linux-kernel - Re: [PATCH -v5][RFC]: mutex: implement adaptive spinning

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Date:	Wed, 7 Jan 2009 18:09:32 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Gregory Haskins <ghaskins@...ell.com>
cc:	Ingo Molnar <mingo@...e.hu>, Andi Kleen <andi@...stfloor.org>,
	Matthew Wilcox <matthew@....cx>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	paulmck@...ux.vnet.ibm.com, Chris Mason <chris.mason@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Nick Piggin <npiggin@...e.de>,
	Peter Morreale <pmorreale@...ell.com>,
	Sven Dietrich <SDietrich@...ell.com>
Subject: Re: [PATCH -v5][RFC]: mutex: implement adaptive spinning


On Wed, 7 Jan 2009, Gregory Haskins wrote:

> Hi Ingo,
> 
> Ingo Molnar wrote:
> > * Gregory Haskins <ghaskins@...ell.com> wrote:
> >
> >   
> >> Can I ask a simple question in light of all this discussion?
> >>
> >> "Is get_task_struct() really that bad?"
> >>     
> >
> > it dirties a cacheline and it also involves atomics.
> >   
> Yes, understood.  But we should note we are always going to be talking
> about thrashing caches here since we are ultimately having one CPU
> observe another.  There's no way to get around that.  I understand that
> there are various degrees of this occurring, and I have no doubt that
> the proposed improvements strive to achieve a reduction of that.  My
> question is really targeted at "at what cost".
> 
> Don't get me wrong.  I am not advocating going back to get/put-task per
> se.  I am simply asking the question of whether we have taken the design
> off into the weeds having lost sight of the actual requirements and/or
> results.  Its starting to smell like we have.  This is just a friendly
> reality check.  Feel free to disregard. ;)

What would be interesting is various benchmarks against all three.

1) no mutex spinning.
2) get_task_struct() implementation.
3) spin_or_sched implementation.

I believe that 2 happens to be the easiest to understand. No need to know 
about the behavior of freed objects. If we see no or negligible 
improvement between 2 and 3 on any benchmark, then I say we stick with 2.

-- Steve

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