Re: [patch 1/2] nohz: only wakeup a single target cpu when kicking a task

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Oct 07, 2020 at 03:01:52PM -0300, Marcelo Tosatti wrote:
> When adding a tick dependency to a task, its necessary to
> wakeup the CPU where the task resides to reevaluate tick
> dependencies on that CPU.
> 
> However the current code wakes up all nohz_full CPUs, which 
> is unnecessary.
> 
> Switch to waking up a single CPU, by using ordering of writes
> to task->cpu and task->tick_dep_mask.
> 
> From: Frederic Weisbecker <frederic@kernel.org>
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> 
> 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
> @@ -274,6 +274,31 @@ void tick_nohz_full_kick_cpu(int cpu)
>  	irq_work_queue_on(&per_cpu(nohz_full_kick_work, cpu), cpu);
>  }
>  
> +static void tick_nohz_kick_task(struct task_struct *tsk)
> +{
> +	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();
> +}

So we need to kick the CPU unconditionally, or only when the task is
actually running? AFAICT we only care about current->tick_dep_mask.