linux-kernel - Re: [PATCH 7/8] nohz: Evaluate tick dependency once on context switch

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Message-ID: <20150612073650.GP3644@twins.programming.kicks-ass.net>
Date:	Fri, 12 Jun 2015 09:36:50 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux.com>,
	Ingo Molnar <mingo@...nel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 7/8] nohz: Evaluate tick dependency once on context switch

On Thu, Jun 11, 2015 at 07:36:07PM +0200, Frederic Weisbecker wrote:
> +static void tick_nohz_full_update_dependencies(void)
> +{
> +	struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);
> +
> +	if (!posix_cpu_timers_can_stop_tick(current))
> +		ts->tick_needed |= TICK_NEEDED_POSIX_CPU_TIMER;
> +
> +	if (!perf_event_can_stop_tick())
> +		ts->tick_needed |= TICK_NEEDED_PERF_EVENT;
> +
> +	if (!sched_can_stop_tick())
> +		ts->tick_needed |= TICK_NEEDED_SCHED;
>  
>  #ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
>  	/*
> +	 * sched_clock_tick() needs us?
> +	 *
>  	 * TODO: kick full dynticks CPUs when
>  	 * sched_clock_stable is set.
>  	 */
>  	if (!sched_clock_stable()) {
> +		ts->tick_needed |= TICK_NEEDED_CLOCK_UNSTABLE;
>  		/*
>  		 * Don't allow the user to think they can get
>  		 * full NO_HZ with this machine.
>  		 */
>  		WARN_ONCE(tick_nohz_full_running,
>  			  "NO_HZ FULL will not work with unstable sched clock");
>  	}
>  #endif
>  }

Colour me confused; why does this function exist at all? Should not
these bits be managed by those respective subsystems?
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