From mboxrd@z Thu Jan 1 00:00:00 1970 From: Joel Fernandes Subject: Re: [PATCH v2] schedutil: Allow cpufreq requests to be made even when kthread kicked Date: Tue, 22 May 2018 15:28:37 -0700 Message-ID: <20180522222837.GA55359@joelaf.mtv.corp.google.com> References: <20180518185501.173552-1-joel@joelfernandes.org> <20180522113844.5rz3skjeck57arft@vireshk-i7> <4237890.zlzv5C60QP@aspire.rjw.lan> <20180522213113.GA40506@joelaf.mtv.corp.google.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org To: "Rafael J. Wysocki" Cc: Viresh Kumar , Linux Kernel Mailing List , "Rafael J . Wysocki" , Peter Zijlstra , Ingo Molnar , Patrick Bellasi , Juri Lelli , Luca Abeni , Todd Kjos , Claudio Scordino , kernel-team@android.com, Linux PM , "Joel Fernandes (Google.)" List-Id: linux-pm@vger.kernel.org On Tue, May 22, 2018 at 11:52:46PM +0200, Rafael J. Wysocki wrote: > On Tue, May 22, 2018 at 11:41 PM, Joel Fernandes wrote: > > On Tue, May 22, 2018 at 05:27:11PM +0200, Rafael J. Wysocki wrote: > >> On Tue, May 22, 2018 at 2:22 PM, Rafael J. Wysocki wrote: > >> > On Tuesday, May 22, 2018 1:42:05 PM CEST Rafael J. Wysocki wrote: > >> >> On Tue, May 22, 2018 at 1:38 PM, Viresh Kumar wrote: > >> >> > On 22-05-18, 13:31, Rafael J. Wysocki wrote: > >> >> >> So below is my (compiled-only) version of the $subject patch, obviously based > >> >> >> on the Joel's work. > >> >> >> > >> >> >> Roughly, what it does is to move the fast_switch_enabled path entirely to > >> >> >> sugov_update_single() and take the spinlock around sugov_update_commit() > >> >> >> in the one-CPU case too. > >> > > >> > [cut] > >> > > >> >> > > >> >> > Why do you assume that fast switch isn't possible in shared policy > >> >> > cases ? It infact is already enabled for few drivers. > >> > > >> > I hope that fast_switch is not used with devfs_possible_from_any_cpu set in the > >> > one-CPU policy case, as that looks racy even without any patching. > >> > >> Which would be the only case in which sugov_update_single() would run > >> on a CPU that is not the target. > >> > >> And running sugov_update_single() concurrently on two different CPUs > >> for the same target is a no-no, as we don't prevent concurrent updates > >> from occurring in that path. > >> > >> Which means that the original patch from Joel will be sufficient as > >> long as we ensure that sugov_update_single() can only run on one CPU > >> at a time. > > > > Since target CPU's runqueue lock is held, I don't see how we can run > > sugov_update_single concurrently with any other CPU for single policy, so > > protecting such race shouldn't be necessary. > > If dvfs_possible_from_any_cpu is set, any CPU can run > sugov_update_single(), but the kthread will only run on the target > itself. So another CPU running sugov_update_single() for the target > may be racing with the target's kthread. > Yes, I agree. I thought you meant the case of sugov_update_single running currently with other sugov_update_single. So just to be on the same page, I'll fix the commit log and repost this one as is. And then I'll post the smp_rmb() patch separately to address the memory order issue (which I believe is in mainline as well). Basically I was thinking to address Viresh's issue, there should be an smp_mb() after the next_freq is read, but before the write to work_in_progress. thanks, - Joel