linux-kernel - Re: [RFC PATCH] qspinlock: Improve performance by reducing load instruction rollback

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Date:	Mon, 19 Oct 2015 11:46:59 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	ling.ma.program@...il.com
Cc:	mingo@...hat.com, linux-kernel@...r.kernel.org,
	Ma Ling <ling.ml@...baba-inc.com>,
	Waiman Long <Waiman.Long@....com>
Subject: Re: [RFC PATCH] qspinlock: Improve performance by reducing load
 instruction rollback

On Mon, Oct 19, 2015 at 10:27:22AM +0800, ling.ma.program@...il.com wrote:
> From: Ma Ling <ling.ml@...baba-inc.com>
> 
> All load instructions can run speculatively but they have to follow
> memory order rule in multiple cores as below:
> _x = _y = 0
> 
> Processor 0				Processor 1
> 
> mov r1, [ _y]  //M1			mov [ _x], 1  //M3
> mov r2, [ _x]  //M2			mov [ _y], 1  //M4
> 
> If r1 = 1, r2 must be 1
> 
> In order to guarantee above rule, although Processor 0 execute
> M1 and M2 instruction out of order, they are kept in ROB,
> when load buffer for _x in Processor 0 received the update
> message from Processor 1, Processor 0 need to roll back
> from M2 instruction, which will flush the whole pipeline,
> the latency is over the penalty from branch prediction miss.
> 
> In this patch we use lock cmpxchg instruction to force load

"lock cmpxchg" makes me think you're working on x86.

> instructions to be serialization,

smp_rmb() does that, and that's 'free' on x86. Because x86 doesn't do
read reordering.

> the destination operand
> receives a write cycle without regard to the result of
> the comparison, which can help us to reduce the penalty
> from load instruction roll back.

And that makes me think I'm not understanding what you're getting at. If
you need to force memory order, a "fence" (or smp_mb()) would still be
cheaper than endlessly pulling the line into exclusive state for no
reason, right?

> Our experiment indicates the performance can be improved by 10%~15%
> for 2 and 3 threads cases, the conflicts from lock cache line
> spend them most of the time.

That just doesn't parse, what?
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