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