linux-kernel - Re: [PATCH v2] locking/pvqspinlock: Relax cmpxchg's to improve performance on some archs

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Date:   Tue, 3 Jan 2017 17:07:54 -0500
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Pan Xinhui <xinhui@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>
Subject: Re: [PATCH v2] locking/pvqspinlock: Relax cmpxchg's to improve
 performance on some archs

On 01/03/2017 11:18 AM, Peter Zijlstra wrote:
> On Sun, Dec 25, 2016 at 03:26:01PM -0500, Waiman Long wrote:
>> A number of cmpxchg calls in qspinlock_paravirt.h were replaced by more
>> relaxed versions to improve performance on architectures that use LL/SC.
> Claim without numbers ;-)

Well it is hard to produce actual numbers here as I don't have the setup
to gather data.
 
>> All the locking related cmpxchg's are replaced with the _acquire
>> variants:
>>  - pv_queued_spin_steal_lock()
>>  - trylock_clear_pending()
> So these seem to make sense in that they're in 'fast' paths..
>
>> The cmpxchg's related to hashing are replaced by either by the _release
>> or the _relaxed variants. See the inline comment for details.
>
> But these not so much, we're going to put the vcpu to sleep, why does it
> make sense to 'optimize' the wait/kick stuff?

I haven't thought too much about fast/slow paths when I was making the
patch. You are right that we properly don't need to do that for the
slowpath cases. I can modify the patch to do just the fast patch change.

Cheers,
Longman