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

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Message-ID: <20201008145940.GG6026@xz-x1>
Date:   Thu, 8 Oct 2020 10:59:40 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Frederic Weisbecker <frederic@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Nitesh Narayan Lal <nitesh@...hat.com>
Subject: Re: [patch 1/2] nohz: only wakeup a single target cpu when kicking a
 task

On Wed, Oct 07, 2020 at 03:01:52PM -0300, Marcelo Tosatti wrote:
> +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();

Pure question: is preempt_disable() required here?  Same question to
tick_nohz_full_kick_all().

> +	if (cpu_online(cpu))
> +		tick_nohz_full_kick_cpu(cpu);
> +	preempt_enable();
> +}

-- 
Peter Xu