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    SubjectRe: [RFC PATCH 2/5] sched: Add NOHZ_STATS_KICK
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    On 01/24/2018 09:25 AM, Vincent Guittot wrote:
    > Hi,
    >
    > Le Thursday 18 Jan 2018 à 10:38:07 (+0000), Morten Rasmussen a écrit :
    >> On Mon, Jan 15, 2018 at 09:26:09AM +0100, Vincent Guittot wrote:
    >>> Le Wednesday 03 Jan 2018 à 10:16:00 (+0100), Vincent Guittot a écrit :

    [...]

    >>>
    >>> Hi Peter,
    >>>
    >>> With the patch below on top of your branch, the blocked loads are updated and
    >>> decayed regularly. The main differences are:
    >>> - It doesn't use a timer to trig ilb but the tick and when a cpu becomes idle.
    >>> The main drawback of this solution is that the load is blocked when the
    >>> system is fully idle with the advantage of not waking up a fully idle
    >>> system. We have to wait for the next tick or newly idle event for updating
    >>> blocked load when the system leaves idle stat which can be up to a tick long.
    >>> If this is too long, we can check for kicking ilb when task wakes up so the
    >>> blocked load will be updated as soon as the system leaves idle state.
    >>> The main advantage is that we don't wake up a fully idle system every 32ms to
    >>> update blocked load that will be not used.
    >>> - I'm working on one more improvement to use nohz_idle_balance in the newly
    >>> idle case when the system is not overloaded and
    >>> (this_rq->avg_idle > sysctl_sched_migration_cost). In this case, we can try to
    >>> use nohz_idle_balance with NOHZ_STATS_KICK and abort as soon as it exceed
    >>> this_rq->avg_idle. This will remove some calls to kick_ilb and some wake up
    >>> of an idle cpus.
    >>
    >> This sound like what I meant in my other reply :-)
    >>
    >> It seems pointless to have a timer to update PELT if the system is
    >> completely idle, and when it isn't we can piggy back other events to
    >> make the updates happen.
    >
    > The patch below implements what has been described above. It calls part of
    > nohz_idle_balance when a cpu becomes idle and kick a ilb if it takes too much
    > time. This removes part of ilb that are kicked on an idle cpu for updating
    > the blocked load but the ratio really depends on when the tick happens compared
    > to a cpu becoming idle and the 32ms boundary. I have an additionnal patch that
    > enables to update the blocked loads when a cpu becomes idle 1 period before
    > kicking an ilb and there is far less ilb because we give more chance to the
    > newly idle case (time_after is replaced by time_after_eq in idle_balance()).
    >
    > The patch also uses a function cfs_rq_has_blocked, which only checks the
    > util/load_avg, instead of the cfs_rq_is_decayed which check *_sum too. This
    > reduce significantly the number of update of blocked load. the *_avg will be
    > fully decayed in around 300~400ms but it's far longer for the *_sum which have
    > a higher resolution and we can easily reach almost seconds. But only the *_avg
    > are used to make decision so keeping some blocked *_sum is acceptable.
    >
    > ---
    > kernel/sched/fair.c | 121 +++++++++++++++++++++++++++++++++++++++-------------
    > 1 file changed, 92 insertions(+), 29 deletions(-)
    >
    > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    > index 898785d..ed90303 100644
    > --- a/kernel/sched/fair.c
    > +++ b/kernel/sched/fair.c
    > @@ -7356,6 +7356,17 @@ static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq)
    > return true;
    > }
    >
    > +static inline bool cfs_rq_has_blocked(struct cfs_rq *cfs_rq)
    > +{
    > + if (cfs_rq->avg.load_avg)
    > + return true;
    > +
    > + if (cfs_rq->avg.util_avg)
    > + return true;
    > +
    > + return false;
    > +}
    > +

    Can we not change cfs_rq_is_decayed() to use avg.foo_avg instead of
    avg.foo_sum ?

    > #ifdef CONFIG_FAIR_GROUP_SCHED
    >
    > static void update_blocked_averages(int cpu)
    > @@ -7393,7 +7404,9 @@ static void update_blocked_averages(int cpu)
    > */
    > if (cfs_rq_is_decayed(cfs_rq))
    > list_del_leaf_cfs_rq(cfs_rq);
    > - else
    > +
    > + /* Don't need periodic decay once load/util_avg are null */
    > + if (cfs_rq_has_blocked(cfs_rq))
    > done = false;
    > }
    >
    > @@ -7463,7 +7476,7 @@ static inline void update_blocked_averages(int cpu)
    > update_cfs_rq_load_avg(cfs_rq_clock_task(cfs_rq), cfs_rq);
    > #ifdef CONFIG_NO_HZ_COMMON
    > rq->last_blocked_load_update_tick = jiffies;
    > - if (cfs_rq_is_decayed(cfs_rq))
    > + if (cfs_rq_has_blocked(cfs_rq))

    Schouldn't this be !cfs_rq_has_blocked(cfs_rq) ?

    > rq->has_blocked_load = 0;
    > #endif
    > rq_unlock_irqrestore(rq, &rf);

    [...]

    > @@ -9438,7 +9451,17 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
    > */
    > if (need_resched()) {
    > has_blocked_load = true;
    > - break;
    > + goto abort;
    > + }
    > +
    > + /*
    > + * If the update is done while CPU becomes idle, we abort
    > + * the update when its cost is higher than the average idle
    > + * time in orde to not delay a possible wake up.
    > + */
    > + if (idle == CPU_NEWLY_IDLE && this_rq->avg_idle < curr_cost) {
    > + has_blocked_load = true;
    > + goto abort;
    > }
    >
    > rq = cpu_rq(balance_cpu);
    > @@ -9453,10 +9476,10 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
    > if (time_after_eq(jiffies, rq->next_balance)) {
    > struct rq_flags rf;
    >
    > - rq_lock_irq(rq, &rf);
    > + rq_lock_irqsave(rq, &rf);
    > update_rq_clock(rq);
    > cpu_load_update_idle(rq);
    > - rq_unlock_irq(rq, &rf);
    > + rq_unlock_irqrestore(rq, &rf);
    >
    > if (flags & NOHZ_BALANCE_KICK)
    > rebalance_domains(rq, CPU_IDLE);
    > @@ -9466,10 +9489,17 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
    > next_balance = rq->next_balance;
    > update_next_balance = 1;
    > }

    Why do you do this cpu_load_update_idle(rq) even this was called with
    CPU_NEWLY_IDLE? Wouldn't it be sufficient to jump to the curr_cost
    calculation in this case?

    > +
    > + domain_cost = sched_clock_cpu(this_cpu) - t0;
    > + curr_cost += domain_cost;
    > +
    > }
    >
    > - update_blocked_averages(this_cpu);
    > - has_blocked_load |= this_rq->has_blocked_load;
    > + /* Newly idle CPU doesn't need an update */
    > + if (idle != CPU_NEWLY_IDLE) {
    > + update_blocked_averages(this_cpu);
    > + has_blocked_load |= this_rq->has_blocked_load;
    > + }
    >
    > if (flags & NOHZ_BALANCE_KICK)
    > rebalance_domains(this_rq, CPU_IDLE);

    [...]

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