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

From: Preeti U Murthy
Date: Wed Jun 17 2015 - 01:59:27 EST

On 06/12/2015 02:16 AM, Rik van Riel wrote:
> 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?

+1. This is not taken care of as far as I can see too.

Preeti U Murthy

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