Re: [PATCH 09/13] nohz: Allow timekeeper's tick to stop when allfull dynticks CPUs are idle

[Paul E. McKenney]

On Wed, Dec 18, 2013 at 03:36:11PM +0100, Frederic Weisbecker wrote:
> On Tue, Dec 17, 2013 at 03:51:17PM -0800, Paul E. McKenney wrote:
> > On Tue, Dec 17, 2013 at 11:51:28PM +0100, Frederic Weisbecker wrote:
> > > +/*
> > > + * Fetch max deferment for the current clockevent source until it overflows.
> > > + * Also in full dynticks environment, make sure the current timekeeper
> > > + * stays periodic until some other CPU can take its timekeeping duty
> > > + * or until all full dynticks go to sleep.
> > > + */
> > > +static u64 tick_timekeeping_max_deferment(struct tick_sched *ts)
> > > +{
> > > +	int cpu;
> > > +	u64 ret = KTIME_MAX;
> > > +
> > > +	/*
> > > +	 * Fast path for full dynticks off-case: skip to
> > > +	 * clockevent max deferment
> > > +	 */
> > > +	if (!tick_nohz_full_enabled())
> > > +		return timekeeping_max_deferment();
> > > +
> > > +	cpu = smp_processor_id();
> > > +
> > > +	/* Full dynticks CPU don't take timekeeping duty */
> > > +	if (!tick_timekeeping_cpu(cpu))
> > > +		return timekeeping_max_deferment();
> > > +
> > > +	/*
> > > +	 * If we are the timekeeper and all full dynticks CPUs are idle,
> > > +	 * then we can finally sleep.
> > > +	 */
> > > +	if (tick_do_timer_cpu == cpu ||
> > > +	    (tick_do_timer_cpu == TICK_DO_TIMER_NONE &&	ts->do_timer_last == 1)) {
> > > +		if (!rcu_sys_is_idle()) {
> > 
> > So multiple CPUs could call rcu_sys_is_idle()?  Seems like this could
> > happen if tick_do_timer_cpu == TICK_DO_TIMER_NONE.  This would be OK only
> > if tick_timekeeping_cpu() returns true for one and only one of the CPUs
> > at any given range of time -- and also that no one calls rcu_sys_is_idle()
> > during a timekeeping CPU handoff.
> 
> Hmm yeah I fear we can have concurrent callers of this at a same time range.

OK, we do need to fix that.

> > If two different CPUs call rcu_sys_is_idle() anywhere nearly concurrently
> > on a small system (CONFIG_NO_HZ_FULL_SYSIDLE_SMALL), rcu_sys_is_idle()
> > will break and you will have voided your warranty.  ;-)
> 
> So it breaks because of concurrent state machine stepping on each other toes, right?
> Like one CPU has reached RCU_SYSIDLE_SHORT and another comes and see only
> RCU_SYSIDLE_NONE, so it can for example overwite to SHORT while the other CPU
> can be already far further the cmpxchg() sequence?

The ugliest possibility is in the CONFIG_NO_HZ_FULL_SYSIDLE_SMALL case.
Three CPUs are independently scanning the CPUs, and by luck, each of these
three CPUs see all the CPUs idle, even though CPUs are going idle and
non-idle continuously throughout.  After all three complete, they each
advance the state.  The state thus becomes RCU_SYSIDLE_FULL even though
there are non-idle CPUs.  The timekeeping tick therefore gets shut off
even though there are non-idle CPUs.  Very bad.

> Aye, I need to think further on how to cope with that...

One way would be to cmpxchg the ownership, and only the winner does
the call.  After that, the winner is tick_do_timer_cpu and no one else
does the call.  I guess you would want to skip the cmpxchg during early
boot, but can't claim to understand your constraints.

> > Also, if tick_timekeeping_cpu() doesn't think that there is a timekeeping
> > CPU, rcu_sys_is_idle() will always return false.  I think that this is
> > what you want to happen, just checking.
> 
> Ah right but that should be fine. tick_timekeeping_cpu() works for all potential
> timekeepers. Basically it's !tick_nohz_full_cpu(cpu).

OK, so we do have to worry about multiple callers.  Like you said above. ;-)

> > > +			/*
> > > +			 * Stop tick for 1 jiffy. In practice we stay periodic
> > > +			 * but that let us possibly delegate our timekeeping duty
> > > +			 * to stop the tick for real in the future.
> > > +			 */
> > > +			ret = tick_period.tv64;
> > > +		}
> > 
> > Do we need to set tick_do_timer_cpu to cpu?  Or is that handled elsewhere?
> > (If this is the boot-safety feature deleted below, could we please have
> > the comment back here?)
> 
> This is done in the patch that calls ..kick_timekeeping()  from sysidle_exit().
> 
> Do you have another case in mind?

Mostly just that if we did that determination here, we could avoid having
multiple callers into the state machine.

Alternatively, I might be able to use a lock to serialize callers, but
that would result in CPUs spinning needlessly in rcu_sys_is_idle().
Hence my suggestion of a cmpxchg() to elect a timekeeping CPU in this
code.

						Thanx, Paul

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