Messages in this thread |  | | Date | Tue, 28 Oct 2014 19:59:19 +0100 | From | Daniel Lezcano <> | Subject | Re: [PATCH V2 1/5] sched: idle: cpuidle: Check the latency req before idle |
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On 10/28/2014 04:51 AM, Preeti Murthy wrote: > Hi Daniel, > > On Thu, Oct 23, 2014 at 2:31 PM, Daniel Lezcano > <daniel.lezcano@linaro.org> wrote: >> When the pmqos latency requirement is set to zero that means "poll in all the >> cases". >> >> That is correctly implemented on x86 but not on the other archs. >> >> As how is written the code, if the latency request is zero, the governor will >> return zero, so corresponding, for x86, to the poll function, but for the >> others arch the default idle function. For example, on ARM this is wait-for- >> interrupt with a latency of '1', so violating the constraint. > > This is not true actually. On PowerPC the idle state 0 has an exit_latency of 0. > >> >> In order to fix that, do the latency requirement check *before* calling the >> cpuidle framework in order to jump to the poll function without entering >> cpuidle. That has several benefits: > > Doing so actually hurts on PowerPC. Because the idle loop defined for > idle state 0 is different from what cpu_relax() does in cpu_idle_loop(). > The spinning is more power efficient in the former case. Moreover we also set > certain register values which indicate an idle cpu. The ppc_runlatch bits > do precisely this. These register values are being read by some user space > tools. So we will end up breaking them with this patch > > My suggestion is very well keep the latency requirement check in > kernel/sched/idle.c > like your doing in this patch. But before jumping to cpu_idle_loop verify if the > idle state 0 has an exit_latency > 0 in addition to your check on the > latency_req == 0. > If not, you can fall through to the regular path of calling into the > cpuidle driver. > The scheduler can query the cpuidle_driver structure anyway. > > What do you think?
Thanks for reviewing the patch and spotting this.
Wouldn't make sense to create:
void __weak_cpu_idle_poll(void) ?
and override it with your specific poll function ?
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