sched/deadline: only set free_cpus for online runqueues

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>

diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
index cdd740b3f774365aa2ad38577fe1d47d769ae65d..37b572cc8aca2e1b2e895560c8a3345546cc5aca 100644 (file)
--- 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);
@@ -237,26 +241,6 @@ void cpudl_set(struct cpudl *cp, int cpu, u64 dl)
        raw_spin_unlock_irqrestore(&cp->lock, flags);
 }
 
-/*
- * 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

diff --git a/kernel/sched/cpudeadline.h b/kernel/sched/cpudeadline.h
index 11c0f1faa7e11d494b48685331fd684b57168908..d7699468eedd57d8424b8dcb7b7612b2b8f58530 100644 (file)
--- a/kernel/sched/cpudeadline.h
+++ b/kernel/sched/cpudeadline.h
@@ -19,8 +19,6 @@ struct cpudl {
 
 int  cpudl_find(struct cpudl *cp, struct task_struct *p, struct cpumask *later_mask);
 void cpudl_set(struct cpudl *cp, int cpu, u64 dl);
-void cpudl_clear(struct cpudl *cp, int cpu);
+void cpudl_clear(struct cpudl *cp, int cpu, bool online);
 int  cpudl_init(struct cpudl *cp);
-void cpudl_set_freecpu(struct cpudl *cp, int cpu);
-void cpudl_clear_freecpu(struct cpudl *cp, int cpu);
 void cpudl_cleanup(struct cpudl *cp);

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 615411a0a8813d44d817bfef6aee945437e362f1..933bd1f82c7456ec7ea147b7e020d886471a3fd7 100644 (file)
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1808,7 +1808,7 @@ static void dec_dl_deadline(struct dl_rq *dl_rq, u64 deadline)
        if (!dl_rq->dl_nr_running) {
                dl_rq->earliest_dl.curr = 0;
                dl_rq->earliest_dl.next = 0;
-               cpudl_clear(&rq->rd->cpudl, rq->cpu);
+               cpudl_clear(&rq->rd->cpudl, rq->cpu, rq->online);
                cpupri_set(&rq->rd->cpupri, rq->cpu, rq->rt.highest_prio.curr);
        } else {
                struct rb_node *leftmost = rb_first_cached(&dl_rq->root);
@@ -2880,9 +2880,10 @@ static void rq_online_dl(struct rq *rq)
        if (rq->dl.overloaded)
                dl_set_overload(rq);
 
-       cpudl_set_freecpu(&rq->rd->cpudl, rq->cpu);
        if (rq->dl.dl_nr_running > 0)
                cpudl_set(&rq->rd->cpudl, rq->cpu, rq->dl.earliest_dl.curr);
+       else
+               cpudl_clear(&rq->rd->cpudl, rq->cpu, true);
 }
 
 /* Assumes rq->lock is held */
@@ -2891,8 +2892,7 @@ static void rq_offline_dl(struct rq *rq)
        if (rq->dl.overloaded)
                dl_clear_overload(rq);
 
-       cpudl_clear(&rq->rd->cpudl, rq->cpu);
-       cpudl_clear_freecpu(&rq->rd->cpudl, rq->cpu);
+       cpudl_clear(&rq->rd->cpudl, rq->cpu, false);
 }
 
 void __init init_sched_dl_class(void)