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 <iwtba...@gmail.com> 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)
