[PATCH sched/urgent] sched/fair: set_load_weight() must also call reweight_task() for SCHED_IDLE tasks

Tejun Heo
2024-06-26T01:30:01Z

When a task's weight is being changed, set_load_weight() is called with
@update_load set. As weight changes aren't trivial for the fair class,
set_load_weight() calls fair.c::reweight_task() for fair class tasks.

However, set_load_weight() first tests task_has_idle_policy() on entry and
skips calling reweight_task() for SCHED_IDLE tasks. This is buggy as
SCHED_IDLE tasks are just fair tasks with a very low weight and they would
incorrectly skip load, vlag and position updates.

Fix it by updating reweight_task() to take struct load_weight as idle weight
can't be expressed with prio and making set_load_weight() call
reweight_task() for SCHED_IDLE tasks too when @update_load is set.

Signed-off-by: Tejun Heo <tj@kernel.org>
Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20240624102331.GI31592@noisy.programming.kicks-ass.net
Fixes: 9059393e4ec1 ("sched/fair: Use reweight_entity() for set_user_nice()")
Cc: stable@vger.kernel.org # v4.15+
---
Hello,

Peter, I took the liberty to use struct load_weight instead of open coding
weight and inv_weight. As it's an obviously user visible bug, I tagged it
with the stable tag but given how long it's been broken, I'm not sure how
urgent it is, so please use your discretion. I verified that the
reweight_task() is called with the right weights on SCHED_IDLE switches.

Thanks.

kernel/sched/core.c | 23 ++++++++++-------------
kernel/sched/fair.c | 7 +++----
kernel/sched/sched.h | 2 +-
3 files changed, 14 insertions(+), 18 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1328,27 +1328,24 @@ int tg_nop(struct task_group *tg, void *
void set_load_weight(struct task_struct *p, bool update_load)
{
int prio = p->static_prio - MAX_RT_PRIO;
- struct load_weight *load = &p->se.load;
+ struct load_weight lw;

- /*
- * SCHED_IDLE tasks get minimal weight:
- */
if (task_has_idle_policy(p)) {
- load->weight = scale_load(WEIGHT_IDLEPRIO);
- load->inv_weight = WMULT_IDLEPRIO;
- return;
+ lw.weight = scale_load(WEIGHT_IDLEPRIO);
+ lw.inv_weight = WMULT_IDLEPRIO;
+ } else {
+ lw.weight = scale_load(sched_prio_to_weight[prio]);
+ lw.inv_weight = sched_prio_to_wmult[prio];
}

/*
* SCHED_OTHER tasks have to update their load when changing their
* weight
*/
- if (update_load && p->sched_class == &fair_sched_class) {
- reweight_task(p, prio);
- } else {
- load->weight = scale_load(sched_prio_to_weight[prio]);
- load->inv_weight = sched_prio_to_wmult[prio];
- }
+ if (update_load && p->sched_class == &fair_sched_class)
+ reweight_task(p, &lw);
+ else
+ p->se.load = lw;
}

#ifdef CONFIG_UCLAMP_TASK
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3835,15 +3835,14 @@ static void reweight_entity(struct cfs_r
}
}

-void reweight_task(struct task_struct *p, int prio)
+void reweight_task(struct task_struct *p, const struct load_weight *lw)
{
struct sched_entity *se = &p->se;
struct cfs_rq *cfs_rq = cfs_rq_of(se);
struct load_weight *load = &se->load;
- unsigned long weight = scale_load(sched_prio_to_weight[prio]);

- reweight_entity(cfs_rq, se, weight);
- load->inv_weight = sched_prio_to_wmult[prio];
+ reweight_entity(cfs_rq, se, lw->weight);
+ load->inv_weight = lw->inv_weight;
}

static inline int throttled_hierarchy(struct cfs_rq *cfs_rq);
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2509,7 +2509,7 @@ extern void init_sched_dl_class(void);
extern void init_sched_rt_class(void);
extern void init_sched_fair_class(void);

-extern void reweight_task(struct task_struct *p, int prio);
+extern void reweight_task(struct task_struct *p, const struct load_weight *lw);

extern void resched_curr(struct rq *rq);
extern void resched_cpu(int cpu);