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X-CRM114-Status: GOOD ( 22.73 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org Ankur Arora writes: > Christoph Lameter (Ampere) writes: > >> On Thu, 26 Jun 2025, Ankur Arora wrote: >> >>> @@ -222,6 +223,53 @@ do { \ >>> #define __smp_timewait_store(ptr, val) \ >>> __cmpwait_relaxed(ptr, val) >>> >>> +/* >>> + * Redefine ARCH_TIMER_EVT_STREAM_PERIOD_US locally to avoid include hell. >>> + */ >>> +#define __ARCH_TIMER_EVT_STREAM_PERIOD_US 100UL >>> +extern bool arch_timer_evtstrm_available(void); >>> + >>> +static inline u64 ___smp_cond_spinwait(u64 now, u64 prev, u64 end, >>> + u32 *spin, bool *wait, u64 slack); >>> +/* >>> + * To minimize time spent spinning, we want to allow a large overshoot. >>> + * So, choose a default slack value of the event-stream period. >>> + */ >>> +#define SMP_TIMEWAIT_DEFAULT_US __ARCH_TIMER_EVT_STREAM_PERIOD_US >>> + >>> +static inline u64 ___smp_cond_timewait(u64 now, u64 prev, u64 end, >>> + u32 *spin, bool *wait, u64 slack) >>> +{ >>> + bool wfet = alternative_has_cap_unlikely(ARM64_HAS_WFXT); >>> + bool wfe, ev = arch_timer_evtstrm_available(); >> >> An unitialized and initialized variable on the same line. Maybe separate >> that. Looks confusing and unusual to me. > > Yeah, that makes sense. Will fix. > >>> + u64 evt_period = __ARCH_TIMER_EVT_STREAM_PERIOD_US; >>> + u64 remaining = end - now; >>> + >>> + if (now >= end) >>> + return 0; >>> + /* >>> + * Use WFE if there's enough slack to get an event-stream wakeup even >>> + * if we don't come out of the WFE due to natural causes. >>> + */ >>> + wfe = ev && ((remaining + slack) > evt_period); >> >> The line above does not matter for the wfet case and the calculation is >> ignored. We hope that in the future wfet will be the default case. > > My assumption was that the compiler would only evaluate the evt_period > comparison if the wfet check evaluates false and it does need to check > if wfe is true or not. > > But let me look at the generated code. So, I was too optimistic. The compiler doesn't do this optimization at all. I'm guessing that's because of at least two reasons: The wfet case is unlikely: bool wfet = alternative_has_cap_unlikely(ARM64_HAS_WFXT); And, I'm testing for: if (wfe || wfet) This last check should have been "if (wfet || wfe)"" since the first clause is pretty much free. But even with that fixed, I don't think the compiler will do the right thing. I could condition the computation on arch_timer_evtstrm_available(), but I would prefer to keep this code straightforward since it will only be evaluated infrequently. But, let me see what I can do. -- ankur