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

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Date:	Tue, 1 Dec 2015 21:41:09 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Chris Metcalf <cmetcalf@...hip.com>,
	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 Fri, Nov 13, 2015 at 03:22:04PM +0100, 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.

Maybe better explain why we need the per task and per signal thingy?

> +static void trace_tick_dependency(unsigned long dep)
> +{
> +	if (dep & TICK_POSIX_TIMER_MASK) {
> +		trace_tick_stop(0, "posix timers running\n");
> +		return;
> +	}
> +
> +	if (dep & TICK_PERF_EVENTS_MASK) {
> +		trace_tick_stop(0, "perf events running\n");
> +		return;
> +	}
> +
> +	if (dep & TICK_SCHED_MASK) {
> +		trace_tick_stop(0, "more than 1 task in runqueue\n");
> +		return;
> +	}
> +
> +	if (dep & TICK_CLOCK_UNSTABLE_MASK)
> +		trace_tick_stop(0, "unstable sched clock\n");
> +}

I would suggest ditching the strings and using the 

> +static void kick_all_work_fn(struct work_struct *work)
> +{
> +       tick_nohz_full_kick_all();
> +}
> +static DECLARE_WORK(kick_all_work, kick_all_work_fn);
> +
> +void __tick_nohz_set_dep_delayed(enum tick_dependency_bit bit, unsigned long *dep)
> +{
> +	unsigned long prev;
> +
> +	prev = fetch_or(dep, BIT_MASK(bit));
> +	if (!prev) {
> +		/*
> +		* We need the IPIs to be sent from sane process context.

Why ?

> +		* The posix cpu timers are always set with irqs disabled.
> +		*/
> +		schedule_work(&kick_all_work);
> +	}
> +}
> +
> +/*
> + * Set a global tick dependency. Lets do the wide IPI kick asynchronously
> + * for callers with irqs disabled.

This seems to suggest you can call this with IRQs disabled

> + */
> +void tick_nohz_set_dep(enum tick_dependency_bit bit)
> +{
> +	unsigned long prev;
> +
> +	prev = fetch_or(&tick_dependency, BIT_MASK(bit));
> +	if (!prev)
> +		tick_nohz_full_kick_all();

But that function seems implemented using smp_call_function_many() which
cannot be called with IRQs disabled.

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