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

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Date:   Thu, 8 Oct 2020 14:54:09 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org,
        Frederic Weisbecker <frederic@...nel.org>,
        Nitesh Narayan Lal <nitesh@...hat.com>,
        Peter Xu <peterx@...hat.com>
Subject: Re: [patch 1/2] nohz: only wakeup a single target cpu when kicking a
 task

On Thu, Oct 08, 2020 at 02:22:56PM +0200, Peter Zijlstra wrote:
> 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@...nel.org>
> > Suggested-by: Peter Zijlstra <peterz@...radead.org>
> > Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> > Signed-off-by: Marcelo Tosatti <mtosatti@...hat.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.

tick is necessary to execute run_posix_cpu_timers, from tick interrupt, 
even if task is not running.