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?
