Messages in this thread | | | Date | Tue, 14 Jan 2014 13:07:32 +0530 | From | "Srivatsa S. Bhat" <> | Subject | Re: [PATCH] cpuidle/menu: Fail cpuidle_idle_call() if no idle state is acceptable |
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On 01/14/2014 12:30 PM, Srivatsa S. Bhat wrote: > On 01/14/2014 11:35 AM, Preeti U Murthy wrote: >> On PowerPC, in a particular test scenario, all the cpu idle states were disabled. >> Inspite of this it was observed that the idle state count of the shallowest >> idle state, snooze, was increasing. >> >> This is because the governor returns the idle state index as 0 even in >> scenarios when no idle state can be chosen. These scenarios could be when the >> latency requirement is 0 or as mentioned above when the user wants to disable >> certain cpu idle states at runtime. In the latter case, its possible that no >> cpu idle state is valid because the suitable states were disabled >> and the rest did not match the menu governor criteria to be chosen as the >> next idle state. >> >> This patch adds the code to indicate that a valid cpu idle state could not be >> chosen by the menu governor and reports back to arch so that it can take some >> default action. >> > > That sounds fair enough. However, the "default" action of pseries idle loop > (pseries_lpar_idle()) surprises me. It enters Cede, which is _deeper_ than doing > a snooze! IOW, a user might "disable" cpuidle or set the PM_QOS_CPU_DMA_LATENCY > to 0 hoping to prevent the CPUs from going to deep idle states, but then the > machine would still end up going to Cede, even though that wont get reflected > in the idle state counts. IMHO that scenario needs some thought as well... >
I checked the git history and found that the default idle was changed (on purpose) to cede the processor, in order to speed up booting.. Hmm..
commit 363edbe2614aa90df706c0f19ccfa2a6c06af0be Author: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com> Date: Fri Sep 6 00:25:06 2013 +0530
powerpc: Default arch idle could cede processor on pseries
Regards, Srivatsa S. Bhat
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