idle, then it is the right target.

> Because apart from the latency spike at 4 procs (and the latency
> numbers look very noisy, so that's probably just noise), it looks
> clearly superior to everything else. On that benchmark, at least.

Yep, I need more results for a more reliable say here.

> How does pgbench look? That's the one that apparently really wants to
> spread out, possibly due to user-level spinlocks. So I assume it will
> show the reverse pattern, with "kill select_idle_sibling" being the
> worst case.

Let me run pgbench tomorrow (I had run it only on an older family 0x10
single-node box) on Bulldozer to check that out. And we haven't started
the multi-node measurements at all.

> Sad, because it really would be lovely to just remove that thing ;)

Right, so why did we need it all, in the first place? There has to be
some reason for it.

Thanks.

-- 
Regards/Gruss,
    Boris.
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