[PATCH 8/9] arm64: support cpuidle-haltpoll

[Ankur Arora]

Sudeep Holla <sudeep.holla at arm.com> writes:

> On Fri, Jun 21, 2024 at 04:59:22PM -0700, Ankur Arora wrote:
>>
>> Sudeep Holla <sudeep.holla at arm.com> writes:
>>
>> > On Tue, Apr 30, 2024 at 11:37:29AM -0700, Ankur Arora wrote:
>> >> Add architectural support for the cpuidle-haltpoll driver by defining
>> >> arch_haltpoll_*(). Also select ARCH_HAS_OPTIMIZED_POLL since we have
>> >> an optimized polling mechanism via smp_cond_load*().
>> >>
>> >> Add the configuration option, ARCH_CPUIDLE_HALTPOLL to allow
>> >> cpuidle-haltpoll to be selected.
>> >>
>> >> Note that we limit cpuidle-haltpoll support to when the event-stream is
>> >> available. This is necessary because polling via smp_cond_load_relaxed()
>> >> uses WFE to wait for a store which might not happen for an prolonged
>> >> period of time. So, ensure the event-stream is around to provide a
>> >> terminating condition.
>> >>
>> >
>> > Currently the event stream is configured 10kHz(1 signal per 100uS IIRC).
>> > But the information in the cpuidle states for exit latency and residency
>> > is set to 0(as per drivers/cpuidle/poll_state.c). Will this not cause any
>> > performance issues ?
>>
>> No I don't think there's any performance issue.
>>
>
> Thanks for the confirmation, that was my assumption as well.
>
>> When the core is waiting in WFE for &thread_info->flags to
>> change, and set_nr_if_polling() happens, the CPU will come out
>> of the wait quickly.
>> So, the exit latency, residency can be reasonably set to 0.
>>
>
> Sure
>
>> If, however, there is no store to &thread_info->flags, then the event
>> stream is what would cause us to come out of the WFE and check if
>> the poll timeout has been exceeded.
>> In that case, there was no work to be done, so there was nothing
>> to wake up from.
>>
>
> This is exactly what I was referring when I asked about performance, but
> it looks like it is not a concern for the reason specified about.
>
>> So, in either circumstance there's no performance loss.
>>
>> However, when we are polling under the haltpoll governor, this might
>> mean that we spend more time polling than determined based on the
>> guest_halt_poll_ns. But, that would only happen in the last polling
>> iteration.
>>
>> So, I'd say, at worst no performance loss. But, we would sometimes
>> poll for longer than necessary before exiting to the host.
>>
>
> Does it make sense to add some comment that implies briefly what we
> have discussed here ? Mainly why 0 exit and target residency values
> are fine and how worst case WFE wakeup doesn't impact the performance.

Yeah let me thresh out the commit message for this patch a bit more.

Thanks for the review!

--
ankur