On Monday, February 09, 2015 04:44:08 PM Peter Zijlstra wrote:
> On Mon, Feb 09, 2015 at 03:54:22AM +0100, Rafael J. Wysocki wrote:
> > Complete patch with that modification is appended. In the next few days I'm
> > going to split it into smaller parts and send along with cpuidle driver
> > patches implementing ->enter_freeze.
> >
> > Please let me know what you think.
>
> > @@ -104,6 +105,21 @@ static void cpuidle_idle_call(void)
> > rcu_idle_enter();
> >
> > /*
> > + * Suspend-to-idle ("freeze") is a system state in which all user space
> > + * has been frozen, all I/O devices have been suspended and the only
> > + * activity happens here and in iterrupts (if any). In that case bypass
> > + * the cpuidle governor and go stratight for the deepest idle state
> > + * available. Possibly also suspend the local tick and the entire
> > + * timekeeping to prevent timer interrupts from kicking us out of idle
> > + * until a proper wakeup interrupt happens.
> > + */
> > + if (idle_should_freeze()) {
> > + cpuidle_enter_freeze();
> > + local_irq_enable();
> > + goto exit_idle;
> > + }
> > +
> > + /*
> > * Ask the cpuidle framework to choose a convenient idle state.
> > * Fall back to the default arch idle method on errors.
> > */
>
> I was hoping to not have to put that into the regular idle path; say
> maybe share a single special branch with the play-dead call. People seem
> to start complaining about the total amount of time it takes to just
> 'run' the idle path.
Well, this check really only replaces one in cpuidle_select() and another
one in cpuidle_reflect() and they both are called from the idle path, so
that's a net gain rather. :-)
> Now I don't think we can do that, because we need the
> arch_cpu_idle_enter() nonsense for the one but not the other; also all
> this really only makes sense in the cpuidle context, so nothing to be
> done about that.
Agreed.
> In any case, you could make that:
>
> static inline bool idle_should_freeze(void)
> {
> return unlikely(suspend_freeze_state == FREEZE_STATE_ENTER);
> }
>
> which should help a bit I suppose.
Won't hurt at least.
> > +static void enter_freeze_proper(struct cpuidle_driver *drv,
> > + struct cpuidle_device *dev, int index)
> > +{
> > + tick_freeze();
> > + drv->states[index].enter_freeze(dev, drv, index);
>
> This is slightly different from cpuidle_enter() in that it does not
> consider the coupled states nonsense, is that on purpose? And if so,
> does that want a comment?
Yes, it is on purpose and the reason why is because the coupled thing re-enables
interrupts (unconditionally). [And it contains a comment about that which does
not make sense to me now that I read it.]
I can add a comment about that, though.
> > + /*
> > + * timekeeping_resume() that will be called by tick_unfreeze() for
> > the
> > + * last CPU executing it calls functions containing RCU read-side
> > + * critical sections, so tell RCU about that.
> > + */
> > + RCU_NONIDLE(tick_unfreeze());
> > +}
>
>
> But over all it looks fine to me.
Cool, thanks!
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