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

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 29f6b2475fdb8833683a511492e5c6ab86f9c974..1dee3f5ef940dc86d3683dd4cfe33d0c5b0ee5a3 100644 (file)
--- 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 fa787c7018a4c1259348d37c902b6fb033648d3b..1c8b8381dd2013542b802ad3b8950c9992f2a5c8 100644 (file)
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -3495,6 +3495,13 @@ 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
@@ -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;