Re: [RFC PATCH v6] sched/fair: select idle cpu from idle cpumask for task wakeup

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Dec 08, 2020 at 09:49:57AM +0800, Aubrey Li wrote:
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index c4da7e17b906..b8af602dea79 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3999,6 +3999,7 @@ void scheduler_tick(void)
>  	rq_lock(rq, &rf);
>  
>  	update_rq_clock(rq);
> +	update_idle_cpumask(rq, false);

Does that really need to be done with rq->lock held?

>  	thermal_pressure = arch_scale_thermal_pressure(cpu_of(rq));
>  	update_thermal_load_avg(rq_clock_thermal(rq), rq, thermal_pressure);

> @@ -6808,6 +6813,51 @@ balance_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>  }
>  #endif /* CONFIG_SMP */
>  
> +/*
> + * Update cpu idle state and record this information
> + * in sd_llc_shared->idle_cpus_span.
> + */
> +void update_idle_cpumask(struct rq *rq, bool set_idle)
> +{
> +	struct sched_domain *sd;
> +	int cpu = cpu_of(rq);
> +	int idle_state;
> +
> +	/*
> +	 * If called from scheduler tick, only update
> +	 * idle cpumask if the CPU is busy, as idle
> +	 * cpumask is also updated on idle entry.
> +	 *
> +	 */
> +	if (!set_idle && idle_cpu(cpu))
> +		return;

scheduler_tick() already calls idle_cpu() when SMP.

> +	/*
> +	 * Also set SCHED_IDLE cpu in idle cpumask to
> +	 * allow SCHED_IDLE cpu as a wakeup target
> +	 */
> +	idle_state = set_idle || sched_idle_cpu(cpu);
> +	/*
> +	 * No need to update idle cpumask if the state
> +	 * does not change.
> +	 */
> +	if (rq->last_idle_state == idle_state)
> +		return;
> +
> +	rcu_read_lock();

This is called with IRQs disabled, surely we can forgo rcu_read_lock()
here.

> +	sd = rcu_dereference(per_cpu(sd_llc, cpu));
> +	if (!sd || !sd->shared)
> +		goto unlock;

I don't think !sd->shared is possible here.

> +	if (idle_state)
> +		cpumask_set_cpu(cpu, sds_idle_cpus(sd->shared));
> +	else
> +		cpumask_clear_cpu(cpu, sds_idle_cpus(sd->shared));
> +
> +	rq->last_idle_state = idle_state;
> +unlock:
> +	rcu_read_unlock();
> +}
> +
>  static unsigned long wakeup_gran(struct sched_entity *se)
>  {
>  	unsigned long gran = sysctl_sched_wakeup_granularity;
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index f324dc36fc43..f995660edf2b 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -156,6 +156,11 @@ static void cpuidle_idle_call(void)
>  		return;
>  	}
>  
> +	/*
> +	 * The CPU is about to go idle, set it in idle cpumask
> +	 * to be a wake up target.
> +	 */
> +	update_idle_cpumask(this_rq(), true);

This should be in do_idle(), right around arch_cpu_idle_enter().

>  	/*
>  	 * The RCU framework needs to be told that we are entering an idle
>  	 * section, so no more rcu read side critical sections and one more
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 8d1ca65db3b0..db460b20217a 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1004,6 +1004,7 @@ struct rq {
>  	/* This is used to determine avg_idle's max value */
>  	u64			max_idle_balance_cost;
>  #endif /* CONFIG_SMP */
> +	unsigned char		last_idle_state;

All of that is pointless for UP. Also, is this the best location?