Commit 53916d5f authored by Juri Lelli's avatar Juri Lelli Committed by Peter Zijlstra
Browse files

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: default avatarJuri Lelli <juri.lelli@redhat.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarPhil Auld <pauld@redhat.com>
Tested-by: default avatarWaiman Long <longman@redhat.com>
Link: https://lore.kernel.org/r/Zzc1DfPhbvqDDIJR@jlelli-thinkpadt14gen4.remote.csb
parent d4742f6e

kernel/sched/core.c

+7 −15
Original line number Diff line number Diff line
@@ -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;

}

kernel/sched/deadline.c

+10 −2
Original line number Diff line number Diff line
@@ -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;