linux-kernel - Re: [PATCH 2/7] nohz: New tick dependency mask

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Date:	Tue, 24 Nov 2015 11:19:15 -0500
From:	Chris Metcalf <cmetcalf@...hip.com>
To:	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Luiz Capitulino <lcapitulino@...hat.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 2/7] nohz: New tick dependency mask

On 11/13/2015 09:22 AM, Frederic Weisbecker wrote:
> The tick dependency is evaluated on every IRQ. This is a batch of checks
> which determine whether it is safe to stop the tick or not. These checks
> are often split in many details: posix cpu timers, scheduler, sched clock,
> perf events. Each of which are made of smaller details: posix cpu
> timer involves checking process wide timers then thread wide timers. Perf
> involves checking freq events then more per cpu details.
>
> Checking these details asynchronously every time we update the full
> dynticks state bring avoidable overhead and a messy layout.
>
> Lets introduce instead tick dependency masks: one for system wide
> dependency (unstable sched clock), one for CPU wide dependency (sched,
> perf), and task/signal level dependencies. The subsystems are responsible
> of setting and clearing their dependency through a set of APIs that will
> take care of concurrent dependency mask modifications and kick targets
> to restart the relevant CPU tick whenever needed.
>
> This new dependency engine stays beside the old one until all subsystems
> having a tick dependency are converted to it.
>
>
> +void tick_nohz_set_dep_cpu(enum tick_dependency_bit bit, int cpu)
> +{
> +	unsigned long prev;
> +	struct tick_sched *ts;
> +
> +	ts = per_cpu_ptr(&tick_cpu_sched, cpu);
> +
> +	prev = fetch_or(&ts->tick_dependency, BIT_MASK(bit));
> +	if (!prev) {
> +		preempt_disable();
> +		/* Perf needs local kick that is NMI safe */
> +		if (cpu == smp_processor_id()) {
> +			tick_nohz_full_kick();
> +		} else {
> +			/* Remote irq work not NMI-safe */
> +			WARN_ON_ONCE(in_nmi());

Better to say "if (!WARN_ON_ONCE(in_nmi()))" here instead so
we don't actually try to kick if we are in an NMI?

-- 
Chris Metcalf, EZChip Semiconductor
http://www.ezchip.com

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