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

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Date:	Mon, 19 Oct 2015 13:24:27 -0400
From:	Waiman Long <waiman.long@....com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	Peter Zijlstra <peterz@...radead.org>, ling.ma.program@...il.com,
	mingo@...hat.com, linux-kernel@...r.kernel.org,
	Ma Ling <ling.ml@...baba-inc.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Jiri Olsa <jolsa@...hat.com>
Subject: Re: [RFC PATCH] qspinlock: Improve performance by reducing load instruction
 rollback

On 10/19/2015 07:24 AM, Ingo Molnar wrote:
> * Peter Zijlstra<peterz@...radead.org>  wrote:
>
>> On Mon, Oct 19, 2015 at 09:58:23AM +0200, Ingo Molnar wrote:
>>> * 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.
>> So its trivial to lift this code into userspace -- in fact, I have that
>> somewhere.
>>
>> The trouble is going to keep them in sync.
> So we can just try this optimistically, and if it keeps breaking, we can use the
> technique perf uses to sync up the rbtree implementation: we copy the kernel
> version into tooling, but run diff against the kernel version and warn at tool
> build time that there's divergence.
>
> I.e. a non-build-fatal force that keeps things in sync.
>
> Thanks,
>
> 	Ingo
>

It is on my to-do list. I just want to wrap up my latest PV qspinlock 
patch before embarking on this adventure.

Cheers,
Longman
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