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Re: [PATCH 2/3] sched: Correctly handle nohz ticks cpu load accounting

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On Fri, Apr 08, 2016 at 03:07:12AM +0200, Frederic Weisbecker wrote:
> +void cpu_load_update_nohz_start(void)
>  {
>  	struct rq *this_rq = this_rq();
> +
> +	/*
> +	 * This is all lockless but should be fine. If weighted_cpuload changes
> +	 * concurrently we'll exit nohz. And cpu_load write can race with
> +	 * cpu_load_update_idle() but both updater would be writing the same.
> +	 */
> +	this_rq->cpu_load[0] = weighted_cpuload(cpu_of(this_rq));
> +}

There is more to this; this also updates ->cpu_load[0] at possibly much
higher frequency than we've done before, while not updating the other
->cpu_load[] members.

Now, I'm not sure we care, but it is a bit odd.

> +/*
> + * Account the tickless load in the end of a nohz frame.
> + */
> +void cpu_load_update_nohz_stop(void)
> +{
>  	unsigned long curr_jiffies = READ_ONCE(jiffies);
> +	struct rq *this_rq = this_rq();
> +	unsigned long load;
>  
>  	if (curr_jiffies == this_rq->last_load_update_tick)
>  		return;
>  
> +	load = weighted_cpuload(cpu_of(this_rq));
>  	raw_spin_lock(&this_rq->lock);
> +	cpu_load_update_nohz(this_rq, curr_jiffies, load);
>  	raw_spin_unlock(&this_rq->lock);
>  }

And this makes us take rq->lock when waking from nohz; a bit
unfortunate. Do we really need this though? Will not a tick be
forthcoming real-soon-now?

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[PATCH 0/3] sched: Fix/improve nohz cpu load updates v2 Frederic Weisbecker <fweisbec@gmail.com> - 2016-04-08 03:10 +0200
  [PATCH 2/3] sched: Correctly handle nohz ticks cpu load accounting Frederic Weisbecker <fweisbec@gmail.com> - 2016-04-08 03:10 +0200
    Re: [PATCH 2/3] sched: Correctly handle nohz ticks cpu load  accounting Peter Zijlstra <peterz@infradead.org> - 2016-04-08 11:50 +0200
      Re: [PATCH 2/3] sched: Correctly handle nohz ticks cpu load  accounting Frederic Weisbecker <fweisbec@gmail.com> - 2016-04-08 15:00 +0200
        Re: [PATCH 2/3] sched: Correctly handle nohz ticks cpu load  accounting Peter Zijlstra <peterz@infradead.org> - 2016-04-08 19:50 +0200
  [PATCH 1/3] sched: Gather cpu load functions under a more conventional namespace Frederic Weisbecker <fweisbec@gmail.com> - 2016-04-08 03:10 +0200
  [PATCH 3/3] sched: Optimize !CONFIG_NO_HZ_COMMON cpu load updates Frederic Weisbecker <fweisbec@gmail.com> - 2016-04-08 03:10 +0200
    Re: [PATCH 3/3] sched: Optimize !CONFIG_NO_HZ_COMMON cpu load updates Peter Zijlstra <peterz@infradead.org> - 2016-04-08 12:50 +0200
      Re: [PATCH 3/3] sched: Optimize !CONFIG_NO_HZ_COMMON cpu load updates Frederic Weisbecker <fweisbec@gmail.com> - 2016-04-08 15:00 +0200
        Re: [PATCH 3/3] sched: Optimize !CONFIG_NO_HZ_COMMON cpu load updates Peter Zijlstra <peterz@infradead.org> - 2016-04-08 19:50 +0200
          Re: [PATCH 3/3] sched: Optimize !CONFIG_NO_HZ_COMMON cpu load updates Frederic Weisbecker <fweisbec@gmail.com> - 2016-04-11 15:20 +0200
            Re: [PATCH 3/3] sched: Optimize !CONFIG_NO_HZ_COMMON cpu load updates Frederic Weisbecker <fweisbec@gmail.com> - 2016-04-11 20:30 +0200
              Re: [PATCH 3/3] sched: Optimize !CONFIG_NO_HZ_COMMON cpu load updates Peter Zijlstra <peterz@infradead.org> - 2016-04-12 16:30 +0200

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