Catalin Marinas <catalin.marinas@xxxxxxx> writes: > 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. AFAICT when polling (which we are since poll_idle() calls current_set_polling_and_test()), the scheduler will elide the IPI by remotely setting the need-resched bit via set_nr_if_polling(). Once we stop polling then the scheduler should take the IPI path because call_function_single_prep_ipi() will fail. -- ankur
