On Tue, Oct 15, 2024 at 10:17:13AM -0700, Christoph Lameter (Ampere) wrote: > On Tue, 15 Oct 2024, Catalin Marinas wrote: > > > Setting of need_resched() from another processor involves sending an IPI > > > after that was set. I dont think we need to smp_cond_load_relaxed since > > > the IPI will cause an event. For ARM a WFE would be sufficient. > > > > I'm not worried about the need_resched() case, even without an IPI it > > would still work. > > > > The loop_count++ side of the condition is supposed to timeout in the > > absence of a need_resched() event. You can't do an smp_cond_load_*() on > > a variable that's only updated by the waiting CPU. Nothing guarantees to > > wake it up to update the variable (the event stream on arm64, yes, but > > that's generic code). > > Hmm... I have WFET implementation here without smp_cond modelled after > the delay() implementation ARM64 (but its not generic and there is > an additional patch required to make this work. Intermediate patch > attached) At least one additional patch ;). But yeah, I suggested hiding all this behind something like smp_cond_load_timeout() which would wait on current_thread_info()->flags but with a timeout. The arm64 implementation would follow some of the logic in __delay(). Others may simply poll with cpu_relax(). Alternatively, if we get an IPI anyway, we can avoid smp_cond_load() and rely on need_resched() and some new delay/cpu_relax() API that waits for a timeout or an IPI, whichever comes first. E.g. cpu_relax_timeout() which on arm64 it's just a simplified version of __delay() without the 'while' loops. -- Catalin
