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From: Vincent Guittot <vincent.guittot@linaro.org>
Date: Wed, 6 Mar 2024 17:17:28 +0100
Message-ID: <CAKfTPtCNnTVFdf85sRdJxPhdNZxc8qKcnHL3sznjg-KBVn8Bew@mail.gmail.com>
Subject: Re: [PATCH 6/9] sched/balancing: Update run_rebalance_domains() comments
To: Valentin Schneider <vschneid@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>, linux-kernel@vger.kernel.org, 
	Shrikanth Hegde <sshegde@linux.ibm.com>, Peter Zijlstra <peterz@infradead.org>, 
	Dietmar Eggemann <dietmar.eggemann@arm.com>, Linus Torvalds <torvalds@linux-foundation.org>
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On Tue, 5 Mar 2024 at 11:50, Valentin Schneider <vschneid@redhat.com> wrote:
>
> On 04/03/24 10:48, Ingo Molnar wrote:
> > The first sentence of the comment explaining run_rebalance_domains()
> > is historic and not true anymore:
> >
> >     * run_rebalance_domains is triggered when needed from the scheduler tick.
> >
> > ... contradicted/modified by the second sentence:
> >
> >     * Also triggered for NOHZ idle balancing (with NOHZ_BALANCE_KICK set).
> >
> > Avoid that kind of confusion straight away and explain from what
> > places sched_balance_softirq() is triggered.
> >
> > Signed-off-by: Ingo Molnar <mingo@kernel.org>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Vincent Guittot <vincent.guittot@linaro.org>
> > Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> > Cc: Linus Torvalds <torvalds@linux-foundation.org>
> > Cc: Valentin Schneider <vschneid@redhat.com>
> > ---
> >  kernel/sched/fair.c | 9 +++++++--
> >  1 file changed, 7 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 4c46bffb6a7a..18b7d2999cff 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -12409,8 +12409,13 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
> >  }
> >
> >  /*
> > - * run_rebalance_domains is triggered when needed from the scheduler tick.
> > - * Also triggered for NOHZ idle balancing (with NOHZ_BALANCE_KICK set).
> > + * The run_rebalance_domains() softirq handler is triggered via SCHED_SOFTIRQ
> > + * from two places:
> > + *
> > + *  - the scheduler_tick(),
> > + *
> > + *  - from the SMP cross-call function nohz_csd_func(),
> > + *    used by NOHZ idle balancing (with NOHZ_BALANCE_KICK set).
>
> Bit of a nit but the CSD is also triggered via the scheduler_tick():
>
>   scheduler_tick()
>   `\
>     trigger_load_balance()
>     `\
>       raise_softirq(SCHED_SOFTIRQ)
>
>   scheduler_tick()
>   `\
>     trigger_load_balance()
>     `\
>       nohz_balance_kick()
>       `\
>         kick_ilb()
>         `\
>           smp_call_function_single_async(ilb_cpu, &cpu_rq(ilb_cpu)->nohz_csd);
>
> I got to the below which is still somewhat confusing, thoughts?
>


> """
> The run_rebalance_domains() softirq handler is triggered via SCHED_SOFTIRQ
> from two places:
>
> - directly from trigger_load_balance() in scheduler_tick(), for periodic
>   load balance
>
> - indirectly from kick_ilb() (invoked down the scheduler_tick() too), which
>   issues an SMP cross-call to nohz_csd_func() which will itself raise the
>   softirq, for NOHZ idle balancing.

I'm not sure that we should provide too many details of the path as
this might change in the future. What about the below ?

 - directly from the local scheduler_tick() for periodic load balance

- indirectly from a remote scheduler_tick() for NOHZ idle balancing
through the SMP cross-call nohz_csd_func()

> """
>
> >   */
> >  static __latent_entropy void run_rebalance_domains(struct softirq_action *h)
> >  {
> > --
> > 2.40.1
>