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

[Sudeep Holla <sudeep.holla@arm.com>]

On Fri, Jun 21, 2024 at 04:59:22PM -0700, Ankur Arora wrote:
> 
> Sudeep Holla <sudeep.holla@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.

--
Regards,
Sudeep