Messages in this thread | | | Date | Mon, 10 Nov 2014 16:17:35 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH V3 1/6] sched: idle: Add a weak arch_cpu_idle_poll function |
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On Mon, Nov 10, 2014 at 07:50:22PM +0530, Preeti U Murthy wrote: > Hi Peter, > > On 11/10/2014 05:59 PM, Peter Zijlstra wrote: > > On Fri, Nov 07, 2014 at 03:31:22PM +0100, Daniel Lezcano wrote: > >> The poll function is called when a timer expired or if we force to poll when > >> the cpu_idle_force_poll option is set. > >> > >> The poll function does: > >> > >> local_irq_enable(); > >> while (!tif_need_resched()) > >> cpu_relax(); > >> > >> This default poll function suits for the x86 arch because its rep; nop; > >> hardware power optimization. But on other archs, this optimization does not > >> exists and we are not saving power. The arch specific bits may want to > >> optimize this loop by adding their own optimization. > > > > This doesn't make sense to me; should an arch not either implement an > > actual idle driver or implement cpu_relax() properly, why allow for a > > third weird option? > > > > The previous version of this patch simply invoked cpu_idle_loop() for > cases where latency_req was 0. This would have changed the behavior > on PowerPC wherein earlier the 0th idle index was returned which is also > a polling loop but differs from cpu_idle_loop() in two ways: > > a. It polls at a relatively lower power state than cpu_relax(). > b. We set certain registers to indicate that the cpu is idle.
So I'm confused; the current code runs the generic cpu_relax idle poll loop for the broadcast case. I suppose you want to retain this because not doing your a-b above will indeed give you a lower latency.
Therefore one could argue that latency_req==0 should indeed use this, and your a-b idle state should be latency_req==1 or higher.
Thus yes it changes behaviour, but I think it actually fixes something. You cannot have a latency_req==0 state which has higher latency than the actual polling loop, as you appear to have.
> Hence for all such cases wherein the cpu is required to poll while idle > (only for cases such as force poll, broadcast ipi to arrive soon and > latency_req = 0), we should be able to call into cpuidle_idle_loop() > only if the cpuidle driver's 0th idle state has an exit_latency > 0. > (The 0th idle state is expected to be a polling loop with > exit_latency = 0). > > If otherwise, it would mean the driver has an optimized polling loop > when idle. But instead of adding in the logic of checking the > exit_latency, we thought it would be simpler to call into an arch > defined polling idle loop under the above circumstances. If that is no > better we could fall back to cpuidle_idle_loop().
That still doesn't make sense to me; suppose the implementation of this special poll state differs on different uarchs for the same arch, then we'll end up with another registration and selection interface parallel to cpuidle.
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