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at 11:05:06AM -0800, Ankur Arora wrote: >> >> Will Deacon writes: >> >> > On Sun, Dec 14, 2025 at 08:49:11PM -0800, Ankur Arora wrote: >> >> Extend __cmpwait_relaxed() to __cmpwait_relaxed_timeout() which takes >> >> an additional timeout value in ns. >> >> >> >> Lacking WFET, or with zero or negative value of timeout we fallback >> >> to WFE. >> >> >> >> Cc: Arnd Bergmann >> >> Cc: Catalin Marinas >> >> Cc: Will Deacon >> >> Cc: linux-arm-kernel@lists.infradead.org >> >> Signed-off-by: Ankur Arora >> >> --- >> >> arch/arm64/include/asm/barrier.h | 8 ++-- >> >> arch/arm64/include/asm/cmpxchg.h | 72 ++++++++++++++++++++++---------- >> >> 2 files changed, 55 insertions(+), 25 deletions(-) >> > >> > Sorry, just spotted something else on this... >> > >> >> diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h >> >> index 6190e178db51..fbd71cd4ef4e 100644 >> >> --- a/arch/arm64/include/asm/barrier.h >> >> +++ b/arch/arm64/include/asm/barrier.h >> >> @@ -224,8 +224,8 @@ do { \ >> >> extern bool arch_timer_evtstrm_available(void); >> >> >> >> /* >> >> - * In the common case, cpu_poll_relax() sits waiting in __cmpwait_relaxed() >> >> - * for the ptr value to change. >> >> + * In the common case, cpu_poll_relax() sits waiting in __cmpwait_relaxed()/ >> >> + * __cmpwait_relaxed_timeout() for the ptr value to change. >> >> * >> >> * Since this period is reasonably long, choose SMP_TIMEOUT_POLL_COUNT >> >> * to be 1, so smp_cond_load_{relaxed,acquire}_timeout() does a >> >> @@ -234,7 +234,9 @@ extern bool arch_timer_evtstrm_available(void); >> >> #define SMP_TIMEOUT_POLL_COUNT 1 >> >> >> >> #define cpu_poll_relax(ptr, val, timeout_ns) do { \ >> >> - if (arch_timer_evtstrm_available()) \ >> >> + if (alternative_has_cap_unlikely(ARM64_HAS_WFXT)) \ >> >> + __cmpwait_relaxed_timeout(ptr, val, timeout_ns); \ >> >> + else if (arch_timer_evtstrm_available()) \ >> >> __cmpwait_relaxed(ptr, val); \ >> > >> > Don't you want to make sure that we have the event stream available for >> > __cmpwait_relaxed_timeout() too? Otherwise, a large timeout is going to >> > cause problems. >> >> Would that help though? If called from smp_cond_load_relaxed_timeout() >> then we would wake up and just call __cmpwait_relaxed_timeout() again. > > Fair enough, I can see that. Is it worth capping the maximum timeout > like we do for udelay()? The DELAY_CONST_MAX thing? So, I'm not sure your concern is about the overall timeout or timeout per WFET iteration? For the overall limit, at least rqspinlock has a pretty large timeout value (NSEC_PER_SEC/4). However, it might be a good idea to attach a DELAY_CONST_MAX like limit when using this interface -- for architectures that do not have an optimized way of polling/define ARCH_HAS_CPU_RELAX. (Currently only x86 defines ARCH_HAS_CPU_RELAX but I had a series which is meant to go after this that renames it to ARCH_HAS_ OPTIMIZED_POLL and selects it for x86 and arm64 [1].) But that still might mean that we could have fairly long WFET iterations. Do you forsee a problem with that? [1] https://lore.kernel.org/lkml/20250218213337.377987-1-ankur.a.arora@oracle.com/ Thanks -- ankur