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

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Date:	Mon, 19 Oct 2015 09:58:23 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	ling.ma.program@...il.com
Cc:	peterz@...radead.org, mingo@...hat.com,
	linux-kernel@...r.kernel.org, Ma Ling <ling.ml@...baba-inc.com>
Subject: Re: [RFC PATCH] qspinlock: Improve performance by reducing load
 instruction rollback


* ling.ma.program@...il.com <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
> instructions to be serialization, 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.
> 
> 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.

So it would be nice to create a new user-space spinlock testing facility, via a 
new 'perf bench spinlock' feature or so. That way others can test and validate 
your results on different hardware as well.

Thanks,

	Ingo
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