Messages in this thread | | | From | "Rafael J. Wysocki" <> | Subject | Re: [PATCH v3]PM/Sleep: Timer quiesce in freeze state | Date | Mon, 26 Jan 2015 15:41:44 +0100 |
| |
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
| |