From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Srivatsa S. Bhat" Subject: Re: [PATCH] cpuidle/menu: Fail cpuidle_idle_call() if no idle state is acceptable Date: Tue, 14 Jan 2014 13:07:32 +0530 Message-ID: <52D4E93C.3050503@linux.vnet.ibm.com> References: <20140114060516.6109.14901.stgit@preeti.in.ibm.com> <52D4E07E.204@linux.vnet.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <52D4E07E.204@linux.vnet.ibm.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: linuxppc-dev-bounces+glppe-linuxppc-embedded-2=m.gmane.org@lists.ozlabs.org Sender: "Linuxppc-dev" To: Preeti U Murthy Cc: deepthi@linux.vnet.ibm.com, linux-pm@vger.kernel.org, daniel.lezcano@linaro.org, rjw@rjwysocki.net, linux-kernel@vger.kernel.org, paulmck@linux.vnet.ibm.com, linuxppc-dev@lists.ozlabs.org, tuukka.tikkanen@linaro.org List-Id: linux-pm@vger.kernel.org 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 Date: Fri Sep 6 00:25:06 2013 +0530 powerpc: Default arch idle could cede processor on pseries Regards, Srivatsa S. Bhat