Re: [RFC 2/3] tick-sched: Keep tick on if hrtimer is due imminently

[Christian Loehle]

On 11/11/24 15:56, Joel Fernandes wrote:
> On Mon, Nov 11, 2024 at 7:38 AM Christian Loehle
> <christian.loehle@xxxxxxx> wrote:
>>
>> On 11/8/24 17:48, Joel Fernandes (Google) wrote:
>>> In highres mode, the kernel only considers timer wheel events when
>>> considering whether to keep the tick on (via get_next_interrupt()).
>>>
>>> This seems odd because it consider several other reasons to keep the
>>> tick on. Further, turning off the tick does not help because once idle
>>> exit happens due to that imminent hrtimer interrupt, the tick hrtimer
>>> interrupt is requeued. That means more hrtimer rbtree operations for not
>>> much benefit.
>>>
>>> Ideally we should not have to do anything because the cpuidle governor
>>> should not try to the stop the tick because it knows about this
>>> situation, but apparently it still does try to stop the tick.
>>
>> Any details on this? Which governor?
> 
> I noticed this in Qemu (virtualized hardware). Actually I need to
> update the commit message. I think it is not because of the governor
> but because of lack of guest cpuidle support.

Ah indeed, then it makes sense.
FYI Anna-Maria proposed something like below a year ago:
https://lore.kernel.org/lkml/20231215130501.24542-1-anna-maria@xxxxxxxxxxxxx/
I have no strong opinion on it either way.

Regards,
Christian

> 
> static void cpuidle_idle_call(void)
> {
> ....
>   if (cpuidle_not_available(drv, dev)) {
>     tick_nohz_idle_stop_tick();
>     default_idle_call();
>     goto exit_idle;
>   }
> ...
> Over here dev and drv are NULL for me. I will also test on real hardware.
> 
> Also maybe the " if (cpuidle_not_available(drv, dev))" condition
> should do some more work to determine if tick_nohz_idle_stop_tick()
> should be called instead of unconditionally calling it?
> 
> Pasting relevant parts of my .config:
> 
> # grep IDLE .config
> CONFIG_NO_HZ_IDLE=y
> CONFIG_ARCH_CPUIDLE_HALTPOLL=y
> CONFIG_ACPI_PROCESSOR_IDLE=y
> # CPU Idle
> CONFIG_CPU_IDLE=y
> # CONFIG_CPU_IDLE_GOV_LADDER is not set
> CONFIG_CPU_IDLE_GOV_MENU=y
> # CONFIG_CPU_IDLE_GOV_TEO is not set
> CONFIG_CPU_IDLE_GOV_HALTPOLL=y
> CONFIG_HALTPOLL_CPUIDLE=y
> # end of CPU Idle
> CONFIG_INTEL_IDLE=y
> 
> thanks,
> 
>  - Joel