Re: [PATCH] sched/fair: Fix nohz.next_balance update

[Peng Liu]

On Tue, May 05, 2020 at 04:27:11PM +0200, Vincent Guittot wrote:
> Le mardi 05 mai 2020 à 21:40:56 (+0800), Peng Liu a écrit :
> > On Mon, May 04, 2020 at 05:17:11PM +0200, Vincent Guittot wrote:
> > > On Sun, 3 May 2020 at 10:34, Peng Liu <iwtbavbm@xxxxxxxxx> wrote:
> > > >

[...]

> > Yes, you're right. When need_resched() returns true, things become out
> > of expectation. We haven't really got the earliest next_balance, abort
> > the update immediately and let the successor to help. Doubtless this
> > will incur some overhead due to the repeating work.
> 
> There should not be some repeating works because CPUs and sched_domain, which
> have already been balanced, will not be rebalanced until the next load balance
> interval.
> 
> Futhermore, there is in fact still work to do bcause not all the idle CPUs got
> a chance to pull work
> 
> >
> > 
> > About the "tick is not stopped when entering idle" case, defer the
> > update to nohz_balance_enter_idle() would be a choice too.
> >
> > 
> > Of course, only update nohz.next_balance in rebalance_domains() is the
> > simpliest way, but as @Valentin put, too many write to it may incur
> > unnecessary overhead. If we can gather the earliest next_balance in
> 
> This is not really possible because we have to move it to the next interval.
> 
> > advance, then a single write is considered to be better.
> > 
> > By the way, remove the redundant check in nohz_idle_balance().
> > 
> > FWIW, how about the below?
> 
> Your proposal below looks quite complex. IMO, one solution would be to move the
> update of nohz.next_balance before calling rebalance_domains(this_rq, CPU_IDLE)
> so you are back to the previous behavior.
> 
> The only difference is that in case of an break because of need_resched, it
> doesn't update nohz.next_balance. But on the other hand, we haven't yet
> finished run rebalance_domains for all CPUs and some load_balance are still
> pending. In fact, this will be done during next tick by an idle CPU.
> 
> So I would be in favor of something as simple as :
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 04098d678f3b..e028bc1c4744 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10457,6 +10457,14 @@ static bool _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
>                 }
>         }
> 
> +       /*
> +        * next_balance will be updated only when there is a need.
> +        * When the CPU is attached to null domain for ex, it will not be
> +        * updated.
> +        */
> +       if (likely(update_next_balance))
> +               nohz.next_balance = next_balance;
> +
>         /* Newly idle CPU doesn't need an update */
>         if (idle != CPU_NEWLY_IDLE) {
>                 update_blocked_averages(this_cpu);
> @@ -10477,14 +10485,6 @@ static bool _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
>         if (has_blocked_load)
>                 WRITE_ONCE(nohz.has_blocked, 1);
> 
> -       /*
> -        * next_balance will be updated only when there is a need.
> -        * When the CPU is attached to null domain for ex, it will not be
> -        * updated.
> -        */
> -       if (likely(update_next_balance))
> -               nohz.next_balance = next_balance;
> -
>         return ret;
>  }
> 

Indeed, simple and straightforward, it's better.

> > ***********************************************
> > * Below code is !!!ENTIRELY UNTESTED!!!, just *

[...]

> > @@ -10354,9 +10350,7 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
> >  {
> >  	int this_cpu = this_rq->cpu;
> >  	unsigned int flags;
> > -
> > -	if (!(atomic_read(nohz_flags(this_cpu)) & NOHZ_KICK_MASK))
> > -		return false;
> 
> why did you remove this ?
> 

It seems that below 'if' do the same thing, isn't?

/* could be _relaxed() */
flags = atomic_fetch_andnot(NOHZ_KICK_MASK, nohz_flags(this_cpu));
if (!(flags & NOHZ_KICK_MASK))
        return false;

> > +	bool done;
> > 
> >  	if (idle != CPU_IDLE) {
> >  		atomic_andnot(NOHZ_KICK_MASK, nohz_flags(this_cpu));
> > @@ -10368,9 +10362,16 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
> >  	if (!(flags & NOHZ_KICK_MASK))
> >  		return false;
> > 

[...]

> >  static void nohz_newidle_balance(struct rq *this_rq)