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SubjectRe: [RFC PATCH 5/6] sched/fair: trigger the update of blocked load on newly idle cpu
On Tue, 9 Feb 2021 at 14:09, Valentin Schneider
<valentin.schneider@arm.com> wrote:
>
> On 05/02/21 12:48, Vincent Guittot wrote:
> > Instead of waking up a random and already idle CPU, we can take advantage
> > of this_cpu being about to enter idle to run the ILB and update the
> > blocked load.
> >
> > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> > ---
> > include/linux/sched/nohz.h | 2 ++
> > kernel/sched/fair.c | 11 ++++++++---
> > kernel/sched/idle.c | 6 ++++++
> > 3 files changed, 16 insertions(+), 3 deletions(-)
> >
> > diff --git a/include/linux/sched/nohz.h b/include/linux/sched/nohz.h
> > index 6d67e9a5af6b..74cdc4e87310 100644
> > --- a/include/linux/sched/nohz.h
> > +++ b/include/linux/sched/nohz.h
> > @@ -9,8 +9,10 @@
> > #if defined(CONFIG_SMP) && defined(CONFIG_NO_HZ_COMMON)
> > extern void nohz_balance_enter_idle(int cpu);
> > extern int get_nohz_timer_target(void);
> > +extern void nohz_run_idle_balance(int cpu);
> > #else
> > static inline void nohz_balance_enter_idle(int cpu) { }
> > +static inline void nohz_run_idle_balance(int cpu) { }
> > #endif
> >
> > #ifdef CONFIG_NO_HZ_COMMON
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 935594cd5430..3d2ab28d5736 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -10461,6 +10461,11 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
> > return true;
> > }
> >
> > +void nohz_run_idle_balance(int cpu)
> > +{
> > + nohz_idle_balance(cpu_rq(cpu), CPU_IDLE);
> > +}
> > +
> > static void nohz_newidle_balance(struct rq *this_rq)
> > {
> > int this_cpu = this_rq->cpu;
> > @@ -10482,10 +10487,10 @@ static void nohz_newidle_balance(struct rq *this_rq)
> > return;
> >
> > /*
> > - * Blocked load of idle CPUs need to be updated.
> > - * Kick an ILB to update statistics.
> > + * Set the need to trigger ILB in order to update blocked load
> > + * before entering idle state.
> > */
> > - kick_ilb(NOHZ_STATS_KICK);
> > + this_rq->nohz_idle_balance = NOHZ_STATS_KICK;
> > }
> >
> > #else /* !CONFIG_NO_HZ_COMMON */
> > diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> > index 305727ea0677..52a4e9ce2f9b 100644
> > --- a/kernel/sched/idle.c
> > +++ b/kernel/sched/idle.c
> > @@ -261,6 +261,12 @@ static void cpuidle_idle_call(void)
> > static void do_idle(void)
> > {
> > int cpu = smp_processor_id();
> > +
> > + /*
> > + * Check if we need to update some blocked load
> > + */
> > + nohz_run_idle_balance(cpu);
> > +
>
> What do we gain from doing this here vs having a stats update in
> newidle_balance()?

As mentioned by Joel, newidle_balance is called in the schedule
context with preempt and irq off which prevent any local activity
like irq/timer. Whereas in this new place, we have the same condition
as during ILB with only preemptoff and _nohz_idle_balance() regularly
checks if it has to abort because something has been scheduled on the
cpu.


>
> The current approach is to have a combined load_balance() + blocked load
> update during newidle, and I get that this can take too long. But then,
> we could still have what you're adding to do_idle() in the tail of
> newidle_balance() itself, no? i.e.
>
> newidle_balance()
> ...
> for_each_domain(this_cpu, sd) {
> ...
> pulled_task = load_balance(...);
> ...
> }
> ...
> if (!pulled_task && !this_rq->nr_running) {
> this_rq->nohz_idle_balance = NOHZ_STATS_KICK;
> _nohz_idle_balance();
> }
>
> or somesuch.
>
> > /*
> > * If the arch has a polling bit, we maintain an invariant:
> > *
> > --
> > 2.17.1

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