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

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Date:   Thu, 10 Sep 2020 15:51:00 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org
Subject: Re: [patch 2/2] nohz: try to avoid IPI when setting tick dependency
 for task

On Thu, Sep 03, 2020 at 05:01:53PM +0200, Frederic Weisbecker wrote:
> 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);

This can also be used for the signal case... thanks.