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sched/deadline: Check bandwidth overflow earlier for hotplug

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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
This commit is contained in:
Juri Lelli 2024-11-15 11:48:29 +00:00 committed by Peter Zijlstra
parent d4742f6ed7
commit 53916d5fd3
2 changed files with 17 additions and 17 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

22
kernel/sched/core.c
View file

@ -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;
}

12
kernel/sched/deadline.c
View file

@ -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;