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

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Message-ID: <20170103161836.GY3107@twins.programming.kicks-ass.net>
Date:   Tue, 3 Jan 2017 17:18:36 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
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 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 ;-)

> 
> 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?