On Monday, January 26, 2015 10:40:24 AM Thomas Gleixner wrote:
> On Mon, 26 Jan 2015, Li, Aubrey wrote:
> > On 2015/1/22 18:15, Thomas Gleixner wrote:
[...]
> > >> + /*
> > >> + * cpuidle_enter will return with interrupt enabled
> > >> + */
> > >> + cpuidle_enter(drv, dev, next_state);
> > >
> > > How is that supposed to work?
> > >
> > > If timekeeping is not yet unfrozen, then any interrupt handling code
> > > which calls anything time related is going to hit lala land.
> > >
> > > You must guarantee that timekeeping is unfrozen before any interrupt
> > > is handled. If you cannot guarantee that, you cannot freeze
> > > timekeeping ever.
> > >
> > > The cpu local tick device is less critical, but it happens to work by
> > > chance, not by design.
> >
> > There are two way to guarantee this: the first way is, disable interrupt
> > before timekeeping frozen and enable interrupt after timekeeping is
> > unfrozen. However, we need to handle wakeup handler before unfreeze
> > timekeeping to wake freeze task up from wait queue.
> >
> > So we have to go the other way, the other way is, we ignore time related
> > calls during freeze, like what I added in irq_enter below.
>
> Groan. You just do not call in irq_enter/exit(), but what prevents any
> interrupt handler or whatever to call into the time/timer code after
> interrupts got reenabled?
>
> Nothing.
>
> > Or, we need to re-implement freeze wait and wake up mechanism?
>
> You need to make sure in the low level idle implementation that this
> cannot happen.
>
> tick_freeze()
> {
> raw_spin_lock(&tick_freeze_lock);
> tick_frozen++;
> if (tick_frozen == num_online_cpus())
> timekeeping_suspend();
> else
> tick_suspend_local();
> raw_spin_unlock(&tick_freeze_lock);
> }
>
> tick_unfreeze()
> {
> raw_spin_lock(&tick_freeze_lock);
> if (tick_frozen == num_online_cpus())
> timekeeping_resume();
> else
> tick_resume_local();
> tick_frozen--;
> raw_spin_unlock(&tick_freeze_lock);
> }
>
> idle_freeze()
> {
> local_irq_disable();
>
> tick_freeze();
>
> /* Must keep interrupts disabled! */
> go_deep_idle()
>
> tick_unfreeze();
>
> local_irq_enable();
> }
>
> That's the only way you can do it proper, everything else will just be
> a horrible mess of bandaids and duct tape.
>
> So that does not need any of the irq_enter/exit conditionals, it does
> not need the real_handler hack. It just works.
As long as go_deep_idle() above does not enable interrupts. This means we won't
be able to use some C-states for suspend-to-idle (hald-induced C1 on some x86
for one example), but that's not a very big deal.
> The only remaining issue might be a NMI calling into
> ktime_get_mono_fast_ns() before timekeeping is resumed. Its probably a
> non issue on x86/tsc, but it might be a problem on other platforms
> which turn off devices, clocks, It's not rocket science to prevent
> that.
I don't see any users of ktime_get_mono_fast_ns() at all, unless some
non-trivial
macros are involved. At least grepping for it only returns the definition,
declarations and the line in trace.c.
Rafael
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