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

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Date:	Mon, 19 Oct 2015 13:20:54 -0400
From:	Waiman Long <waiman.long@....com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	ling.ma.program@...il.com, 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

On 10/19/2015 05:33 AM, Peter Zijlstra wrote:
> 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
>> 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.
> On what hardware? Also, you forgot to Cc Waiman, who is a prime author
> of this code. Excessive quoting for his benefit.

Thanks for letting me aware of this patch. I had commented on the patch 
in a separate mail.

Cheers,
Longman

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