linux-kernel - Re: [RFC][PATCH 6/8] mm: handle_speculative

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Date:	Tue, 5 Jan 2010 09:55:43 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andi Kleen <andi@...stfloor.org>
cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>, cl@...ux-foundation.org,
	"hugh.dickins" <hugh.dickins@...cali.co.uk>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [RFC][PATCH 6/8] mm: handle_speculative_fault()

On Tue, 5 Jan 2010, Andi Kleen wrote:
> 
> > Oh well. Somebody who is bored might look at trying to make the wrapper 
> > code in arch/x86/lib/semaphore_32.S work on x86-64 too. It should make the 
> > successful rwsem cases much faster.
> 
> Maybe, maybe not.

If there is actual contention on the lock, but mainly just readers (which 
is what the profile indicates: since there is no scheduler footprint, the 
actual writer-vs-reader case is probably very rare), then the xadd is 
likely to be _much_ faster than the spinlock.

Sure, the cacheline is going to bounce regardless (since it's a shared 
per-mm data structure), but the spinlock is going to bounce wildly 
back-and-forth between everybody who _tries_ to get it, while the regular 
xadd is going to bounce just once per actual successful xadd.

So a spinlock is as cheap as an atomic when there is no contention (which 
is the common single-thread case - the real cost of both lock and atomic 
is simply the fact that CPU serialization is expensive), but when there is 
actual lock contention, I bet the atomic xadd is going to be shown to be 
superior.

Remember: we commonly claim that 'spin_unlock' is basically free on x86 - 
and that's true, but it is _only_ true for the uncontended state. 

		Linus
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