linux-kernel - Re: RFC on fixing mutex spinning on owner

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Date:	Wed, 16 Mar 2016 22:17:51 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Joel Fernandes <agnel.joel@...il.com>
Cc:	Greg Kroah-Hartman <greg@...ah.com>,
	linux-rt-users@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	kernelnewbies <kernelnewbies@...linux.org>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: RFC on fixing mutex spinning on owner

On Wed, 16 Mar 2016 16:38:56 -0700
Joel Fernandes <agnel.joel@...il.com> 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 for the report.

-- Steve