Sudeep Holla <sudeep.holla@xxxxxxx> writes: > 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. Yeah let me thresh out the commit message for this patch a bit more. Thanks for the review! -- ankur
