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

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Date:	Wed, 25 Nov 2015 12:32:33 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Chris Metcalf <cmetcalf@...hip.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	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 Tue, Nov 24, 2015 at 11:19:15AM -0500, Chris Metcalf wrote:
> 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?

Makes sense yeah. I'll fix that.

Thanks.
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