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

[Wanpeng Li]

Hi Frederic,
2016-04-13 21:56 GMT+08:00 Frederic Weisbecker <fweisbec@xxxxxxxxx>:
> Ticks can happen while the CPU is in dynticks-idle or dynticks-singletask
> mode. In fact "nohz" or "dynticks" only mean that we exit the periodic
> mode and we try to minimize the ticks as much as possible. The nohz
> subsystem uses a confusing terminology with the internal state
> "ts->tick_stopped" which is also available through its public interface
> with tick_nohz_tick_stopped(). This is a misnomer as the tick is instead
> reduced with the best effort rather than stopped. In the best case the
> tick can indeed be actually stopped but there is no guarantee about that.
> If a timer needs to fire one second later, a tick will fire while the
> CPU is in nohz mode and this is a very common scenario.
>
> Now this confusion happens to be a problem with cpu load updates:
> cpu_load_update_active() doesn't handle nohz ticks correctly because it
> assumes that ticks are completely stopped in nohz mode and that
> cpu_load_update_active() can't be called in dynticks mode. When that
> happens, the whole previous tickless load is ignored and the function
> just records the load for the current tick, ignoring potentially long
> idle periods behind.

This is for one timer interrupt per second scenario, right?

Regards,
Wanpeng Li