Re: [PATCH v2 0/2] Fix DEADLINE bandwidth accounting in root domain changes and hotplug

[Juri Lelli]

Thanks Waiman and Phil for the super quick review/test of this v2!

On 14/11/24 14:28, Juri Lelli wrote:

...

> In all honesty, I still see intermittent issues that seems to however be
> related to the dance we do in sched_cpu_deactivate(), where we first
> turn everything related to a cpu/rq off and revert that if
> cpuset_cpu_inactive() reveals failing DEADLINE checks. But, since these
> seem to be orthogonal to the original discussion we started from, I
> wanted to send this out as an hopefully meaningful update/improvement
> since yesterday. Will continue looking into this.

About this that I mentioned, it looks like the below cures it (and
hopefully doesn't regress wrt the other 2 patches).

What do everybody think?

---
Subject: [PATCH] sched/deadline: Check bandwidth overflow earlier for hotplug

Currently we check for bandwidth overflow potentially due to hotplug
operations at the end of sched_cpu_deactivate(), after the cpu going
offline has already been removed from scheduling, active_mask, etc.
This can create issues for DEADLINE tasks, as there is a substantial
race window between the start of sched_cpu_deactivate() and the moment
we possibly decide to roll-back the operation if dl_bw_deactivate()
returns failure in cpuset_cpu_inactive(). An example is a throttled
task that sees its replenishment timer firing while the cpu it was
previously running on is considered offline, but before
dl_bw_deactivate() had a chance to say no and roll-back happened.

Fix this by directly calling dl_bw_deactivate() first thing in
sched_cpu_deactivate() and do the required calculation in the former
function considering the cpu passed as an argument as offline already.

Signed-off-by: Juri Lelli <juri.lelli@xxxxxxxxxx>
---
 kernel/sched/core.c     |  9 +++++----
 kernel/sched/deadline.c | 12 ++++++++++--
 2 files changed, 15 insertions(+), 6 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d1049e784510..43dfb3968eb8 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8057,10 +8057,6 @@ static void cpuset_cpu_active(void)
 static int cpuset_cpu_inactive(unsigned int cpu)
 {
 	if (!cpuhp_tasks_frozen) {
-		int ret = dl_bw_deactivate(cpu);
-
-		if (ret)
-			return ret;
 		cpuset_update_active_cpus();
 	} else {
 		num_cpus_frozen++;
@@ -8128,6 +8124,11 @@ int sched_cpu_deactivate(unsigned int cpu)
 	struct rq *rq = cpu_rq(cpu);
 	int ret;
 
+	ret = dl_bw_deactivate(cpu);
+
+	if (ret)
+		return ret;
+
 	/*
 	 * Remove CPU from nohz.idle_cpus_mask to prevent participating in
 	 * load balancing when not active
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 267ea8bacaf6..6e988d4cd787 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -3505,6 +3505,13 @@ static int dl_bw_manage(enum dl_bw_request req, int cpu, u64 dl_bw)
 		}
 		break;
 	case dl_bw_req_deactivate:
+		/*
+		 * cpu is not off yet, but we need to do the math by
+		 * considering it off already (i.e., what would happen if we
+		 * turn cpu off?).
+		 */
+		cap -= arch_scale_cpu_capacity(cpu);
+
 		/*
 		 * cpu is going offline and NORMAL tasks will be moved away
 		 * from it. We can thus discount dl_server bandwidth
@@ -3522,9 +3529,10 @@ static int dl_bw_manage(enum dl_bw_request req, int cpu, u64 dl_bw)
 		if (dl_b->total_bw - fair_server_bw > 0) {
 			/*
 			 * Leaving at least one CPU for DEADLINE tasks seems a
-			 * wise thing to do.
+			 * wise thing to do. As said above, cpu is not offline
+			 * yet, so account for that.
 			 */
-			if (dl_bw_cpus(cpu))
+			if (dl_bw_cpus(cpu) - 1)
 				overflow = __dl_overflow(dl_b, cap, fair_server_bw, 0);
 			else
 				overflow = 1;