Catalin Marinas <catalin.marinas@xxxxxxx> writes: > On Thu, Oct 17, 2024 at 09:56:13AM -0700, Christoph Lameter (Ampere) wrote: >> On Wed, 16 Oct 2024, Catalin Marinas wrote: >> > The behaviour above is slightly different from the current poll_idle() >> > implementation. The above is more like poll every timeout period rather >> > than continuously poll until either the need_resched() condition is true >> > _or_ the timeout expired. From Ankur's email, an IPI may not happen so >> > we don't have any guarantee that WFET will wake up before the timeout. >> > The only way for WFE/WFET to wake up on need_resched() is to use LDXR to >> > arm the exclusive monitor. That's what smp_cond_load_relaxed() does. >> >> Sorry no. The IPI will cause the WFE to continue immediately and not wait >> till the end of the timeout period. > > *If* there is an IPI. The scheduler is not really my area but some > functions like wake_up_idle_cpu() seem to elide the IPI if > TIF_NR_POLLING is set. > > But even if we had an IPI, it still feels like abusing the semantics of > smp_cond_load_relaxed() when relying on it to increment a variable in > the condition check as a result of some unrelated wake-up event. This > API is meant to wait for a condition on a single variable. It cannot > wait on multiple variables and especially not one it updates itself > (even if it happens to work on arm64 under certain conditions). Yeah that makes sense. smp_cond_load_relaxed() uses two separate side-effects to make sure things work: the event-stream and the increment in the conditional. I do want to thresh out smp_cond_load_timeout() a bit more but let me reply to your other mail for that. > My strong preference would be to revive the smp_cond_load_timeout() > proposal from Ankur earlier in the year. Ack that. -- ankur
