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Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.6.0 Subject: Re: [PATCH v2] sched: cpuset: Don't rebuild sched domains on suspend-resume Content-Language: en-US From: Waiman Long To: Qais Yousef Cc: Peter Zijlstra , Ingo Molnar , Juri Lelli , Steven Rostedt , tj@kernel.org, linux-kernel@vger.kernel.org, luca.abeni@santannapisa.it, claudio@evidence.eu.com, tommaso.cucinotta@santannapisa.it, bristot@redhat.com, mathieu.poirier@linaro.org, Dietmar Eggemann , cgroups@vger.kernel.org, Vincent Guittot , Wei Wang , Rick Yiu , Quentin Perret References: <20230120194822.962958-1-qyousef@layalina.io> <20230125163546.pspvigh4groiwjy7@airbuntu> <45e0f8ea-d229-1ae7-5c12-7f0a64c6767a@redhat.com> In-Reply-To: <45e0f8ea-d229-1ae7-5c12-7f0a64c6767a@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.1 on 10.11.54.3 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 1/29/23 21:49, Waiman Long wrote: > 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. BTW, you also need to expand the comment to explain why we need to check for cpuhp_tasks_frozen. Cheers, Longman