linux-kernel - Re: [PATCH v2 02/11] locking/ww_mutex: Re-check ww->ctx in the inner optimistic spin loop

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Message-ID: <20161206182935.GD3107@twins.programming.kicks-ass.net>
Date:   Tue, 6 Dec 2016 19:29:35 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Nicolai Hähnle <nhaehnle@...il.com>,
        linux-kernel@...r.kernel.org,
        Nicolai Hähnle <Nicolai.Haehnle@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Maarten Lankhorst <dev@...ankhorst.nl>,
        Daniel Vetter <daniel@...ll.ch>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v2 02/11] locking/ww_mutex: Re-check ww->ctx in the inner
 optimistic spin loop

On Tue, Dec 06, 2016 at 11:03:28AM -0500, Waiman Long wrote:
> 
> The mutex_spin_on_owner() function was originally marked noinline
> because it could be a major consumer of CPU cycles in a contended lock.
> Having it shown separately in the perf output will help the users have a
> better understanding of what is consuming all the CPU cycles. So I would
> still like to keep it this way.

ah!, I tried to dig through history but couldn't find a reason for it.

> 
> If you have concern about additional latency for non-ww_mutex calls, one
> alternative can be:

That's pretty horrific :/