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Message-ID: <CAKfTPtBsfneVVdT5UhtysUGACqPp7YUGKzSTKu4D8UYKwDbykg@mail.gmail.com>
Date: Wed, 29 Apr 2020 10:27:51 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Scott Wood <swood@...hat.com>
Cc: Steven Rostedt <rostedt@...dmis.org>,
Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Rik van Riel <riel@...riel.com>,
Mel Gorman <mgorman@...e.de>,
linux-kernel <linux-kernel@...r.kernel.org>,
linux-rt-users <linux-rt-users@...r.kernel.org>
Subject: Re: [RFC PATCH 1/3] sched/fair: Call newidle_balance() from finish_task_switch()
On Tue, 28 Apr 2020 at 07:02, Scott Wood <swood@...hat.com> wrote:
>
> Thus, newidle_balance() is entered with interrupts enabled, which allows
> (in the next patch) enabling interrupts when the lock is dropped.
The comment fails to explain which changes have been done to
newidle_balance(), for which reasons and what are the impact for CFS
scheduler
>
> Signed-off-by: Scott Wood <swood@...hat.com>
> ---
> kernel/sched/core.c | 7 ++++---
> kernel/sched/fair.c | 45 ++++++++++++++++----------------------------
> kernel/sched/sched.h | 6 ++----
> 3 files changed, 22 insertions(+), 36 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 9a2fbf98fd6f..0294beb8d16c 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3241,6 +3241,10 @@ static struct rq *finish_task_switch(struct task_struct *prev)
> }
>
> tick_nohz_task_switch();
> +
> + if (is_idle_task(current))
> + newidle_balance();
> +
> return rq;
> }
>
> @@ -3919,8 +3923,6 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> rq->nr_running == rq->cfs.h_nr_running)) {
>
> p = pick_next_task_fair(rq, prev, rf);
> - if (unlikely(p == RETRY_TASK))
> - goto restart;
>
> /* Assumes fair_sched_class->next == idle_sched_class */
> if (!p) {
> @@ -3931,7 +3933,6 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> return p;
> }
>
> -restart:
> #ifdef CONFIG_SMP
> /*
> * We must do the balancing pass before put_next_task(), such
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 02f323b85b6d..74c3c5280d6b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6758,8 +6758,6 @@ balance_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> {
> if (rq->nr_running)
> return 1;
> -
> - return newidle_balance(rq, rf) != 0;
Missing return ?
> }
> #endif /* CONFIG_SMP */
>
> @@ -6934,9 +6932,7 @@ pick_next_task_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf
> struct cfs_rq *cfs_rq = &rq->cfs;
> struct sched_entity *se;
> struct task_struct *p;
> - int new_tasks;
>
> -again:
> if (!sched_fair_runnable(rq))
> goto idle;
>
> @@ -7050,19 +7046,6 @@ done: __maybe_unused;
> if (!rf)
> return NULL;
>
> - new_tasks = newidle_balance(rq, rf);
> -
> - /*
> - * Because newidle_balance() releases (and re-acquires) rq->lock, it is
> - * possible for any higher priority task to appear. In that case we
> - * must re-start the pick_next_entity() loop.
> - */
> - if (new_tasks < 0)
> - return RETRY_TASK;
> -
> - if (new_tasks > 0)
> - goto again;
> -
> /*
> * rq is about to be idle, check if we need to update the
> * lost_idle_time of clock_pelt
> @@ -10425,14 +10408,23 @@ static inline void nohz_newidle_balance(struct rq *this_rq) { }
> * 0 - failed, no new tasks
> * > 0 - success, new (fair) tasks present
> */
> -int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
> +int newidle_balance(void)
> {
> unsigned long next_balance = jiffies + HZ;
> - int this_cpu = this_rq->cpu;
> + int this_cpu;
> struct sched_domain *sd;
> + struct rq *this_rq;
> int pulled_task = 0;
> u64 curr_cost = 0;
>
> + preempt_disable();
> + this_rq = this_rq();
> + this_cpu = this_rq->cpu;
> + local_bh_disable();
> + raw_spin_lock_irq(&this_rq->lock);
> +
> + update_rq_clock(this_rq);
> +
> update_misfit_status(NULL, this_rq);
> /*
> * We must set idle_stamp _before_ calling idle_balance(), such that we
> @@ -10444,15 +10436,7 @@ int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
> * Do not pull tasks towards !active CPUs...
> */
> if (!cpu_active(this_cpu))
> - return 0;
> -
> - /*
> - * This is OK, because current is on_cpu, which avoids it being picked
> - * for load-balance and preemption/IRQs are still disabled avoiding
> - * further scheduler activity on it and we're being very careful to
> - * re-start the picking loop.
> - */
> - rq_unpin_lock(this_rq, rf);
> + goto out_unlock;
>
> if (this_rq->avg_idle < sysctl_sched_migration_cost ||
> !READ_ONCE(this_rq->rd->overload)) {
> @@ -10534,7 +10518,10 @@ int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
> if (pulled_task)
> this_rq->idle_stamp = 0;
>
> - rq_repin_lock(this_rq, rf);
> +out_unlock:
> + raw_spin_unlock_irq(&this_rq->lock);
> + local_bh_enable();
> + preempt_enable();
>
> return pulled_task;
> }
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index db3a57675ccf..3d97c51544d7 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1504,13 +1504,13 @@ static inline void unregister_sched_domain_sysctl(void)
> }
> #endif
>
> -extern int newidle_balance(struct rq *this_rq, struct rq_flags *rf);
> +extern int newidle_balance(void);
>
> #else
>
> static inline void sched_ttwu_pending(void) { }
>
> -static inline int newidle_balance(struct rq *this_rq, struct rq_flags *rf) { return 0; }
> +static inline int newidle_balance(void) { return 0; }
>
> #endif /* CONFIG_SMP */
>
> @@ -1742,8 +1742,6 @@ extern const u32 sched_prio_to_wmult[40];
> #define ENQUEUE_MIGRATED 0x00
> #endif
>
> -#define RETRY_TASK ((void *)-1UL)
> -
> struct sched_class {
> const struct sched_class *next;
>
> --
> 2.18.2
>
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