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.
>
> 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?
Aye, I need to think further on how to cope with that...
>
> 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).
>
> > + /*
> > + * 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?
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