Messages in this thread |  | | From | Vincent Guittot <> | Date | Tue, 9 Feb 2021 14:57:59 +0100 | Subject | Re: [RFC PATCH 5/6] sched/fair: trigger the update of blocked load on newly idle cpu |
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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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