Messages in this thread | | | Date | Tue, 17 Dec 2013 15:40:23 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 10/13] nohz: Hand over timekeeping duty on cpu offlining |
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On Tue, Dec 17, 2013 at 11:51:29PM +0100, Frederic Weisbecker wrote: > When there are full dynticks CPUs around and the timekeeper goes > offline, we have to hand over the timekeeping duty to another potential > timekeeper. > > The default timekeeper (aka CPU 0) is the perfect candidate for this > task since it can't be offlined itself. > > So lets send an IPI to the default timekeeping when the current > timekeeper goes offline, so that the duty is relayed.
A few comments below.
Thanx, Paul
> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com> > Cc: Thomas Gleixner <tglx@linutronix.de> > Cc: Ingo Molnar <mingo@kernel.org> > Cc: Peter Zijlstra <peterz@infradead.org> > Cc: Steven Rostedt <rostedt@goodmis.org> > Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > Cc: John Stultz <john.stultz@linaro.org> > Cc: Alex Shi <alex.shi@linaro.org> > Cc: Kevin Hilman <khilman@linaro.org> > --- > include/linux/tick.h | 2 ++ > kernel/time/tick-sched.c | 31 +++++++++++++++++++++++++++++++ > 2 files changed, 33 insertions(+) > > diff --git a/include/linux/tick.h b/include/linux/tick.h > index af98d2c..bd3c32e 100644 > --- a/include/linux/tick.h > +++ b/include/linux/tick.h > @@ -218,6 +218,7 @@ extern void tick_nohz_init(void); > extern void __tick_nohz_full_check(void); > extern void tick_nohz_full_kick(void); > extern void tick_nohz_full_kick_all(void); > +extern void tick_nohz_full_kick_timekeeping(void); > extern void __tick_nohz_task_switch(struct task_struct *tsk); > # else > static inline void tick_nohz_init(void) { } > @@ -227,6 +228,7 @@ static inline bool tick_timekeeping_cpu(int cpu) { return true; } > static inline void __tick_nohz_full_check(void) { } > static inline void tick_nohz_full_kick(void) { } > static inline void tick_nohz_full_kick_all(void) { } > +static inline void tick_nohz_full_kick_timekeeping(void) { } > static inline void __tick_nohz_task_switch(struct task_struct *tsk) { } > #endif > > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c > index 527b501..94b6901 100644 > --- a/kernel/time/tick-sched.c > +++ b/kernel/time/tick-sched.c > @@ -217,6 +217,12 @@ static u64 tick_timekeeping_max_deferment(struct tick_sched *ts) > return timekeeping_max_deferment(); > > /* > + * Order tick_do_timer_cpu read against the IPI, pairs with > + * tick_nohz_full_kick_timekeeping() > + */ > + smp_rmb();
If this is the handler for the smp_send_reschedule(), then the above memory barrier is not needed. (See my comment below.)
> + > + /* > * If we are the timekeeper and all full dynticks CPUs are idle, > * then we can finally sleep. > */ > @@ -293,6 +299,22 @@ void tick_nohz_full_kick_all(void) > preempt_enable(); > } > > +/** > + * tick_nohz_full_kick_timekeeping - kick the default timekeeper > + * > + * kick the default timekeeper when a secondary timekeeper goes offline. > + */ > +void tick_nohz_full_kick_timekeeping(void) > +{ > + tick_do_timer_cpu = tick_timekeeping_default_cpu(); > + /* > + * Order tick_do_timer_cpu against the IPI, pairs with > + * tick_timekeeping_max_deferment on irq exit. > + */ > + smp_wmb();
But the IPI is supposed to provide full ordering between the CPU invoking the IPI and the IPI handler, right? I do not believe that you need the above smp_wmb() -- though keeping the comment stating that you are relying on the implicit barrier in IPI would be good.
> + smp_send_reschedule(tick_timekeeping_default_cpu());
Again, smp_send_reschedule()'s IPI hander does not necessarily do anything if there is nothing for the scheduler to do, so any needed actions are taking in the return-from-interrupt code?
> +} > + > /* > * Re-evaluate the need for the tick as we switch the current task. > * It might need the tick due to per task/process properties: > @@ -351,6 +373,15 @@ static int tick_nohz_cpu_down_callback(struct notifier_block *nfb, > if (tick_nohz_full_running && tick_timekeeping_default_cpu() == cpu) > return NOTIFY_BAD; > break; > + > + case CPU_DYING: > + /* > + * Notify default timekeeper if we are giving up > + * timekeeping duty > + */ > + if (tick_nohz_full_running && tick_do_timer_cpu == cpu) > + tick_nohz_full_kick_timekeeping(); > + break; > } > return NOTIFY_OK; > } > -- > 1.8.3.1 >
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