Re: RFC on fixing mutex spinning on owner

From	Peter Zijlstra <peterz@infradead.org>
Newsgroups	linux.kernel
Subject	Re: RFC on fixing mutex spinning on owner
Date	2016-03-17 11:20 +0100
Message-ID	<rdDc6-65q-11@gated-at.bofh.it> (permalink)
References	(1 earlier) <rdtcK-7ER-13@gated-at.bofh.it> <rdtcK-7ER-27@gated-at.bofh.it> <rdvHA-UH-7@gated-at.bofh.it> <rdAHg-4j7-7@gated-at.bofh.it> <rdC6m-5mE-15@gated-at.bofh.it>
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On Thu, Mar 17, 2016 at 08:05:26AM +0000, Nicholas Mc Guire wrote:
> scanning for that patter with a quite relaxed spatch did not
> turn up more than a hand full:
> 
> @resched_spin exists@
> position p;
> @@
> 
> (
> * while@p (...) {
>           ...
>           if (need_resched() || ...)
>                   break;
>           ...
>           \(cpu_relax\|cpu_relax_lowlatency\)();
>   }
> |
> * while@p (!need_resched()) {
>           ...
>           \(cpu_relax\|cpu_relax_lowlatency\)();
>   }
> )
> 
> @script:python@
> p << resched_spin.p;
> @@
> print "%s:%s " % (p[0].file,p[0].line)                                          
> 
> 
> is this making some wrong assumptions here or is this
> really so infrequent ? 

Ah, a tool, nice! :-)

There should be one in the osq_lock, one in mutex, one in rwsem-xadd,
davidlohr was adding one to rt_mutex; and these are the one I can
remember from the top of my head.

But I would not be surprised if there are a fair few more. Also for (;;)
is a frequent loop pattern.

My biggest worry is how to not forget adding this annotation if we
create yet another instance of this. The tool could help I suppose.

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.

Thread

RFC on fixing mutex spinning on owner Joel Fernandes <agnel.joel@gmail.com> - 2016-03-17 00:30 +0100
  Re: RFC on fixing mutex spinning on owner Greg Kroah-Hartman <greg@kroah.com> - 2016-03-17 00:40 +0100
    Re: RFC on fixing mutex spinning on owner Joel Fernandes <agnel.joel@gmail.com> - 2016-03-17 00:40 +0100
      Re: RFC on fixing mutex spinning on owner Steven Rostedt <rostedt@goodmis.org> - 2016-03-17 03:20 +0100
        Re: RFC on fixing mutex spinning on owner Joel Fernandes <agnel.joel@gmail.com> - 2016-03-17 06:40 +0100
          Re: RFC on fixing mutex spinning on owner Steven Rostedt <rostedt@goodmis.org> - 2016-03-17 13:10 +0100
        Re: RFC on fixing mutex spinning on owner Peter Zijlstra <peterz@infradead.org> - 2016-03-17 08:40 +0100
          Re: RFC on fixing mutex spinning on owner Nicholas Mc Guire <der.herr@hofr.at> - 2016-03-17 10:10 +0100
            Re: RFC on fixing mutex spinning on owner Peter Zijlstra <peterz@infradead.org> - 2016-03-17 11:20 +0100
              Re: RFC on fixing mutex spinning on owner Thomas Gleixner <tglx@linutronix.de> - 2016-03-17 12:20 +0100
                Re: RFC on fixing mutex spinning on owner Steven Rostedt <rostedt@goodmis.org> - 2016-03-17 13:10 +0100
                Re: RFC on fixing mutex spinning on owner Peter Zijlstra <peterz@infradead.org> - 2016-03-17 13:20 +0100
                Re: RFC on fixing mutex spinning on owner Thomas Gleixner <tglx@linutronix.de> - 2016-03-17 14:40 +0100
                Re: RFC on fixing mutex spinning on owner Nicholas Mc Guire <der.herr@hofr.at> - 2016-03-17 17:50 +0100

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