Re: [PATCH v3 1/2] sched/fair: Add NOHZ balancer flag for nohz.next_balance updates

[Vincent Guittot <vincent.guittot@linaro.org>]

On Mon, 23 Aug 2021 at 13:17, Valentin Schneider
<valentin.schneider@arm.com> wrote:
>
> A following patch will trigger NOHZ idle balances as a means to update
> nohz.next_balance. Vincent noted that blocked load updates can have
> non-negligible overhead, which should be avoided if the intent is to only
> update nohz.next_balance.
>
> Add a new NOHZ balance kick flag, NOHZ_NEXT_KICK. Gate NOHZ blocked load
> update by the presence of NOHZ_STATS_KICK - currently all NOHZ balance
> kicks will have the NOHZ_STATS_KICK flag set, so no change in behaviour is
> expected.
>
> Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>

Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>

> ---
>  kernel/sched/fair.c  | 24 ++++++++++++++----------
>  kernel/sched/sched.h |  8 +++++++-
>  2 files changed, 21 insertions(+), 11 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 6cd05f1d77ef..4a91f3027c92 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10342,7 +10342,7 @@ static void nohz_balancer_kick(struct rq *rq)
>                 goto out;
>
>         if (rq->nr_running >= 2) {
> -               flags = NOHZ_KICK_MASK;
> +               flags = NOHZ_STATS_KICK | NOHZ_BALANCE_KICK;
>                 goto out;
>         }
>
> @@ -10356,7 +10356,7 @@ static void nohz_balancer_kick(struct rq *rq)
>                  * on.
>                  */
>                 if (rq->cfs.h_nr_running >= 1 && check_cpu_capacity(rq, sd)) {
> -                       flags = NOHZ_KICK_MASK;
> +                       flags = NOHZ_STATS_KICK | NOHZ_BALANCE_KICK;
>                         goto unlock;
>                 }
>         }
> @@ -10370,7 +10370,7 @@ static void nohz_balancer_kick(struct rq *rq)
>                  */
>                 for_each_cpu_and(i, sched_domain_span(sd), nohz.idle_cpus_mask) {
>                         if (sched_asym_prefer(i, cpu)) {
> -                               flags = NOHZ_KICK_MASK;
> +                               flags = NOHZ_STATS_KICK | NOHZ_BALANCE_KICK;
>                                 goto unlock;
>                         }
>                 }
> @@ -10383,7 +10383,7 @@ static void nohz_balancer_kick(struct rq *rq)
>                  * to run the misfit task on.
>                  */
>                 if (check_misfit_status(rq, sd)) {
> -                       flags = NOHZ_KICK_MASK;
> +                       flags = NOHZ_STATS_KICK | NOHZ_BALANCE_KICK;
>                         goto unlock;
>                 }
>
> @@ -10410,7 +10410,7 @@ static void nohz_balancer_kick(struct rq *rq)
>                  */
>                 nr_busy = atomic_read(&sds->nr_busy_cpus);
>                 if (nr_busy > 1) {
> -                       flags = NOHZ_KICK_MASK;
> +                       flags = NOHZ_STATS_KICK | NOHZ_BALANCE_KICK;
>                         goto unlock;
>                 }
>         }
> @@ -10572,7 +10572,8 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
>          * setting the flag, we are sure to not clear the state and not
>          * check the load of an idle cpu.
>          */
> -       WRITE_ONCE(nohz.has_blocked, 0);
> +       if (flags & NOHZ_STATS_KICK)
> +               WRITE_ONCE(nohz.has_blocked, 0);
>
>         /*
>          * Ensures that if we miss the CPU, we must see the has_blocked
> @@ -10594,13 +10595,15 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
>                  * balancing owner will pick it up.
>                  */
>                 if (need_resched()) {
> -                       has_blocked_load = true;
> +                       if (flags & NOHZ_STATS_KICK)
> +                               has_blocked_load = true;
>                         goto abort;
>                 }
>
>                 rq = cpu_rq(balance_cpu);
>
> -               has_blocked_load |= update_nohz_stats(rq);
> +               if (flags & NOHZ_STATS_KICK)
> +                       has_blocked_load |= update_nohz_stats(rq);
>
>                 /*
>                  * If time for next balance is due,
> @@ -10631,8 +10634,9 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
>         if (likely(update_next_balance))
>                 nohz.next_balance = next_balance;
>
> -       WRITE_ONCE(nohz.next_blocked,
> -               now + msecs_to_jiffies(LOAD_AVG_PERIOD));
> +       if (flags & NOHZ_STATS_KICK)
> +               WRITE_ONCE(nohz.next_blocked,
> +                          now + msecs_to_jiffies(LOAD_AVG_PERIOD));
>
>  abort:
>         /* There is still blocked load, enable periodic update */
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index e7e2bba5b520..30b7bd2ef25d 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2706,12 +2706,18 @@ extern void cfs_bandwidth_usage_dec(void);
>  #define NOHZ_BALANCE_KICK_BIT  0
>  #define NOHZ_STATS_KICK_BIT    1
>  #define NOHZ_NEWILB_KICK_BIT   2
> +#define NOHZ_NEXT_KICK_BIT     3
>
> +/* Run rebalance_domains() */
>  #define NOHZ_BALANCE_KICK      BIT(NOHZ_BALANCE_KICK_BIT)
> +/* Update blocked load */
>  #define NOHZ_STATS_KICK                BIT(NOHZ_STATS_KICK_BIT)
> +/* Update blocked load when entering idle */
>  #define NOHZ_NEWILB_KICK       BIT(NOHZ_NEWILB_KICK_BIT)
> +/* Update nohz.next_balance */
> +#define NOHZ_NEXT_KICK         BIT(NOHZ_NEXT_KICK_BIT)
>
> -#define NOHZ_KICK_MASK (NOHZ_BALANCE_KICK | NOHZ_STATS_KICK)
> +#define NOHZ_KICK_MASK (NOHZ_BALANCE_KICK | NOHZ_STATS_KICK | NOHZ_NEXT_KICK)
>
>  #define nohz_flags(cpu)        (&cpu_rq(cpu)->nohz_flags)
>
> --
> 2.25.1
>