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

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Message-ID: <20160317073605.GM6344@twins.programming.kicks-ass.net>
Date:	Thu, 17 Mar 2016 08:36:05 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Joel Fernandes <agnel.joel@...il.com>,
	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>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: RFC on fixing mutex spinning on owner

On Wed, Mar 16, 2016 at 10:17:51PM -0400, Steven Rostedt wrote:
> 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.

Urgh, there's a lot of such spin loops all over, that's going to be a
pain to annotate all.