linux-kernel - Re: AIM7 40% regression with 2.6.26-rc1

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Date:	Wed, 7 May 2008 20:49:21 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Matthew Wilcox <matthew@....cx>, Andi Kleen <andi@...stfloor.org>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Alexander Viro <viro@....linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: AIM7 40% regression with 2.6.26-rc1


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> .. Hmm ... Time passes. Linus looks at git history.
> 
> It does look like "cond_resched()" has not worked with the BKL since 
> 2005, and hasn't taken the BKL into account. Commit 5bbcfd9000:
> 
>     [PATCH] cond_resched(): fix bogus might_sleep() warning
> 
> +       if (unlikely(preempt_count()))
> +               return;
> 
> which talks about the BKS, ie it only took the *semaphore* 
> implementation into account. Never the spinlock-with-preemption-count 
> one.
> 
> Or am I blind?

hm, i think you are right.

most latency reduction was concentrated on the PREEMPT+PREEMPT_BKL case, 
and not getting proper cond_resched() behavior in case of !PREEMPT_BKL 
would certainly not be noticed by distros or users.

We made CONFIG_PREEMPT_BKL=y the default on SMP in v2.6.8, in this 
post-2.6.7 commit that introduced the feature:

|  commit fb8f6499abc6a847109d9602b797aa6afd2d5a3d
|  Author: Ingo Molnar <mingo@...e.hu>
|  Date:   Fri Jan 7 21:59:57 2005 -0800
|
|     [PATCH] remove the BKL by turning it into a semaphore

There was constant trouble around all these variations of preemptability 
and their combination with debugging helpers. (So i was rather happy to 
get rid of !PREEMPT_BKL - in the (apparently wrong) assumption that no 
tears will be shed.)

	Ingo
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