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

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Message-ID: <20170119083252.6cbb0c8d@gandalf.local.home>
Date:   Thu, 19 Jan 2017 08:32:52 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Chris Wilson <chris@...is-wilson.co.uk>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: __mutex_lock_common() unlikely very likely

On Thu, 19 Jan 2017 08:55:07 +0000
Chris Wilson <chris@...is-wilson.co.uk> wrote:

> 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.
>

Then I suggest that we remove the unlikely, as it's only "unlikely" if
you have the right hardware. If you don't (and I appear to have three
boxes that don't) then it becomes very likely.

"unlikely" is not about how likely you have the right hardware. It's
about high likely the logic is. If it's a hardware issue, it shouldn't
have a likely or unlikely attached to it.

I'll send a patch to nuke it. When drm is no longer a special case,
because it's a popular platform, we can add it back.

-- Steve