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sched/deadline: only set free_cpus for online runqueues

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Commit 16b269436b ("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 9659e1eeee ("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>
This commit is contained in:
Doug Berger 2025-08-14 18:22:36 -07:00 • committed by Peter Zijlstra
parent 79104becf4
commit 382748c05e
3 changed files with 14 additions and 32 deletions
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34
kernel/sched/cpudeadline.c
View file

@ -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

4
kernel/sched/cpudeadline.h
View file

@ -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);

8
kernel/sched/deadline.c
View file

@ -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)