[tip: sched/core] sched/deadline: Check bandwidth overflow earlier for hotplug

tip-bot2 for Juri Lelli
2024-12-02T11:14:44Z

The following commit has been merged into the sched/core branch of tip:

Commit-ID: 53916d5fd3c0b658de3463439dd2b7ce765072cb
Gitweb: https://git.kernel.org/tip/53916d5fd3c0b658de3463439dd2b7ce765072cb
Author: Juri Lelli <juri.lelli@redhat.com>
AuthorDate: Fri, 15 Nov 2024 11:48:29
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Mon, 02 Dec 2024 12:01:31 +01:00

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.

By doing so we also simplify sched_cpu_deactivate(), as there is no need
anymore for any kind of roll-back if we fail early.

Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Phil Auld <pauld@redhat.com>
Tested-by: Waiman Long <longman@redhat.com>
Link: https://lore.kernel.org/r/Zzc1DfPhbvqDDIJR@jlelli-thinkpadt14gen4.remote.csb
---
kernel/sched/core.c | 22 +++++++---------------
kernel/sched/deadline.c | 12 ++++++++++--
2 files changed, 17 insertions(+), 17 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 29f6b24..1dee3f5 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8182,19 +8182,14 @@ static void cpuset_cpu_active(void)
cpuset_update_active_cpus();
}

-static int cpuset_cpu_inactive(unsigned int cpu)
+static void 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++;
partition_sched_domains(1, NULL, NULL);
}
- return 0;
}

static inline void sched_smt_present_inc(int cpu)
@@ -8256,6 +8251,11 @@ 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
@@ -8301,15 +8301,7 @@ int sched_cpu_deactivate(unsigned int cpu)
return 0;

sched_update_numa(cpu, false);
- ret = cpuset_cpu_inactive(cpu);
- if (ret) {
- sched_smt_present_inc(cpu);
- sched_set_rq_online(rq, cpu);
- balance_push_set(cpu, false);
- set_cpu_active(cpu, true);
- sched_update_numa(cpu, true);
- return ret;
- }
+ cpuset_cpu_inactive(cpu);
sched_domains_numa_masks_clear(cpu);
return 0;
}
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index fa787c7..1c8b838 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -3496,6 +3496,13 @@ 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
* contribution as it won't need to be servicing tasks after
@@ -3512,9 +3519,10 @@ 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;