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This waiting is via some mix of looping around, dereferencing an address, or waiting in a WFE until the CPU gets an event due to a store to the address, or because of periodic events from the event-stream. Define __smp_cond_timewait_coarse() for usecases where the caller can tolerate a relatively large overshoot. This allows us to minimize the time spent spinning at the cost of spending extra time in the WFE state. This would result in a worst case delay of ARCH_TIMER_EVT_STREAM_PERIOD_US and a spin period of no more than SMP_TIMEWAIT_CHECK_US. Cc: Will Deacon Cc: Catalin Marinas Cc: Kumar Kartikeya Dwivedi Cc: Alexei Starovoitov Signed-off-by: Ankur Arora --- arch/arm64/include/asm/barrier.h | 66 ++++++++++++++++++++++++++++++++ 1 file changed, 66 insertions(+) diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barr= ier.h index eaeb78dd48c0..f4a184a96933 100644 --- a/arch/arm64/include/asm/barrier.h +++ b/arch/arm64/include/asm/barrier.h @@ -10,6 +10,7 @@ #ifndef __ASSEMBLY__ =20 #include +#include =20 #include =20 @@ -219,6 +220,71 @@ do { \ #define __smp_timewait_store(ptr, val) \ __cmpwait_relaxed(ptr, val) =20 +/* + * 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); + +/* + * For coarse grained waits, allow overshoot by the event-stream period. + * Defined without reference to ARCH_TIMER_EVT_STREAM_PERIOD_US to avoid + * include hell. + */ +#define SMP_TIMEWAIT_SLACK_COARSE_US __ARCH_TIMER_EVT_STREAM_PERIOD_US + +#define SMP_TIMEWAIT_SPIN_BASE 16 +#define SMP_TIMEWAIT_CHECK_US 2UL + +static inline u64 ___cond_timewait(u64 now, u64 prev, u64 end, + u32 *spin, bool *wait, u64 slack) +{ + bool wfet =3D alternative_has_cap_unlikely(ARM64_HAS_WFXT); + bool wfe, ev =3D arch_timer_evtstrm_available(); + u64 evt_period =3D __ARCH_TIMER_EVT_STREAM_PERIOD_US; + u64 remaining =3D end - now; + + if (now >=3D 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 =3D ev && ((remaining + slack) > evt_period); + + if (wfe || wfet) { + *wait =3D true; + *spin =3D 0; + return now; + } + + /* + * Our wait period is shorter than our best granularity. Spin. + * + * A time-check is expensive but not too expensive. Scale the + * spin-count so we stay close to the fine-grained slack period. + */ + *wait =3D false; + if ((now - prev) < SMP_TIMEWAIT_CHECK_US) + *spin <<=3D 1; + else + *spin =3D max((*spin >> 1) + (*spin >> 2), SMP_TIMEWAIT_SPIN_BASE); + return now; +} + +/* + * Coarse wait_policy: minimizes the duration spent spinning at the cost of + * potentially spending the available slack in a WFE wait state. + * + * The resultant worst case timeout delay is SMP_TIMEWAIT_SLACK_COARSE_US + * (same as ARCH_TIMER_EVT_STREAM_PERIOD_US) and a spin period of no more + * than SMP_TIMEWAIT_CHECK_US. + */ +#define __smp_cond_timewait_coarse(now, prev, end, spin, wait) \ + ___cond_timewait(now, prev, end, spin, wait, \ + SMP_TIMEWAIT_SLACK_COARSE_US) + #include =20 #endif /* __ASSEMBLY__ */ --=20 2.43.5