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conditional variable, while watching the clock. The generic code presents the simple case where the waiting is done via a cpu_relax() spin-wait loop. To keep the pipeline as idle as possible, we want to do the relatively expensive time check only intermittently. Add ___smp_cond_spinwait() which handles adjustments to the spin-count based on the deadline. Cc: Arnd Bergmann Cc: Will Deacon Cc: Catalin Marinas Cc: Peter Zijlstra Cc: linux-arch@vger.kernel.org Signed-off-by: Ankur Arora --- include/asm-generic/barrier.h | 24 ++++++++++++++++++++++-- 1 file changed, 22 insertions(+), 2 deletions(-) diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h index d33c2701c9ee..8299c57d1110 100644 --- a/include/asm-generic/barrier.h +++ b/include/asm-generic/barrier.h @@ -15,6 +15,7 @@ =20 #include #include +#include #include =20 #ifndef nop @@ -286,11 +287,30 @@ do { \ static inline u64 ___smp_cond_spinwait(u64 now, u64 prev, u64 end, u32 *spin, bool *wait, u64 slack) { + u64 time_check; + u64 remaining =3D end - now; + if (now >=3D end) return 0; - - *spin =3D SMP_TIMEWAIT_SPIN_BASE; + /* + * Use a floor spin-count as it might be artificially low if we are + * transitioning from wait to spin, or because we got interrupted. + */ + *spin =3D min(*spin, SMP_TIMEWAIT_SPIN_BASE); *wait =3D false; + + /* + * We will map the time_check interval to the spin-count by scaling + * based on the previous time-check interval. This is imprecise, so + * use a safety margin. + */ + time_check =3D min(remaining/4, 1UL); + + if ((now - prev) < time_check) + *spin <<=3D 1; + else + *spin =3D ((*spin >> 1) + (*spin >> 2)); + return now; } =20 --=20 2.43.5