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

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Date:	Thu, 17 Mar 2016 14:32:54 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Steven Rostedt <rostedt@...dmis.org>,
	Nicholas Mc Guire <der.herr@...r.at>,
	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>
Subject: Re: RFC on fixing mutex spinning on owner

On Thu, 17 Mar 2016, Peter Zijlstra wrote:
> On Thu, Mar 17, 2016 at 08:06:29AM -0400, Steven Rostedt wrote:
> > On Thu, 17 Mar 2016 12:16:11 +0100 (CET)
> > Thomas Gleixner <tglx@...utronix.de> wrote:
> > 
> > > On Thu, 17 Mar 2016, Peter Zijlstra wrote:
> > > > Also, maybe the tracer should measure the time from need_resched()
> > > > getting true until the next preemption point, instead of the entire time
> > > > preemption was disabled. Which would avoid the entire issue altogether.  
> > > 
> > > Well, that only gives you the information on a actual preemption, but not
> > > information about long preempt disabled regions which can cause a problem
> > > eventually.
> > > 
> > 
> > Actually, I was thinking the reverse. If need_resched() is called and
> > is false, then do a reset of the preemption time. But if need_resched()
> > is true, then do nothing, as that would measure the total time preempt
> > disable was set and a task could not schedule.
> > 
> > Question is, should this be a hook and each location audited, or add
> > this to need_resched() itself?
> 
> Is anybody calling need_resched() and then not doing anything with the
> value?

Probably not. So Stevens idea makes a lot of sense.

Thanks,

	tglx