linux-kernel - Re: __mutex_lock_common() unlikely very likely

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Message-ID: <20170119085507.GA28607@nuc-i3427.alporthouse.com>
Date:   Thu, 19 Jan 2017 08:55:07 +0000
From:   Chris Wilson <chris@...is-wilson.co.uk>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: __mutex_lock_common() unlikely very likely

On Wed, Jan 18, 2017 at 03:58:24PM -0500, Steven Rostedt wrote:
> Chris,
> 
> My branch tracer flagged the unlikely in __mutex_lock_common() as
> always hit. That's the:
> 
> 	if (use_ww_ctx) {
> 		[...]
> 		if (unlikely(ww_ctx == READ_ONCE(ww->ctx)))
> 			return -EALREADY;
> 	}
> 
> This is hit 100% of the time, and its coming from the drm logic:

By design this is an exceptional case. In practice, drm modesetting is a
little slapsidasical when it comes to locking. However, it is the
minority use case, just that on intel, the more prevalent users do not
hit this path - though they will with the ww_mutex refactoring work. ttm
drivers (amdgpu, nouveau etc) will be demonstrating that this is the
unlikely branch.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre