linux-kernel - Re: [PATCH][RFC]: mutex: adaptive spin

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Date:	Tue, 6 Jan 2009 07:55:58 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Matthew Wilcox <matthew@....cx>,
	Andi Kleen <andi@...stfloor.org>,
	Chris Mason <chris.mason@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Gregory Haskins <ghaskins@...ell.com>,
	Nick Piggin <npiggin@...e.de>
Subject: Re: [PATCH][RFC]: mutex: adaptive spin

On Tue, 6 Jan 2009, Ingo Molnar wrote:
> 
> The thing i like most about Peter's patch (compared to most other adaptive 
> spinning approaches i've seen, which all sucked as they included various 
> ugly heuristics complicating the whole thing) is that it solves the "how 
> long should we spin" question elegantly: we spin until the owner runs on a 
> CPU.

The other way around, you mean: we spin until the owner is no longer 
holding a cpu.

I agree that it's better than the normal "spin for some random time" 
model, but I can't say I like the "return 0" cases where it just retries 
the whole loop if the semaphore was gotten by somebody else instead. 
Sounds like an easyish live-lock to me. 

I also still strongly suspect that whatever lock actually needs this, 
should be seriously re-thought. 

But apart from the "return 0" craziness I at least dont' _hate_ this 
patch. Do we have numbers? Do we know which locks this matters on?

		Linus
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