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 <rickyiu@xxxxxxxxxx>
Signed-off-by: Qais Yousef (Google) <qyousef@xxxxxxxxxxx>
---
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
