linux-kernel - Re: [PATCH v2 1/2] sched/fair: Add NOHZ balancer flag for nohz.next

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Date:   Tue, 10 Aug 2021 15:33:36 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>
Subject: Re: [PATCH v2 1/2] sched/fair: Add NOHZ balancer flag for
 nohz.next_balance updates

On Mon, 19 Jul 2021 at 12:31, Valentin Schneider
<valentin.schneider@....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@...aro.org>
> Signed-off-by: Valentin Schneider <valentin.schneider@....com>
> ---
>  kernel/sched/fair.c  | 9 ++++++---
>  kernel/sched/sched.h | 8 +++++++-
>  2 files changed, 13 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 11d22943753f..5c88698c3664 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10506,7 +10506,8 @@ 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
> @@ -10528,13 +10529,15 @@ 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,

You forgot to skip the update of nohz.next_blocked if NOHZ_STATS_KICK
is not set:

                 WRITE_ONCE(nohz.next_blocked,
                                  now + msecs_to_jiffies(LOAD_AVG_PERIOD));


> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 9a1c6aeb9165..b0f38b5d2550 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2695,12 +2695,18 @@ 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
>