netdev - Re: [PATCH] atomic: add atomic_inc_not_zero

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Message-ID: <alpine.DEB.2.00.1011150821000.19175@router.home>
Date:	Mon, 15 Nov 2010 08:25:37 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Ingo Molnar <mingo@...e.hu>, Andi Kleen <andi@...stfloor.org>,
	Nick Piggin <npiggin@...nel.dk>
Subject: Re: [PATCH] atomic: add atomic_inc_not_zero_hint()

On Mon, 15 Nov 2010, Eric Dumazet wrote:

> Exclusive access ? As soon as another cpu takes it again, you lose.

Sure but you want to avoid the fetch in shared mode here.

> Its not really the same thing... Maybe you miss the 'hint' intention at
> all. We know the probable value of the counter, we dont want to read it.

Ok may be in thise case you can predict the value but in general it is
difficult to always provide an expected value. It would be easier to be
able to tell the processor that the cacheline should not be fetched as
shared but immediately in exclusive state.

> atomic_read() and atomic_cmpxchg(). We tried prefetchw() and it was a
> performance drop. It was with only 16 cpus contending on neighbour

Does prefetchw work? Andi claims that prefetchw is not working on
x86 and I doubt that you ran tests on Itanium.
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