Messages in this thread | | | Date | Fri, 21 Apr 2023 18:47:29 +0200 | From | Frederic Weisbecker <> | Subject | Re: Loongson (and other $ARCHs?) idle VS timer enqueue |
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On Fri, Apr 21, 2023 at 04:24:46PM +0200, Peter Zijlstra wrote: > On Fri, Apr 21, 2023 at 02:36:52PM +0200, Frederic Weisbecker wrote: > > Hi, > > > > I'm looking at the __arch_cpu_idle() implementation in Loongarch > > and I'm wondering about the rollback code. I don't understand well > > that code but with the help from PeterZ I might have seen something, > > so tell me if I'm wrong: when an interrupt happens within > > __arch_cpu_idle(), handle_vint() rolls back the return value to the > > beginning of __arch_cpu_idle(), so that TIF_NEED_RESCHED is checked > > again. Is that correct? > > Loongson copied this crap from MIPS, so they are direct affected too.
Right.
> > > Because if an interrupt fires while in __arch_cpu_idle(), that IRQ > > might enqueue a new timer and that new timer needs to be reprogrammed > > from the main idle loop and just checking TIF_NEED_RESCHED doesn't > > tell about that information. > > Notably; this is only relevant to NOHZ, right?
Indeed.
> > And set that from the timer enqueue in idle time and check TIF_IDLE_EXIT > > on idle callback. It depends how many architectures are concerned by this. > > All I know so far is: > > The alternative is changing kernel/entry/common.c:irqentry_exit() to add > a nohz callback next to ct_irq_exit(), and have that reprogram the timer > if/when we're in NOHZ mode.
We used to do that but Rafael rewrote the thing a few years ago in order for the cpuidle governor to know about the next timer event as a heuristic to predict the best c-state, and actually decide if it's worth stopping the tick.
So cpuidle_select() eventually calls tick_nohz_get_sleep_length() in the beginning of the idle loop to know the next timer event (without stopping the tick yet), on top of that and other informations, tick is stopped or not (cf: stop_tick argument in cpuidle_select()).
If an IRQ wakes up the CPU and queues a timer, we need to go through that whole process again, otherwise we shortcut cpuidle C-state update.
> *HOWEVER* > > intel_idle_irq() is affected -- except that we only (normally) use that > for very shallow idle states and it won't interact with NOHZ (because we > only disable the tick for deep idle states).
Well I don't know, that doesn't look comfortable... :)
Also why does it need to enable IRQs if ecx=1 ?
> > * Need to check all other archs > > > > I'm trying to find an automated way to debug this kind of issue but it's not > > easy... > > Yeah, too much arch code :/ Easiest might be to check if our idea of > where the timer should be and the hardware agree on IRQ entry or so -- > *any* IRQ. That will miss a lot of cases, but at least it's something.
Hmm, not sure I understand what you're suggesting...
Thanks.
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