Re: RFC on fixing mutex spinning on owner

[Joel Fernandes]

On Wed, Mar 16, 2016 at 7:17 PM, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> On Wed, 16 Mar 2016 16:38:56 -0700
> Joel Fernandes <agnel.joel@xxxxxxxxx> wrote:
>
> > I am not sure if the problem is with the i915 driver, because the
> > mutex spin on owner stuff is mutex related so the mutex design may
> > potentially need a tweak (I mentioned a proposal of adding mutex
> > spinning time outs).
> > Also since this is latency issue related (I mentioned preemptoff
> > tracer and preempt disabled), I sent it to linux-rt-users. Thanks for
> > the tip about sending it to i915 developers, incase no one here has a
> > say in the matter, I can drop them a note later as well.
>
> Actually, the preempt off section here is not really an issue:
>
>         rcu_read_lock();
>         while (owner_running(lock, owner)) {
>                 if (need_resched())
>                         break;
>
>                 cpu_relax_lowlatency();
>         }
>         rcu_read_unlock();
>
> Although preemption may be disabled, that "need_resched()" check will
> break out of the loop if a higher priority task were to want to run on
> this CPU.
>
> I probably should add a hook there to let the preemptoff tracer know
> that this is not an issue.

Thanks Steve! That makes sense. If you do end up adding this hook to
the tracer, I would appreciate it if you could Cc me on the patch so I
could back port it to my kernel as well.

Also, since we are on the topic of preemptoff tracer, I posted a patch
[1] few days ago fixing another issue, if it looks Ok to you could you
pick it up? I had CC'd you on it.

[1] http://www.mail-archive.com/linux-kernel@xxxxxxxxxxxxxxx/msg1099561.html

Thanks,
Joel