Re: [PATCH v3] sched: cpuset: Don't rebuild root domains on suspend-resume

[Qais Yousef]

On 03/01/23 08:31, Juri Lelli wrote:
> Hi,
> 
> On 28/02/23 17:46, Qais Yousef wrote:
> > On 02/28/23 15:09, Dietmar Eggemann wrote:
> > 
> > > > IIUC you're suggesting to introduce some new mechanism to detect if hotplug has
> > > > lead to a cpu to disappear or not and use that instead? Are you saying I can
> > > > use arch_update_cpu_topology() for that? Something like this?
> > > > 
> > > > 	diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> > > > 	index e5ddc8e11e5d..60c3dcf06f0d 100644
> > > > 	--- a/kernel/cgroup/cpuset.c
> > > > 	+++ b/kernel/cgroup/cpuset.c
> > > > 	@@ -1122,7 +1122,7 @@ partition_and_rebuild_sched_domains(int ndoms_new, cpumask_var_t doms_new[],
> > > > 	 {
> > > > 		mutex_lock(&sched_domains_mutex);
> > > > 		partition_sched_domains_locked(ndoms_new, doms_new, dattr_new);
> > > > 	-       if (update_dl_accounting)
> > > > 	+       if (arch_update_cpu_topology())
> > > > 			update_dl_rd_accounting();
> > > > 		mutex_unlock(&sched_domains_mutex);
> > > > 	 }
> > > 
> > > No, this is not what I meant. I'm just saying the:
> > > 
> > >   partition_sched_domains_locked()
> > >     new_topology = arch_update_cpu_topology();
> > > 
> > > has to be considered here as well since we do a
> > > `dl_clear_root_domain(rd)` (1) in partition_sched_domains_locked() for
> > > !new_topology.
> > 
> > Ah you're referring to the dl_clear_root_domain() call there. I thought this
> > doesn't trigger.
> > 
> > > 
> > > And (1) requires the `update_tasks_root_domain()` to happen later.
> > > 
> > > So there are cases now, e.g. `rebuild_sched_domains_energy()` in which
> > > `new_topology=0` and `update_dl_accounting=false` which now clean the rd
> > > but don't do a new DL accounting anymore.
> > > rebuild_root_domains() itself cleans the `default root domain`, not the
> > > other root domains which could exists as well.
> > > 
> > > Example: Switching CPUfreq policy [0,3-5] performance to schedutil (slow
> > > switching, i.e. we have sugov:X DL task(s)):
> > > 
> > > [  862.479906] CPU4 partition_sched_domains_locked() new_topology=0
> > > [  862.499073] Workqueue: events rebuild_sd_workfn
> > > [  862.503646] Call trace:
> > > ...
> > > [  862.520789]  partition_sched_domains_locked+0x6c/0x670
> > > [  862.525962]  rebuild_sched_domains_locked+0x204/0x8a0
> > > [  862.531050]  rebuild_sched_domains+0x2c/0x50
> > > [  862.535351]  rebuild_sd_workfn+0x38/0x54                        <-- !
> > > ...
> > > [  862.554047] CPU4 dl_clear_root_domain() rd->span=0-5 total_bw=0
> > > def_root_domain=0                                                  <-- !
> > > [  862.561597] CPU4 dl_clear_root_domain() rd->span= total_bw=0
> > > def_root_domain=1
> > > [  862.568960] CPU4 dl_add_task_root_domain() [sugov:0 1801]
> > > total_bw=104857 def_root_domain=0 rd=0xffff0008015f0000            <-- !
> > > 
> > > The dl_clear_root_domain() of the def_root_domain and the
> > > dl_add_task_root_domain() to the rd in use won't happen.
> > > 
> > > [sugov:0 1801] is only a simple example here. I could have spawned a
> > > couple of DL tasks before this to illustrate the issue more obvious.
> > > 
> > > ---
> > > 
> > > The same seems to happen during suspend/resume (system with 2 frequency
> > > domains, both with slow switching schedutil CPUfreq gov):
> > > 
> > > [   27.735821] CPU5 partition_sched_domains_locked() new_topology=0
> > > ...
> > > [   27.735864] Workqueue: events cpuset_hotplug_workfn
> > > [   27.735894] Call trace:
> > > ...
> > > [   27.735984]  partition_sched_domains_locked+0x6c/0x670
> > > [   27.736004]  rebuild_sched_domains_locked+0x204/0x8a0
> > > [   27.736026]  cpuset_hotplug_workfn+0x254/0x52c                  <-- !
> > > ...
> > > [   27.736155] CPU5 dl_clear_root_domain() rd->span=0-5 total_bw=0
> > > def_root_domain=0                                                  <-- !
> > > [   27.736178] CPU5 dl_clear_root_domain() rd->span= total_bw=0
> > > def_root_domain=1
> > > [   27.736296] CPU5 dl_add_task_root_domain() [sugov:0 80]         <-- !
> > >  total_bw=104857 def_root_domain=0 rd=0xffff000801728000
> > > [   27.736318] CPU5 dl_add_task_root_domain() [sugov:1 81]
> > > total_bw=209714 def_root_domain=0 rd=0xffff000801728000            <-- !
> > > ...
> > > 
> > > > I am not keen on this. arm64 seems to just read a value without a side effect.
> > > 
> > > Arm64 (among others) sets `update_topology=1` before
> > > `rebuild_sched_domains()` and `update_topology=0` after it in
> > > update_topology_flags_workfn(). This then makes `new_topology=1` in
> > > partition_sched_domains_locked().
> > > 
> > > > But x86 does reset this value so we can't read it twice in the same call tree
> > > > and I'll have to extract it.
> > > > 
> > > > The better solution that was discussed before is to not iterate through every
> > > > task in the system and let cpuset track when dl tasks are added to it and do
> > > > smarter iteration. ATM even if there are no dl tasks in the system we'll
> > > > blindly go through every task in the hierarchy to update nothing.
> > > 
> > > Yes, I can see the problem. And IMHO this solution approach seems to be
> > > better than parsing update_dl_accounting` through the stack of involved
> > > functions.
> > 
> > The best I can do is protect this dl_clear_root_domain() too. I really don't
> > have my heart in this but trying my best to help, but it has taken a lot of my
> > time already and would prefer to hand over to Juri to address this regression
> > if what I am proposing is not good enough.
> > 
> > FWIW, there are 0 dl tasks in the system where this was noticed. And this delay
> > is unbounded because it'll depend on how many tasks there are in the hierarchy.
> 
> Not ignoring you guys here, but it turns out I'm quite bogged down with
> other stuff at the moment. :/ So, apologies and I'll try to get to this
> asap. Thanks a lot for all your efforts and time reviewing so far!

Np, I can feel you :-)


Thanks

--
Qais Yousef