linux-kernel - Re: [PATCH 2/2] mutex: Apply adaptive spinning on mutex

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Date:	Fri, 25 Mar 2011 09:29:30 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Mason <chris.mason@...cle.com>,
	linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org
Subject: Re: [PATCH 2/2] mutex: Apply adaptive spinning on mutex_trylock()

On Fri, 2011-03-25 at 09:10 -0400, Steven Rostedt wrote:

> One solution is to have this be only done on explicit trylocks. Perhaps
> introduce a mutex_trylock_spin()? Then when the developer knows that
> this scenario does not exist, they can convert mutex_trylocks() into
> this spinning version.
> 

I'm not sure this is even worth it, as I'm looking at the
btfs/extend-tree.c code, this is the main reason to use mutex_trylock().

I guess what you see in your benchmarks is that trylock contention
happens mostly in the non-deadlock scenario. But I bet you have
latencies when it does happen, but the benefit seems to out weigh it in
the results.

I wonder what happens if you run dbench as an RT task.

-- Steve


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