Re: [PATCH v12 3/4] cpuidle: Export the next timer/tick expiration for a CPU

[Rafael J. Wysocki]

On Mon, Mar 25, 2019 at 3:24 PM Ulf Hansson <ulf.hansson@xxxxxxxxxx> wrote:
>
> On Mon, 25 Mar 2019 at 13:21, Rafael J. Wysocki <rjw@xxxxxxxxxxxxx> wrote:
> >
> > On Wednesday, February 27, 2019 8:58:35 PM CET Ulf Hansson wrote:
> > > To be able to predict the sleep duration for a CPU that is entering idle,
> > > knowing when the next timer/tick is going to expire, is extremely useful.
> > > Both the teo and the menu cpuidle governors already makes use of this
> > > information, while selecting an idle state.
> > >

[cut]

> >
> > > +
> > >       if (cpuidle_state_is_coupled(drv, index))
> > >               return cpuidle_enter_state_coupled(dev, drv, index);
> > >       return cpuidle_enter_state(dev, drv, index);
> >
> > Also I would clear next_hrtimer here to avoid dragging stale values
> > around.
>
> Right, I can do that.
>
> However, at least in my case it would be an unnecessary update of the
> variable, as I am never in a path where the value can be "stale".

It easily can AFAICS.  After all, cpu_power_down_ok() need not run on
the same CPU that is setting next_hrtimer here.

> Even if one theoretically could use a stale value, it's seems likely to not
> be an issue, don't you think?

That would be because of the locking in the ->enter() callback I
suppose?  But is it actually universally guaranteed that setting
next_hrtimer will never be reordered with acquiring the lock?

Also, there is some overhead to be avoided if cpu_power_down_ok()
checked the next_hrtimer of the other CPUs against 0 explicitly, isn't
it?