Re: [PATCH 02/32 RESEND] nohz: Drop ts->idle_active

Frederic Weisbecker
2011-08-29T16:15:23Z

On Mon, Aug 29, 2011 at 04:23:12PM +0200, Peter Zijlstra wrote:
> On Mon, 2011-08-15 at 17:51 +0200, Frederic Weisbecker wrote:
> > ts->idle_active is used to know if we want to account the idle sleep
> > time. But ts->inidle is enough to check that.
> >
> While possibly true, its not immediately obvious and no hints are
> supplied. For example: tick_check_nohz() would disable ->idle_active..
> where is this mirrored in the ->inidle state.

Hmm, you're right. By the time we call tick_check_nohz() (irq_enter())
and tick_nohz_stop_sched_tick() (irq_exit()) there may be a softirq
and then another hard irq that could update the idle time spuriously.

So the mapping inidle - idle_active is wrong.

Let's drop that patch.

>
> Also, tick_nohz_stop_sched_tick() has this comment:
>
> /*
> * Set ts->inidle unconditionally. Even if the system did not
> * switch to NOHZ mode the cpu frequency governers rely on the
> * update of the idle time accounting in tick_nohz_start_idle().
> */
> ts->inidle = 1;
>
> Which suggest the ->inidle state doesn't accurately reflect things.
>
> This is all rather hairy code, such changes really want more in terms of
> explanation.
>
>