Catalin Marinas <catalin.marinas@xxxxxxx> writes: > On Thu, Oct 17, 2024 at 03:47:31PM -0700, Ankur Arora wrote: >> Catalin Marinas <catalin.marinas@xxxxxxx> writes: >> > On Wed, Oct 16, 2024 at 03:13:33PM +0000, Okanovic, Haris wrote: >> >> On Tue, 2024-10-15 at 13:04 +0100, Catalin Marinas wrote: >> >> > On Wed, Sep 25, 2024 at 04:24:15PM -0700, Ankur Arora wrote: >> >> > > + smp_cond_load_relaxed(¤t_thread_info()->flags, >> >> > > + VAL & _TIF_NEED_RESCHED || >> >> > > + loop_count++ >= POLL_IDLE_RELAX_COUNT); >> >> > >> >> > The above is not guaranteed to make progress if _TIF_NEED_RESCHED is >> >> > never set. With the event stream enabled on arm64, the WFE will >> >> > eventually be woken up, loop_count incremented and the condition would >> >> > become true. However, the smp_cond_load_relaxed() semantics require that >> >> > a different agent updates the variable being waited on, not the waiting >> >> > CPU updating it itself. Also note that the event stream can be disabled >> >> > on arm64 on the kernel command line. >> >> >> >> Alternately could we condition arch_haltpoll_want() on >> >> arch_timer_evtstrm_available(), like v7? >> > >> > No. The problem is about the smp_cond_load_relaxed() semantics - it >> > can't wait on a variable that's only updated in its exit condition. We >> > need a new API for this, especially since we are changing generic code >> > here (even it was arm64 code only, I'd still object to such >> > smp_cond_load_*() constructs). >> >> Right. The problem is that smp_cond_load_relaxed() used in this context >> depends on the event-stream side effect when the interface does not >> encode those semantics anywhere. >> >> So, a smp_cond_load_timeout() like in [1] that continues to depend on >> the event-stream is better because it explicitly accounts for the side >> effect from the timeout. >> >> This would cover both the WFxT and the event-stream case. > > Indeed. > >> The part I'm a little less sure about is the case where WFxT and the >> event-stream are absent. >> >> As you said earlier, for that case on arm64, we use either short >> __delay() calls or spin in cpu_relax(), both of which are essentially >> the same thing. > Something derived from __delay(), not exactly this function. We can't > use it directly as we also want it to wake up if an event is generated > as a result of a memory write (like the current smp_cond_load(). > >> Now on x86 cpu_relax() is quite optimal. The spec explicitly recommends >> it and from my measurement a loop doing "while (!cond) cpu_relax()" gets >> an IPC of something like 0.1 or similar. >> >> On my arm64 systems however the same loop gets an IPC of 2. Now this >> likely varies greatly but seems like it would run pretty hot some of >> the time. > > For the cpu_relax() fall-back, it wouldn't be any worse than the current > poll_idle() code, though I guess in this instance we'd not enable idle > polling. > > I expect the event stream to be on in all production deployments. The > reason we have a way to disable it is for testing. We've had hardware > errata in the past where the event on spin_unlock doesn't cross the > cluster boundary. We'd not notice because of the event stream. Ah, interesting. Thanks, that helps. >> So maybe the right thing to do would be to keep smp_cond_load_timeout() >> but only allow polling if WFxT or event-stream is enabled. And enhance >> cpuidle_poll_state_init() to fail if the above condition is not met. > > We could do this as well. Maybe hide this behind another function like > arch_has_efficient_smp_cond_load_timeout() (well, some shorter name), > checked somewhere in or on the path to cpuidle_poll_state_init(). Well, > it might be simpler to do this in haltpoll_want(), backed by an > arch_haltpoll_want() function. Yeah, checking in arch_haltpoll_want() would mean that we can leave all the cpuidle_poll_state_init() call sites unchanged. However, I suspect that even acpi-idle on arm64 might end up using poll_idle() (as this patch tries to do: https://lore.kernel.org/lkml/f8a1f85b-c4bf-4c38-81bf-728f72a4f2fe@xxxxxxxxxx/). So, let me try doing it both ways to see which one is simpler. Given that the event-stream can be assumed to be always-on it might just be more straight-forward to fallback to cpu_relax() in that edge case. > I assume we want poll_idle() to wake up as soon as a task becomes > available. Otherwise we could have just used udelay() for some fraction > of cpuidle_poll_time() instead of cpu_relax(). Yeah, agreed. Thanks -- ankur
