Re: [PATCH v3 4/8] sched/deadline: Rebuild root domain accounting after every update

[Waiman Long]

On 3/11/25 9:29 AM, Dietmar Eggemann wrote:
> On 11/03/2025 13:34, Waiman Long wrote:
> > On 3/11/25 7:59 AM, Juri Lelli wrote:
> > > On 10/03/25 20:16, Waiman Long wrote:
> > > > On 3/10/25 3:18 PM, Waiman Long wrote:
> > > > > On 3/10/25 2:54 PM, Dietmar Eggemann wrote:
> > > > > > On 10/03/2025 10:37, Juri Lelli wrote:
> > > > > > > Rebuilding of root domains accounting information (total_bw) is
> > > > > > > currently broken on some cases, e.g. suspend/resume on aarch64.
> > > > > > > Problem
> > > > > > Nit: Couldn't spot any arch dependency here. I guess it was just
> > > > > > tested
> > > > > > on Arm64 platforms so far.
> > > > > >
> > > > > > [...]
> > > > > >
> > > > > > > diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> > > > > > > index 44093339761c..363ad268a25b 100644
> > > > > > > --- a/kernel/sched/topology.c
> > > > > > > +++ b/kernel/sched/topology.c
> > > > > > > @@ -2791,6 +2791,7 @@ void partition_sched_domains_locked(int
> > > > > > > ndoms_new, cpumask_var_t doms_new[],
> > > > > > >         ndoms_cur = ndoms_new;
> > > > > > >           update_sched_domain_debugfs();
> > > > > > > +    dl_rebuild_rd_accounting();
> > > > > > Won't dl_rebuild_rd_accounting()'s lockdep_assert_held(&cpuset_mutex)
> > > > > > barf when called via cpuhp's:
> > > > > >
> > > > > > sched_cpu_deactivate()
> > > > > >
> > > > > >      cpuset_cpu_inactive()
> > > > > >
> > > > > >        partition_sched_domains()
> > > > > >
> > > > > >          partition_sched_domains_locked()
> > > > > >
> > > > > >            dl_rebuild_rd_accounting()
> > > > > >
> > > > > > ?
> > > Good catch. Guess I didn't notice while testing with LOCKDEP as I was
> > > never able to hit this call path on my systems.
> > >
> > > > > Right. If cpuhp_tasks_frozen is true, partition_sched_domains() will be
> > > > > called without holding cpuset mutex.
> > > > >
> > > > > Well, I think we will need an additional wrapper in cpuset.c that
> > > > > acquires the cpuset_mutex first before calling
> > > > > partition_sched_domains()
> > > > > and use the new wrapper in these cases.
> > > > Actually, partition_sched_domains() is called with the special
> > > > arguments (1,
> > > > NULL, NULL) to reset the domain to a single one. So perhaps something
> > > > like
> > > > the following will be enough to avoid this problem.
> > > I think this would work, as we will still rebuild the accounting after
> > > last CPU comes back from suspend. The thing I am still not sure about is
> > > what we want to do in case we have DEADLINE tasks around, since with
> > > this I belive we would be ignoring them and let suspend proceed.
> > That is the current behavior. You can certainly create a test case to
> > trigger such condition and see what to do about it. Alternatively, you
> > can document that and come up with a follow-up patch later on.
> But don't we rely on that partition_sched_domains_locked() calls
> dl_rebuild_rd_accounting() even in the reset_domain=1 case?
>
> Testcase: suspend/resume
>
> on Arm64 big.LITTLE cpumask=[LITTLE][big]=[0,3-5][1-2]
> plus cmd line option 'isolcpus=3,4'.
>
> with Waiman's snippet:
> https://lkml.kernel.org/r/fd4d6143-9bd2-4a7c-80dc-1e19e4d1b2d1@xxxxxxxxxx
>
> ...
> [  234.831675] --- > partition_sched_domains_locked() reset_domain=1
> [  234.835966] psci: CPU4 killed (polled 0 ms)
> [  234.838912] Error taking CPU3 down: -16
> [  234.838952] Non-boot CPUs are not disabled
> [  234.838986] Enabling non-boot CPUs ...
> ...
>
> IIRC, that's the old DL accounting issue.

You are right. cpuhp_tasks_frozen will be set in the suspend/resume 
case. In that case, we do need to add a cpuset helper to acquire the 
cpuset_mutex. A test patch as follows (no testing done yet):

diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
index c414daa7d503..ef1ffb9c52b0 100644
--- a/include/linux/cpuset.h
+++ b/include/linux/cpuset.h
@@ -129,6 +129,7 @@ extern void dl_rebuild_rd_accounting(void);
 extern void rebuild_sched_domains(void);

 extern void cpuset_print_current_mems_allowed(void);
+extern void cpuset_reset_sched_domains(void)

 /*
  * read_mems_allowed_begin is required when making decisions involving
@@ -269,6 +270,11 @@ static inline void rebuild_sched_domains(void)
        partition_sched_domains(1, NULL, NULL);
 }

+static inline void cpuset_reset_sched_domains(void)
+{
+       partition_sched_domains(1, NULL, NULL);
+}
+
 static inline void cpuset_print_current_mems_allowed(void)
 {
 }
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 7995cd58a01b..a51099e5d587 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1076,6 +1076,13 @@ void rebuild_sched_domains(void)
        cpus_read_unlock();
 }

+void cpuset_reset_sched_domains(void)
+{
+       mutex_lock(&cpuset_mutex);
+       partition_sched_domains(1, NULL, NULL);
+       mutex_unlock(&cpuset_mutex);
+}
+
 /**
  * cpuset_update_tasks_cpumask - Update the cpumasks of tasks in the 
cpuset.
  * @cs: the cpuset in which each task's cpus_allowed mask needs to be 
changed
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 58593f4d09a1..dbf44ddbb6b4 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8183,7 +8183,7 @@ static void cpuset_cpu_active(void)
                 * operation in the resume sequence, just build a 
single sched
                 * domain, ignoring cpusets.
                 */
-               partition_sched_domains(1, NULL, NULL);
+               cpuset_reset_sched_domains();
                if (--num_cpus_frozen)
                        return;
                /*
@@ -8202,7 +8202,7 @@ static void cpuset_cpu_inactive(unsigned int cpu)
                cpuset_update_active_cpus();
        } else {
                num_cpus_frozen++;
-               partition_sched_domains(1, NULL, NULL);
+               cpuset_reset_sched_domains();
        }
 }

Cheers,
Longman

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