linux-kernel - Re: [PATCH 2/3] context_tracking: Inherit TIF_NOHZ through forks instead of context switches

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Message-ID: <20150402180803.GA20608@redhat.com>
Date:	Thu, 2 Apr 2015 20:08:03 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Dave Jones <davej@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 2/3] context_tracking: Inherit TIF_NOHZ through forks
	instead of context switches

On 04/02, Frederic Weisbecker wrote:
>
> +void context_tracking_cpu_set(int cpu)
>  {
> -	clear_tsk_thread_flag(prev, TIF_NOHZ);
> -	set_tsk_thread_flag(next, TIF_NOHZ);
> +	static bool initialized = false;
> +	struct task_struct *p, *t;
> +	unsigned long flags;
> +
> +	if (!per_cpu(context_tracking.active, cpu)) {
> +		per_cpu(context_tracking.active, cpu) = true;
> +		static_key_slow_inc(&context_tracking_enabled);
> +	}
> +
> +	if (initialized)
> +		return;
> +
> +	set_tsk_thread_flag(&init_task, TIF_NOHZ);
> +
> +	/*
> +	 * There shouldn't be any thread at this early boot stage
> +	 * but the scheduler is ready to host any. So lets walk
> +	 * the tasklist  just in case. tasklist_lock isn't necessary
> +	 * either that early but take it for correctness checkers.
> +	 */
> +	read_lock_irqsave(&tasklist_lock, flags);
> +	for_each_process_thread(p, t)
> +		set_tsk_thread_flag(t, TIF_NOHZ);
> +	read_unlock_irqrestore(&tasklist_lock, flags);
> +
> +	initialized = true;
>  }

Agreed, but _irqsave is not needed. read_lock(tasklist) should work
just fine.

Any reason 3/3 comes as a separate change? I won't argue, just curious.

Oleg.

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