sched/deadline: only set free_cpus for online runqueues

[ Upstream commit 382748c05e58a9f1935f5a653c352422375566ea ]

Commit 16b269436b72 ("sched/deadline: Modify cpudl::free_cpus
to reflect rd->online") introduced the cpudl_set/clear_freecpu
functions to allow the cpu_dl::free_cpus mask to be manipulated
by the deadline scheduler class rq_on/offline callbacks so the
mask would also reflect this state.

Commit 9659e1eeee28 ("sched/deadline: Remove cpu_active_mask
from cpudl_find()") removed the check of the cpu_active_mask to
save some processing on the premise that the cpudl::free_cpus
mask already reflected the runqueue online state.

Unfortunately, there are cases where it is possible for the
cpudl_clear function to set the free_cpus bit for a CPU when the
deadline runqueue is offline. When this occurs while a CPU is
connected to the default root domain the flag may retain the bad
state after the CPU has been unplugged. Later, a different CPU
that is transitioning through the default root domain may push a
deadline task to the powered down CPU when cpudl_find sees its
free_cpus bit is set. If this happens the task will not have the
opportunity to run.

One example is outlined here:
https://lore.kernel.org/lkml/20250110233010.2339521-1-opendmb@gmail.com

Another occurs when the last deadline task is migrated from a
CPU that has an offlined runqueue. The dequeue_task member of
the deadline scheduler class will eventually call cpudl_clear
and set the free_cpus bit for the CPU.

This commit modifies the cpudl_clear function to be aware of the
online state of the deadline runqueue so that the free_cpus mask
can be updated appropriately.

It is no longer necessary to manage the mask outside of the
cpudl_set/clear functions so the cpudl_set/clear_freecpu
functions are removed. In addition, since the free_cpus mask is
now only updated under the cpudl lock the code was changed to
use the non-atomic __cpumask functions.

Signed-off-by: Doug Berger <opendmb@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

1 files changed, 9 insertions, 25 deletions

diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
index cdd740b3f774..37b572cc8aca 100644
--- a/kernel/sched/cpudeadline.c
+++ b/kernel/sched/cpudeadline.c
@@ -166,12 +166,13 @@ int cpudl_find(struct cpudl *cp, struct task_struct *p,
  * cpudl_clear - remove a CPU from the cpudl max-heap
  * @cp: the cpudl max-heap context
  * @cpu: the target CPU
+ * @online: the online state of the deadline runqueue
  *
  * Notes: assumes cpu_rq(cpu)->lock is locked
  *
  * Returns: (void)
  */
-void cpudl_clear(struct cpudl *cp, int cpu)
+void cpudl_clear(struct cpudl *cp, int cpu, bool online)
 {
 	int old_idx, new_cpu;
 	unsigned long flags;
@@ -184,7 +185,7 @@ void cpudl_clear(struct cpudl *cp, int cpu)
 	if (old_idx == IDX_INVALID) {
 		/*
 		 * Nothing to remove if old_idx was invalid.
-		 * This could happen if a rq_offline_dl is
+		 * This could happen if rq_online_dl or rq_offline_dl is
 		 * called for a CPU without -dl tasks running.
 		 */
 	} else {
@@ -195,9 +196,12 @@ void cpudl_clear(struct cpudl *cp, int cpu)
 		cp->elements[new_cpu].idx = old_idx;
 		cp->elements[cpu].idx = IDX_INVALID;
 		cpudl_heapify(cp, old_idx);
-
-		cpumask_set_cpu(cpu, cp->free_cpus);
 	}
+	if (likely(online))
+		__cpumask_set_cpu(cpu, cp->free_cpus);
+	else
+		__cpumask_clear_cpu(cpu, cp->free_cpus);
+
 	raw_spin_unlock_irqrestore(&cp->lock, flags);
 }
 
@@ -228,7 +232,7 @@ void cpudl_set(struct cpudl *cp, int cpu, u64 dl)
 		cp->elements[new_idx].cpu = cpu;
 		cp->elements[cpu].idx = new_idx;
 		cpudl_heapify_up(cp, new_idx);
-		cpumask_clear_cpu(cpu, cp->free_cpus);
+		__cpumask_clear_cpu(cpu, cp->free_cpus);
 	} else {
 		cp->elements[old_idx].dl = dl;
 		cpudl_heapify(cp, old_idx);
@@ -238,26 +242,6 @@ void cpudl_set(struct cpudl *cp, int cpu, u64 dl)
 }
 
 /*
- * cpudl_set_freecpu - Set the cpudl.free_cpus
- * @cp: the cpudl max-heap context
- * @cpu: rd attached CPU
- */
-void cpudl_set_freecpu(struct cpudl *cp, int cpu)
-{
-	cpumask_set_cpu(cpu, cp->free_cpus);
-}
-
-/*
- * cpudl_clear_freecpu - Clear the cpudl.free_cpus
- * @cp: the cpudl max-heap context
- * @cpu: rd attached CPU
- */
-void cpudl_clear_freecpu(struct cpudl *cp, int cpu)
-{
-	cpumask_clear_cpu(cpu, cp->free_cpus);
-}
-
-/*
  * cpudl_init - initialize the cpudl structure
  * @cp: the cpudl max-heap context
  */