Re: [patch V4 part 1 03/36] sched: Clean up scheduler_ipi()

Alexandre Chartre
2020-05-06T12:39:34Z

On 5/5/20 3:16 PM, Thomas Gleixner wrote:
> The scheduler IPI has grown weird and wonderful over the years, time
> for spring cleaning.
>
> Move all the non-trivial stuff out of it and into a regular smp function
> call IPI. This then reduces the schedule_ipi() to most of it's former MOP
> glory and ensures to keep the interrupt vector lean and mean.
>
> Aside of that avoiding the full irq_enter() in the x86 IPI implementation
> is incorrect as scheduler_ipi() can be instrumented. To work around that
> scheduler_ipi() had an irq_enter/exit() hack when heavy work was
> pending. This is gone now.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
> kernel/sched/core.c | 64 +++++++++++++++++++++++----------------------------
> kernel/sched/fair.c | 5 +--
> kernel/sched/sched.h | 6 +++-
> 3 files changed, 36 insertions(+), 39 deletions(-)
>
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -219,6 +219,13 @@ void update_rq_clock(struct rq *rq)
> update_rq_clock_task(rq, delta);
> }
>
> +static inline void
> +rq_csd_init(struct rq *rq, call_single_data_t *csd, smp_call_func_t func)
> +{
> + csd->flags = 0;
> + csd->func = func;
> + csd->info = rq;
> +}
>
> #ifdef CONFIG_SCHED_HRTICK
> /*
> @@ -314,16 +321,14 @@ void hrtick_start(struct rq *rq, u64 del
> hrtimer_start(&rq->hrtick_timer, ns_to_ktime(delay),
> HRTIMER_MODE_REL_PINNED_HARD);
> }
> +
> #endif /* CONFIG_SMP */
>
> static void hrtick_rq_init(struct rq *rq)
> {
> #ifdef CONFIG_SMP
> - rq->hrtick_csd.flags = 0;
> - rq->hrtick_csd.func = __hrtick_start;
> - rq->hrtick_csd.info = rq;
> + rq_csd_init(rq, &rq->hrtick_csd, __hrtick_start);
> #endif
> -
> hrtimer_init(&rq->hrtick_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL_HARD);
> rq->hrtick_timer.function = hrtick;
> }
> @@ -650,6 +655,16 @@ static inline bool got_nohz_idle_kick(vo
> return false;
> }
>
> +static void nohz_csd_func(void *info)
> +{
> + struct rq *rq = info;
> +
> + if (got_nohz_idle_kick()) {
> + rq->idle_balance = 1;
> + raise_softirq_irqoff(SCHED_SOFTIRQ);
> + }
> +}
> +
> #else /* CONFIG_NO_HZ_COMMON */
>
> static inline bool got_nohz_idle_kick(void)
> @@ -2292,6 +2307,11 @@ void sched_ttwu_pending(void)
> rq_unlock_irqrestore(rq, &rf);
> }
>
> +static void wake_csd_func(void *info)
> +{
> + sched_ttwu_pending();
> +}
> +
> void scheduler_ipi(void)
> {
> /*
> @@ -2300,34 +2320,6 @@ void scheduler_ipi(void)
> * this IPI.
> */
> preempt_fold_need_resched();
> -
> - if (llist_empty(&this_rq()->wake_list) && !got_nohz_idle_kick())
> - return;
> -
> - /*
> - * Not all reschedule IPI handlers call irq_enter/irq_exit, since
> - * traditionally all their work was done from the interrupt return
> - * path. Now that we actually do some work, we need to make sure
> - * we do call them.
> - *
> - * Some archs already do call them, luckily irq_enter/exit nest
> - * properly.
> - *
> - * Arguably we should visit all archs and update all handlers,
> - * however a fair share of IPIs are still resched only so this would
> - * somewhat pessimize the simple resched case.
> - */
> - irq_enter();
> - sched_ttwu_pending();
> -
> - /*
> - * Check if someone kicked us for doing the nohz idle load balance.
> - */
> - if (unlikely(got_nohz_idle_kick())) {
> - this_rq()->idle_balance = 1;
> - raise_softirq_irqoff(SCHED_SOFTIRQ);
> - }
> - irq_exit();
> }
>
> static void ttwu_queue_remote(struct task_struct *p, int cpu, int wake_flags)
> @@ -2336,9 +2328,9 @@ static void ttwu_queue_remote(struct tas
>
> p->sched_remote_wakeup = !!(wake_flags & WF_MIGRATED);
>
> - if (llist_add(&p->wake_entry, &cpu_rq(cpu)->wake_list)) {
> + if (llist_add(&p->wake_entry, &rq->wake_list)) {
> if (!set_nr_if_polling(rq->idle))
> - smp_send_reschedule(cpu);
> + smp_call_function_single_async(cpu, &rq->wake_csd);
> else
> trace_sched_wake_idle_without_ipi(cpu);
> }
> @@ -6685,12 +6677,16 @@ void __init sched_init(void)
> rq->avg_idle = 2*sysctl_sched_migration_cost;
> rq->max_idle_balance_cost = sysctl_sched_migration_cost;
>
> + rq_csd_init(rq, &rq->wake_csd, wake_csd_func);
> +
> INIT_LIST_HEAD(&rq->cfs_tasks);
>
> rq_attach_root(rq, &def_root_domain);
> #ifdef CONFIG_NO_HZ_COMMON
> rq->last_blocked_load_update_tick = jiffies;
> atomic_set(&rq->nohz_flags, 0);
> +
> + rq_csd_init(rq, &rq->nohz_csd, nohz_csd_func);
> #endif
> #endif /* CONFIG_SMP */
> hrtick_rq_init(rq);
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10009,12 +10009,11 @@ static void kick_ilb(unsigned int flags)
> return;
>
> /*
> - * Use smp_send_reschedule() instead of resched_cpu().
> - * This way we generate a sched IPI on the target CPU which
> + * This way we generate an IPI on the target CPU which
> * is idle. And the softirq performing nohz idle load balance
> * will be run before returning from the IPI.
> */
> - smp_send_reschedule(ilb_cpu);
> + smp_call_function_single_async(ilb_cpu, &cpu_rq(ilb_cpu)->wake_csd);

This should be nohz_csd instead of wake_csd, no? I.e.:

smp_call_function_single_async(ilb_cpu, &cpu_rq(ilb_cpu)->nohz_csd);

With that:

Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>

alex.

> }
>
> /*
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -890,9 +890,10 @@ struct rq {
> #ifdef CONFIG_SMP
> unsigned long last_blocked_load_update_tick;
> unsigned int has_blocked_load;
> + call_single_data_t nohz_csd;
> #endif /* CONFIG_SMP */
> unsigned int nohz_tick_stopped;
> - atomic_t nohz_flags;
> + atomic_t nohz_flags;
> #endif /* CONFIG_NO_HZ_COMMON */
>
> unsigned long nr_load_updates;
> @@ -979,7 +980,7 @@ struct rq {
>
> /* This is used to determine avg_idle's max value */
> u64 max_idle_balance_cost;
> -#endif
> +#endif /* CONFIG_SMP */
>
> #ifdef CONFIG_IRQ_TIME_ACCOUNTING
> u64 prev_irq_time;
> @@ -1021,6 +1022,7 @@ struct rq {
> #endif
>
> #ifdef CONFIG_SMP
> + call_single_data_t wake_csd;
> struct llist_head wake_list;
> #endif
>
>