Re: RFC on fixing mutex spinning on owner

[Thomas Gleixner]

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@xxxxxxxxxxxxx> 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