Re: [patch 2/2] nohz: try to avoid IPI when setting tick dependency for task

[Frederic Weisbecker]

On Tue, Aug 25, 2020 at 03:41:49PM -0300, Marcelo Tosatti wrote:
> When enabling per-CPU posix timers, an IPI to nohz_full CPUs might be
> performed (to re-read the dependencies and possibly not re-enter
> nohz_full on a given CPU).
> 
> A common case is for applications that run on nohz_full= CPUs
> to not use POSIX timers (eg DPDK).
> 
> This patch optimizes tick_nohz_dep_set_task to avoid kicking
> all nohz_full= CPUs in case the task allowed mask does not
> intersect with nohz_full= CPU mask,
> when going through tick_nohz_dep_set_task.
> 
> This reduces interruptions to nohz_full= CPUs.
> 
> ---
>  kernel/time/tick-sched.c |    9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> Index: linux-2.6/kernel/time/tick-sched.c
> ===================================================================
> --- linux-2.6.orig/kernel/time/tick-sched.c
> +++ linux-2.6/kernel/time/tick-sched.c
> @@ -383,11 +383,16 @@ void tick_nohz_dep_set_task(struct task_
>  			tick_nohz_full_kick();
>  			preempt_enable();
>  		} else {
> +			unsigned long flags;
> +
>  			/*
>  			 * Some future tick_nohz_full_kick_task()
> -			 * should optimize this.
> +			 * should further optimize this.
>  			 */
> -			tick_nohz_full_kick_all();
> +			raw_spin_lock_irqsave(&tsk->pi_lock, flags);
> +			if (cpumask_intersects(&tsk->cpus_mask, tick_nohz_full_mask))
> +				tick_nohz_full_kick_all();
> +			raw_spin_unlock_irqrestore(&tsk->pi_lock, flags);
>  		}
>  	}
>  }
> 
> 

Not long ago, Peterz suggested that we simply do:

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index f0199a4ba1ad..42ce8e458013 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -357,17 +357,26 @@ EXPORT_SYMBOL_GPL(tick_nohz_dep_clear_cpu);
 void tick_nohz_dep_set_task(struct task_struct *tsk, enum tick_dep_bits bit)
 {
 	if (!atomic_fetch_or(BIT(bit), &tsk->tick_dep_mask)) {
-		if (tsk == current) {
-			preempt_disable();
-			tick_nohz_full_kick();
-			preempt_enable();
-		} else {
-			/*
-			 * Some future tick_nohz_full_kick_task()
-			 * should optimize this.
-			 */
-			tick_nohz_full_kick_all();
-		}
+		int cpu = task_cpu(tsk);
+
+		/*
+		 * If the task concurrently migrates to another cpu,
+		 * we guarantee it sees the new tick dependency upon
+		 * schedule.
+		 *
+		 * set_task_cpu(p, cpu);
+		 *   STORE p->cpu = @cpu
+		 * __schedule() (switch to task 'p')
+		 *   LOCK rq->lock
+		 *   smp_mb__after_spin_lock()		STORE p->tick_dep_mask
+		 *   tick_nohz_task_switch()		smp_mb() (atomic_fetch_or())
+		 *      LOAD p->tick_dep_mask		LOAD p->cpu
+		 */
+
+		preempt_disable();
+		if (cpu_online(cpu))
+			tick_nohz_full_kick_cpu(cpu);
+		preempt_enable();
 	}
 }
 EXPORT_SYMBOL_GPL(tick_nohz_dep_set_task);