Re: [PATCH v2 7/7] sched/core: Add debug code to catch missing update_rq_clock()

From	Jan Kara <jack@suse.cz>
Newsgroups	linux.kernel
Subject	Re: [PATCH v2 7/7] sched/core: Add debug code to catch missing update_rq_clock()
Date	2016-09-22 10:40 +0200
Message-ID	<sk7HX-3Yu-1@gated-at.bofh.it> (permalink)
References	<sjPUJ-17j-7@gated-at.bofh.it> <sjPUK-17j-21@gated-at.bofh.it> <sjS6e-2q3-21@gated-at.bofh.it> <sk7eV-3OW-5@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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On Thu 22-09-16 10:04:36, Peter Zijlstra wrote:
> On Wed, Sep 21, 2016 at 05:58:27PM +0200, Petr Mladek wrote:
> > > +static inline void assert_clock_updated(struct rq *rq)
> > > +{
> > > +#ifdef CONFIG_SCHED_DEBUG
> > > +	/*
> > > +	 * The only reason for not seeing a clock update since the
> > > +	 * last rq_pin_lock() is if we're currently skipping updates.
> > > +	 */
> > > +	WARN_ON_ONCE(rq->clock_update_flags < RQCF_ACT_SKIP);
> > > +#endif
> > > +}
> > 
> > I am afraid that it might eventually create a deadlock.
> > For example, there is the following call chain:
> > 
> 
> Yeah, meh. There's already plenty WARNs in the sched code. The idea of
> course being that they should not trigger. If they do, something
> buggered already, so who bloody cares about a deadlock later ;-)

Yeah, the trouble is that you usually won't see the WARN message
before deadlocking. So WARN_ON in scheduler is usually equivalent to

	if (condition)
		while (1);

;) Not really helping debugging much...

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Thread

[PATCH v2 0/7] sched: Diagnostic checks for missing rq clock updates Matt Fleming <matt@codeblueprint.co.uk> - 2016-09-21 15:40 +0200
  [PATCH v2 5/7] sched/core: Reset RQCF_ACT_SKIP before unpinning rq->lock Matt Fleming <matt@codeblueprint.co.uk> - 2016-09-21 15:40 +0200
  [PATCH v2 3/7] sched/fair: Update rq clock in task_hot() Matt Fleming <matt@codeblueprint.co.uk> - 2016-09-21 15:40 +0200
  [PATCH v2 2/7] sched/fair: Update rq clock before waking up new task Matt Fleming <matt@codeblueprint.co.uk> - 2016-09-21 15:40 +0200
  [PATCH v2 7/7] sched/core: Add debug code to catch missing update_rq_clock() Matt Fleming <matt@codeblueprint.co.uk> - 2016-09-21 15:40 +0200
    Re: [PATCH v2 7/7] sched/core: Add debug code to catch missing  update_rq_clock() Petr Mladek <pmladek@suse.com> - 2016-09-21 18:00 +0200
      Re: [PATCH v2 7/7] sched/core: Add debug code to catch missing  update_rq_clock() Matt Fleming <matt@codeblueprint.co.uk> - 2016-09-21 21:10 +0200
        Re: [PATCH v2 7/7] sched/core: Add debug code to catch missing  update_rq_clock() Thomas Gleixner <tglx@linutronix.de> - 2016-09-21 21:50 +0200
        Re: [PATCH v2 7/7] sched/core: Add debug code to catch missing  update_rq_clock() Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2016-09-22 02:50 +0200
      Re: [PATCH v2 7/7] sched/core: Add debug code to catch missing  update_rq_clock() Peter Zijlstra <peterz@infradead.org> - 2016-09-22 10:10 +0200
        Re: [PATCH v2 7/7] sched/core: Add debug code to catch missing  update_rq_clock() Jan Kara <jack@suse.cz> - 2016-09-22 10:40 +0200
          Re: [PATCH v2 7/7] sched/core: Add debug code to catch missing  update_rq_clock() Peter Zijlstra <peterz@infradead.org> - 2016-09-22 11:40 +0200
            Re: [PATCH v2 7/7] sched/core: Add debug code to catch missing  update_rq_clock() Peter Zijlstra <peterz@infradead.org> - 2016-09-22 13:30 +0200
  [PATCH v2 4/7] sched: Add wrappers for lockdep_(un)pin_lock() Matt Fleming <matt@codeblueprint.co.uk> - 2016-09-21 15:40 +0200
  [PATCH v2 6/7] sched/fair: Push rq lock pin/unpin into idle_balance() Matt Fleming <matt@codeblueprint.co.uk> - 2016-09-21 15:40 +0200
  [PATCH v2 1/7] sched/fair: Update the rq clock before detaching tasks Matt Fleming <matt@codeblueprint.co.uk> - 2016-09-21 15:50 +0200

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