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

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Date:	Thu, 11 Jun 2015 16:46:29 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
CC:	Peter Zijlstra <peterz@...radead.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>
Subject: Re: [PATCH 7/8] nohz: Evaluate tick dependency once on context switch

On 06/11/2015 01:36 PM, 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 every time we update the full dynticks state
> bring avoidable overhead.
> 
> So lets evaluate these dependencies once on context switch. Then the
> further dependency checks will be performed through a single state check.
> 
> This is a first step that can be later optimized by dividing task level
> dependency, CPU level dependency and global dependency and update
> each at the right time.

> +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;
>  

I see this getting kicked from task work and from ipi
context, but does it get kicked on task wakeup, when
we have a second runnable task on a CPU, but we decide
not to preempt the currently running task to switch to
it yet, but we will want to preempt the currently running
task at a later point in time?
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