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. > 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. 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(). -- Catalin
