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Date: Tue, 6 Feb 2018 14:32:27 +0000
From: Valentin Schneider <valentin.schneider@....com>
To: Vincent Guittot <vincent.guittot@...aro.org>, peterz@...radead.org,
mingo@...nel.org, linux-kernel@...r.kernel.org
Cc: morten.rasmussen@...s.arm.com, brendan.jackman@....com,
dietmar.eggemann@....com
Subject: Re: [PATCH 3/3] sched: update blocked load when newly idle
Hi Vincent,
On 02/06/2018 08:32 AM, Vincent Guittot wrote:
> When NEWLY_IDLE load balance is not triggered, we might need to update the
> blocked load anyway. We can kick an ilb so an idle CPU will take care of
> updating blocked load or we can try to update them locally before entering
> idle. In the latter case, we reuse part of the nohz_idle_balance.
>
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
> kernel/sched/fair.c | 102 ++++++++++++++++++++++++++++++++++++++++++----------
> 1 file changed, 84 insertions(+), 18 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 6998528..256defe 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8829,6 +8829,9 @@ update_next_balance(struct sched_domain *sd, unsigned long *next_balance)
> *next_balance = next;
> }
>
> +static bool _nohz_idle_balance(struct rq *this_rq, unsigned int flags, enum cpu_idle_type idle);
> +static void kick_ilb(unsigned int flags);
> +
> /*
> * idle_balance is called by schedule() if this_cpu is about to become
> * idle. Attempts to pull tasks from other CPUs.
> @@ -8863,12 +8866,26 @@ static int idle_balance(struct rq *this_rq, struct rq_flags *rf)
>
> if (this_rq->avg_idle < sysctl_sched_migration_cost ||
> !this_rq->rd->overload) {
> + unsigned long has_blocked = READ_ONCE(nohz.has_blocked);
> + unsigned long next = READ_ONCE(nohz.next_blocked);
Ditto on 'next' - there's next_balance referenced in here so it'd be nice to make clear which is which.
> +
> rcu_read_lock();
> sd = rcu_dereference_check_sched_domain(this_rq->sd);
> if (sd)
> update_next_balance(sd, &next_balance);
> rcu_read_unlock();
>
> + /*
> + * Update blocked idle load if it has not been done for a
> + * while. Try to do it locally before entering idle but kick a
> + * ilb if it takes too much time and/or might delay next local
> + * wake up
> + */
> + if (has_blocked && time_after_eq(jiffies, next) &&
> + (this_rq->avg_idle < sysctl_sched_migration_cost ||
> + !_nohz_idle_balance(this_rq, NOHZ_STATS_KICK, CPU_NEWLY_IDLE)))
"this_rq->avg_idle < sysctl_sched_migration_cost" is used twice now, how about storing it in an "idle_too_short" variable ?
> + kick_ilb(NOHZ_STATS_KICK);
> +
> goto out;
> }
>
> @@ -9393,30 +9410,24 @@ static void rebalance_domains(struct rq *rq, enum cpu_idle_type idle)
>
> #ifdef CONFIG_NO_HZ_COMMON
> /*
> - * In CONFIG_NO_HZ_COMMON case, the idle balance kickee will do the
> - * rebalancing for all the cpus for whom scheduler ticks are stopped.
> + * Internal function that runs load balance for all idle cpus. The load balance
> + * can be a simple update of blocked load or a complete load balance with
> + * tasks movement depending of flags.
> + * For newly idle mode, we abort the loop if it takes too much time and return
> + * false to notify that the loop has not be completed and a ilb should be kick.
> */
> -static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
> +static bool _nohz_idle_balance(struct rq *this_rq, unsigned int flags, enum cpu_idle_type idle)
> {
> /* Earliest time when we have to do rebalance again */
> unsigned long now = jiffies;
> unsigned long next_balance = now + 60*HZ;
> - unsigned long next_stats = now + msecs_to_jiffies(LOAD_AVG_PERIOD);
> + bool has_blocked_load = false;
> int update_next_balance = 0;
> int this_cpu = this_rq->cpu;
> - unsigned int flags;
> int balance_cpu;
> + int ret = false;
> struct rq *rq;
> -
> - if (!(atomic_read(nohz_flags(this_cpu)) & NOHZ_KICK_MASK))
> - return false;
> -
> - if (idle != CPU_IDLE) {
> - atomic_andnot(NOHZ_KICK_MASK, nohz_flags(this_cpu));
> - return false;
> - }
> -
> - flags = atomic_fetch_andnot(NOHZ_KICK_MASK, nohz_flags(this_cpu));
> + u64 curr_cost = 0;
>
> SCHED_WARN_ON((flags & NOHZ_KICK_MASK) == NOHZ_BALANCE_KICK);
>
> @@ -9431,6 +9442,10 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
> WRITE_ONCE(nohz.has_blocked, 0);
>
> for_each_cpu(balance_cpu, nohz.idle_cpus_mask) {
> + u64 t0, domain_cost;
> +
> + t0 = sched_clock_cpu(this_cpu);
> +
> if (balance_cpu == this_cpu || !idle_cpu(balance_cpu))
> continue;
>
> @@ -9444,6 +9459,16 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
> 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);
>
> update_blocked_averages(rq->cpu);
> @@ -9456,10 +9481,10 @@ 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);
> @@ -9469,15 +9494,27 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
> next_balance = rq->next_balance;
> update_next_balance = 1;
> }
> +
> + domain_cost = sched_clock_cpu(this_cpu) - t0;
> + curr_cost += domain_cost;
> +
> + }
> +
> + /* 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;
> }
>
> - update_blocked_averages(this_cpu);
> if (flags & NOHZ_BALANCE_KICK)
> rebalance_domains(this_rq, CPU_IDLE);
>
> WRITE_ONCE(nohz.next_blocked,
> now + msecs_to_jiffies(LOAD_AVG_PERIOD));
>
> + /* The full idle balance loop has been done */
> + ret = true;
> +
> abort:
> /* There is still blocked load, enable periodic update */
> if (has_blocked_load)
> @@ -9491,6 +9528,35 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
> if (likely(update_next_balance))
> nohz.next_balance = next_balance;
>
> + return ret;
> +}
> +
> +/*
> + * In CONFIG_NO_HZ_COMMON case, the idle balance kickee will do the
> + * rebalancing for all the cpus for whom scheduler ticks are stopped.
> + */
> +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;
> +
> + if (idle != CPU_IDLE) {
> + atomic_andnot(NOHZ_KICK_MASK, nohz_flags(this_cpu));
> + return false;
> + }
> +
> + /*
> + * barrier, pairs with nohz_balance_enter_idle(), ensures ...
> + */
> + flags = atomic_fetch_andnot(NOHZ_KICK_MASK, nohz_flags(this_cpu));
> + if (!(flags & NOHZ_KICK_MASK))
> + return false;
> +
> + _nohz_idle_balance(this_rq, flags, idle);
> +
> return true;
> }
> #else
>
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