From mboxrd@z Thu Jan 1 00:00:00 1970 From: Waiman Long Subject: Re: [PATCH v2] sched: cpuset: Don't rebuild sched domains on suspend-resume Date: Sun, 29 Jan 2023 21:49:49 -0500 Message-ID: <45e0f8ea-d229-1ae7-5c12-7f0a64c6767a@redhat.com> References: <20230120194822.962958-1-qyousef@layalina.io> <20230125163546.pspvigh4groiwjy7@airbuntu> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1675046999; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=ua0a+7FLRjiYvvWmSYrY3NETYCBxmwh5dh1sUQ6IfFk=; b=hk7h5uPdqgl8y6CukA8/vJdTpCkspMRuNKltKK15T712RS3lAFX7bKSqpv6meOWaqzgRf8 3+Mh7iKLYtGtCTH/R96n8UPb2AEW5kIOsyO27BmCFRxejeUlesEVWOwJb+zKRRkIPYsM6F pgK0FEC5RGUU/ffn1cJ1u4V8EWvJzAw= Content-Language: en-US In-Reply-To: <20230125163546.pspvigh4groiwjy7@airbuntu> List-ID: Content-Type: text/plain; charset="us-ascii"; format="flowed" To: Qais Yousef Cc: Peter Zijlstra , Ingo Molnar , Juri Lelli , Steven Rostedt , tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, luca.abeni-5rdYK369eBLQB0XuIGIEkQ@public.gmane.org, claudio-YOzL5CV4y4YG1A2ADO40+w@public.gmane.org, tommaso.cucinotta-5rdYK369eBLQB0XuIGIEkQ@public.gmane.org, bristot-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org, mathieu.poirier-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org, Dietmar Eggemann , cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Vincent Guittot , Wei Wang , Rick Yiu , Quentin Perret On 1/25/23 11:35, Qais Yousef wrote: > On 01/20/23 17:16, Waiman Long wrote: >> On 1/20/23 14:48, Qais Yousef wrote: >>> Commit f9a25f776d78 ("cpusets: Rebuild root domain deadline accounting information") >>> enabled rebuilding sched domain on cpuset and hotplug operations to >>> correct deadline accounting. >>> >>> Rebuilding sched domain is a slow operation and we see 10+ ms delay on >>> suspend-resume because of that. >>> >>> Since nothing is expected to change on suspend-resume operation; skip >>> rebuilding the sched domains to regain the time lost. >>> >>> Debugged-by: Rick Yiu >>> Signed-off-by: Qais Yousef (Google) >>> --- >>> >>> Changes in v2: >>> * Remove redundant check in update_tasks_root_domain() (Thanks Waiman) >>> v1 link: >>> https://lore.kernel.org/lkml/20221216233501.gh6m75e7s66dmjgo@airbuntu/ >>> >>> kernel/cgroup/cpuset.c | 3 +++ >>> kernel/sched/deadline.c | 3 +++ >>> 2 files changed, 6 insertions(+) >>> >>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c >>> index a29c0b13706b..9a45f083459c 100644 >>> --- a/kernel/cgroup/cpuset.c >>> +++ b/kernel/cgroup/cpuset.c >>> @@ -1088,6 +1088,9 @@ static void rebuild_root_domains(void) >>> lockdep_assert_cpus_held(); >>> lockdep_assert_held(&sched_domains_mutex); >>> + if (cpuhp_tasks_frozen) >>> + return; >>> + >>> rcu_read_lock(); >>> /* >>> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c >>> index 0d97d54276cc..42c1143a3956 100644 >>> --- a/kernel/sched/deadline.c >>> +++ b/kernel/sched/deadline.c >>> @@ -2575,6 +2575,9 @@ void dl_clear_root_domain(struct root_domain *rd) >>> { >>> unsigned long flags; >>> + if (cpuhp_tasks_frozen) >>> + return; >>> + >>> raw_spin_lock_irqsave(&rd->dl_bw.lock, flags); >>> rd->dl_bw.total_bw = 0; >>> raw_spin_unlock_irqrestore(&rd->dl_bw.lock, flags); >> cpuhp_tasks_frozen is set when thaw_secondary_cpus() or >> freeze_secondary_cpus() is called. I don't know the exact suspend/resume >> calling sequences, will cpuhp_tasks_frozen be cleared at the end of resume >> sequence? Maybe we should make sure that rebuild_root_domain() is called at >> least once at the end of resume operation. > Very good questions. It made me look at the logic again and I realize now that > the way force_build behaves is causing this issue. > > I *think* we should just make the call rebuild_root_domains() only if > cpus_updated in cpuset_hotplug_workfn(). > > cpuset_cpu_active() seems to be the source of force_rebuild in my case; which > seems to be called only after the last cpu is back online (what you suggest). > In this case we can end up with cpus_updated = false, but force_rebuild = true. > > Now you added a couple of new users to force_rebuild in 4b842da276a8a; I'm > trying to figure out what the conditions would be there. It seems we can have > corner cases for cpus_update might not trigger correctly? > > Could the below be a good cure? > > AFAICT we must rebuild the root domains if something has changed in cpuset. > Which should be captured by either having: > > * cpus_updated = true > * force_rebuild && !cpuhp_tasks_frozen > > /me goes to test the patch > > --->8--- > > diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c > index a29c0b13706b..363e4459559f 100644 > --- a/kernel/cgroup/cpuset.c > +++ b/kernel/cgroup/cpuset.c > @@ -1079,6 +1079,8 @@ static void update_tasks_root_domain(struct cpuset *cs) > css_task_iter_end(&it); > } > > +static bool need_rebuild_rd = true; > + > static void rebuild_root_domains(void) > { > struct cpuset *cs = NULL; > @@ -1088,6 +1090,9 @@ static void rebuild_root_domains(void) > lockdep_assert_cpus_held(); > lockdep_assert_held(&sched_domains_mutex); > > + if (!need_rebuild_rd) > + return; > + > rcu_read_lock(); > > /* > @@ -3627,7 +3632,9 @@ static void cpuset_hotplug_workfn(struct work_struct *work) > /* rebuild sched domains if cpus_allowed has changed */ > if (cpus_updated || force_rebuild) { > force_rebuild = false; > + need_rebuild_rd = cpus_updated || (force_rebuild && !cpuhp_tasks_frozen); > rebuild_sched_domains(); > + need_rebuild_rd = true; You do the force_check check after it is set to false in the previous statement which is definitely not correct. So it will be false whenever cpus_updated is false. If you just want to skip rebuild_sched_domains() call for hotplug, why don't just skip the call here if the condition is right? Like /* rebuild sched domains if cpus_allowed has changed */ if (cpus_updated || (force_rebuild && !cpuhp_tasks_frozen)) { force_rebuild = false; rebuild_sched_domains(); } Still, we will need to confirm that cpuhp_tasks_frozen will be cleared outside of the suspend/resume cycle. Cheers, Longman