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Date: Wed, 7 May 2008 20:49:21 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matthew Wilcox <matthew@wil.cx>, Andi Kleen <andi@firstfloor.org>,
       "Zhang, Yanmin" <yanmin_zhang@linux.intel.com>,
       LKML <linux-kernel@vger.kernel.org>,
       Alexander Viro <viro@ftp.linux.org.uk>,
       Andrew Morton <akpm@linux-foundation.org>
Subject: Re: AIM7 40% regression with 2.6.26-rc1
Message-ID: <20080507184921.GA15452@elte.hu>
References: <87hcdab8zp.fsf@basil.nowhere.org> <alpine.LFD.1.10.0805070728280.32269@woody.linux-foundation.org> <20080507162012.GA10096@elte.hu> <alpine.LFD.1.10.0805070929310.3024@woody.linux-foundation.org> <20080507170528.GA11511@elte.hu> <alpine.LFD.1.10.0805071017130.3024@woody.linux-foundation.org> <20080507173612.GA13591@elte.hu> <alpine.LFD.1.10.0805071049180.3024@woody.linux-foundation.org> <20080507175959.GZ19219@parisc-linux.org> <alpine.LFD.1.10.0805071104290.3024@woody.linux-foundation.org>
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* Linus Torvalds <torvalds@linux-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@elte.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