From mboxrd@z Thu Jan 1 00:00:00 1970 From: Quentin Perret Subject: Re: [RFC PATCH 5/6] sched/fair: Select an energy-efficient CPU on task wake-up Date: Sat, 24 Mar 2018 01:22:06 +0000 Message-ID: <20180324012205.GA1317@queper01-VirtualBox> References: <20180320094312.24081-1-dietmar.eggemann@arm.com> <20180320094312.24081-6-dietmar.eggemann@arm.com> <20180321153518.GC13951@e110439-lin> <20180323154745.GP4589@e105550-lin.cambridge.arm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20180323154745.GP4589@e105550-lin.cambridge.arm.com> Sender: linux-kernel-owner@vger.kernel.org To: Morten Rasmussen Cc: Joel Fernandes , Patrick Bellasi , Dietmar Eggemann , LKML , Peter Zijlstra , Thara Gopinath , Linux PM , Chris Redpath , Valentin Schneider , "Rafael J . Wysocki" , Greg Kroah-Hartman , Vincent Guittot , Viresh Kumar , Todd Kjos List-Id: linux-pm@vger.kernel.org On Friday 23 Mar 2018 at 15:47:45 (+0000), Morten Rasmussen wrote: > On Thu, Mar 22, 2018 at 01:10:22PM -0700, Joel Fernandes wrote: > > On Wed, Mar 21, 2018 at 8:35 AM, Patrick Bellasi > > wrote: > > > [...] > > > > > >> @@ -6555,6 +6613,14 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f > > >> break; > > >> } > > >> > > >> + /* > > >> + * Energy-aware task placement is performed on the highest > > >> + * non-overutilized domain spanning over cpu and prev_cpu. > > >> + */ > > >> + if (want_energy && !sd_overutilized(tmp) && > > >> + cpumask_test_cpu(prev_cpu, sched_domain_span(tmp))) > > >> + energy_sd = tmp; > > >> + > > > > > > Not entirely sure, but I was trying to understand if we can avoid to > > > modify the definition of want_affine (in the previous chunk) and move > > > this block before the previous "if (want_affine..." (in mainline but > > > not in this chunk), which will became an else, e.g. > > > > > > if (want_energy && !sd_overutilized(tmp) && > > > // ... > > > else if (want_energy && !sd_overutilized(tmp) && > > > // ... > > > > > > Isn't that the same? > > > > > > Maybe there is a code path I'm missing... but otherwise it seems a > > > more self contained modification of select_task_rq_fair... > > > > Just replying to this here Patrick instead of the other thread. > > > > I think this is the right place for the block from Quentin quoted > > above because we want to search for the highest domain that is > > !overutilized and look among those for the candidates. So from that > > perspective, we can't move the block to the beginning and it seems to > > be in the right place. My main concern on the other thread was > > different, I was talking about the cases where sd_flag & tmp->flags > > don't match. In that case, sd = NULL would trump EAS and I was > > wondering if that's the right thing to do... > > You mean if SD_BALANCE_WAKE isn't set on sched_domains? > > The current code seems to rely on that flag to be set to work correctly. > Otherwise, the loop might bail out on !want_affine and we end up doing > the find_energy_efficient_cpu() on the lowest level sched_domain even if > there is higher level one which isn't over-utilized. > > However, SD_BALANCE_WAKE should be set if SD_ASYM_CPUCAPACITY is set so > sd == NULL shouldn't be possible? This only holds as long as we only > want EAS for asymmetric systems. That's correct, we are under the assumption that the SD_ASYM_CPUCAPACITY flag is set somewhere in the hierarchy here. If a sched domain has this flag set, SD_BALANCE_WAKE is propagated to all lower sched domains (see sd_init() in kernel/sched/topology.c) so we should be fine.