netdev - Re: [PATCHv5 2/2] memory barrier: adding smp_mb__after

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Date:	Tue, 7 Jul 2009 16:34:16 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Jiri Olsa <jolsa@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	fbl@...hat.com, nhorman@...hat.com, davem@...hat.com,
	htejun@...il.com, jarkao2@...il.com, davidel@...ilserver.org
Subject: Re: [PATCHv5 2/2] memory barrier: adding smp_mb__after_lock

On 07/07, Mathieu Desnoyers wrote:
>
> As with any optimization (and this is one that adds a semantic that will
> just grow the memory barrier/locking rule complexity), it should come
> with performance benchmarks showing real-life improvements.

Well, the same applies to smp_mb__xxx_atomic_yyy or smp_mb__before_clear_bit.

Imho the new helper is not worse, and it could be also used by
try_to_wake_up(), __pollwake(), insert_work() at least.

> Otherwise I'd recommend sticking to smp_mb() if this execution path is
> not that critical, or to move to RCU if it's _that_ critical.
>
> A valid argument would be if the data structures protected are so
> complex that RCU is out of question but still the few cycles saved by
> removing a memory barrier are really significant.

Not sure I understand how RCU can help,

> And even then, the
> proper solution would be more something like a
> __read_lock()+smp_mb+smp_mb+__read_unlock(), so we get the performance
> improvements on architectures other than x86 as well.

Hmm. could you explain what you mean?

Oleg.

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