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Date:   Mon, 04 May 2020 01:10:46 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Peng Liu <iwtbavbm@...il.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de
Subject: Re: [PATCH] sched/fair: Fix nohz.next_balance update


Hi,

On 03/05/20 09:34, Peng Liu wrote:
> commit c5afb6a87f23 ("sched/fair: Fix nohz.next_balance update")

I got confused because this has the same topic as your patch, but that's a
genuine commit from 2015. Is this meant to be a "Fixes:" reference?

> During idle load balance, this_cpu(ilb) do load balance for the other
> idle CPUs, also gather the earliest (nohz.)next_balance.
>
> Since commit:
>   'b7031a02ec75 ("sched/fair: Add NOHZ_STATS_KICK")'
>
> We update nohz.next_balance like this:
>
>   _nohz_idle_balance() {
>       for_each_cpu(nohz.idle_cpus_mask) {
>         rebalance_domains() {
>                     update nohz.next_balance <-- compare and update
>         }
>       }
>       rebalance_domains(this_cpu) {
>         update nohz.next_balance <-- compare and update
>       }
>       update nohz.next_balance <-- unconditionally update
>   }
>
> For instance, nohz.idle_cpus_mask spans {cpu2,3,5,8}, and this_cpu is
> cpu5. After the above loop we could gather the earliest *next_balance*
> among {cpu2,3,8}, then rebalance_domains(this_cpu) update
> nohz.next_balance with this_rq->next_balance, but finally overwrite
> nohz.next_balance with the earliest *next_balance* among {cpu2,3,8},
> we may end up with not getting the earliest next_balance.
>

That does look like it, nice catch!

> Since we can gather all the updated rq->next_balance, including this_cpu,
> in _nohz_idle_balance(), it's safe to remove the extra lines in
> rebalance_domains() which are originally intended for this_cpu. And
> finally the updating only happen in _nohz_idle_balance().
>

One added benefit of this is that we get rid of extra writes to
nohz.next_balance, since that special case in rebalance_domains() could be
hit by all NOHZ CPUs, not just the ILB.

With the below comment taken into account:

Reviewed-by: Valentin Schneider <valentin.schneider@....com>

> Signed-off-by: Peng Liu <iwtbavbm@...il.com>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Juri Lelli <juri.lelli@...hat.com>
> Cc: Vincent Guittot <vincent.guittot@...aro.org>
> Cc: Dietmar Eggemann <dietmar.eggemann@....com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Ben Segall <bsegall@...gle.com>
> Cc: Mel Gorman <mgorman@...e.de>
> Cc: Valentin Schneider <valentin.schneider@....com>
> ---
>  kernel/sched/fair.c | 24 ++++++++----------------
>  1 file changed, 8 insertions(+), 16 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 02f323b85b6d..1d0cf33fefad 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9943,22 +9943,8 @@ static void rebalance_domains(struct rq *rq, enum cpu_idle_type idle)
>        * When the cpu is attached to null domain for ex, it will not be
>        * updated.
>        */
> -	if (likely(update_next_balance)) {
> +	if (likely(update_next_balance))
>               rq->next_balance = next_balance;
> -
> -#ifdef CONFIG_NO_HZ_COMMON
> -		/*
> -		 * If this CPU has been elected to perform the nohz idle
> -		 * balance. Other idle CPUs have already rebalanced with
> -		 * nohz_idle_balance() and nohz.next_balance has been
> -		 * updated accordingly. This CPU is now running the idle load
> -		 * balance for itself and we need to update the
> -		 * nohz.next_balance accordingly.
> -		 */
> -		if ((idle == CPU_IDLE) && time_after(nohz.next_balance, rq->next_balance))
> -			nohz.next_balance = rq->next_balance;
> -#endif
> -	}
>  }
>
>  static inline int on_null_domain(struct rq *rq)
> @@ -10321,9 +10307,15 @@ static bool _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
>               has_blocked_load |= this_rq->has_blocked_load;
>       }
>
> -	if (flags & NOHZ_BALANCE_KICK)
> +	if (flags & NOHZ_BALANCE_KICK) {
>               rebalance_domains(this_rq, CPU_IDLE);
>
> +		if (time_after(next_balance, this_rq->next_balance)) {
> +			next_balance = this_rq->next_balance;
> +			update_next_balance = 1;
> +		}
> +	}

To align with what we do for the other NOHZ CPUs, shouldn't this update
be outside of the NOHZ_BALANCE_KICK condition? That way we can update
nohz.next_balance with just NOHZ_STATS_KICK, which IMO is the expected
course of action.

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

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