linux-kernel - Re: [PATCH 30/33] sched: Debug nohz rq clock

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Date:	Thu, 11 Apr 2013 18:47:26 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Kevin Hilman <khilman@...prootsystems.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Alessio Igor Bogani <abogani@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Metcalf <cmetcalf@...era.com>,
	Christoph Lameter <cl@...ux.com>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Li Zhong <zhong@...ux.vnet.ibm.com>,
	Namhyung Kim <namhyung.kim@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 30/33] sched: Debug nohz rq clock

On Wed, Mar 20, 2013 at 04:23:34PM -0700, Kevin Hilman wrote:
> Hi Frederic,
> 
> On 01/07/2013 06:08 PM, Frederic Weisbecker wrote:
> > The runqueue clock is supposed to be periodically updated by the
> > tick. On full dynticks CPU we call update_nohz_rq_clock() before
> > reading it. Now the scheduler code is complicated enough that we
> > may miss some update_nohz_rq_clock() calls before reading the
> > runqueue clock.
> > 
> > This therefore introduce a new debugging feature that detects
> > when the rq clock is stale due to missing updates on full
> > dynticks CPUs.
> > 
> > This can be later expanded to debug stale clocks on dynticks-idle
> > CPUs.
> 
> [...]
> 
> > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > index e1bac76..0fef0b3 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -502,16 +502,39 @@ DECLARE_PER_CPU(struct rq, runqueues);
> >  #define cpu_curr(cpu)		(cpu_rq(cpu)->curr)
> >  #define raw_rq()		(&__raw_get_cpu_var(runqueues))
> >  
> > +static inline void rq_clock_check(struct rq *rq)
> > +{
> > +#if defined(CONFIG_SCHED_DEBUG) && defined(CONFIG_NO_HZ_FULL)
> > +	unsigned long long clock;
> > +	unsigned long flags;
> > +	int cpu;
> > +
> > +	cpu = cpu_of(rq);
> > +	if (!tick_nohz_full_cpu(cpu) || rq->curr == rq->idle)
> > +		return;
> > +
> > +	local_irq_save(flags);
> > +	clock = sched_clock_cpu(cpu_of(rq));
> > +	local_irq_restore(flags);
> > +
> > +	if (abs(clock - rq->clock) > (TICK_NSEC * 3))
> > +		WARN_ON_ONCE(1);
> > +#endif
> > +}
> 
> In working on the ARM port for full nohz, I'm hitting this
> warning early in the kernel boot, well before userspace starts
> (dump below[2].)
> 
> I've seen a few different variations of this, but the common
> thing for all of them is the use of wait_for_completion().
> 
> During boot, only swapper is running so it seems
> any waiting of sufficient length during boot will always trigger
> this warning.  The hack below[1] avoids checking for the init task,
> but I'm not sure if it's the right fix.
> 
> Kevin
> 
> [1]
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index f96329b..56e74df 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -512,7 +512,8 @@ static inline void rq_clock_check(struct rq *rq)
>  	int cpu;
> 
>  	cpu = cpu_of(rq);
> -	if (!tick_nohz_full_cpu(cpu) || rq->curr == rq->idle)
> +	if (!tick_nohz_full_cpu(cpu) || rq->curr == rq->idle ||
> +	    is_global_init(current))

Makes sense. But we seem to be taking a new direction there after feedback from Ingo
and Peterz: tag scheduler entry and exit points and invalidate on top of missing rq clock
updates since the last scheduler entry point.

thanks.
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