* [PATCH v4 1/2] sched/fair: Rebuild load weight when switching to fair
2026-06-24 9:16 ` [PATCH v4 0/2] sched/fair: Fix fair load state when switching to fair Zicheng Qu
@ 2026-06-24 9:16 ` Zicheng Qu
2026-06-24 9:16 ` [PATCH v4 2/2] sched/fair: Keep load_avg in sync after reweighting Zicheng Qu
2026-06-24 11:57 ` [PATCH v4 0/2] sched/fair: Fix fair load state when switching to fair Andrea Righi
2 siblings, 0 replies; 4+ messages in thread
From: Zicheng Qu @ 2026-06-24 9:16 UTC (permalink / raw)
To: sashiko-bot
Cc: sashiko-reviews, linux-kernel, sched-ext, peterz, arighi, brho,
bsegall, changwoo, dietmar.eggemann, haoluo, joshdon, juri.lelli,
kprateek.nayak, mgorman, mingo, rostedt, tj, vincent.guittot,
void, vschneid, tanghui20, zhangqiao22, quzicheng, quzicheng315
Tasks that run outside fair may not keep p->se.load in sync with their
current scheduling policy and static priority. sched_ext, for example,
uses p->scx.weight as the active scheduling weight, so p->se.load can be
stale when a task moves back to fair.
The fair_sched_class expects the sched_entity load weight to be valid
before the task is enqueued. Rebuild it from fair's switching_to hook,
which runs after the class has been changed to fair and before enqueue,
so both sched_ext disable and SCHED_EXT to SCHED_NORMAL transitions get
a native fair load weight.
Fixes: f0e1a0643a59 ("sched_ext: Implement BPF extensible scheduler class")
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Zicheng Qu <quzicheng@huawei.com>
Acked-by: Tejun Heo <tj@kernel.org>
---
Changes in v4:
- No code changes.
- Add Tejun's Acked-by.
- Split the PELT load_avg synchronization into patch 2.
Changes in v3:
- Move the rebuild into fair's switching_to hook, as suggested by Peter.
This lets fair prepare its own state before enqueue and avoids adding a
sched_ext/fair-specific fixup to the generic sched_change_end() path.
Changes in v2:
- Move the fix from scx_root_disable() to the class switch path so it also
covers partial-mode SCHED_EXT to SCHED_NORMAL transitions through
sched_setscheduler(). Andrea identified this missing case in the v1
discussion.
kernel/sched/fair.c | 10 ++++++++++
1 file changed, 10 insertions(+)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d78467ec6ee1..edb11065a9fc 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -14998,6 +14998,15 @@ static void switching_from_fair(struct rq *rq, struct task_struct *p)
dequeue_task(rq, p, DEQUEUE_SLEEP | DEQUEUE_DELAYED | DEQUEUE_NOCLOCK);
}
+static void switching_to_fair(struct rq *rq, struct task_struct *p)
+{
+ /*
+ * Tasks may come from classes that don't keep se.load up to date.
+ * Rebuild it before the task is enqueued.
+ */
+ set_load_weight(p, false);
+}
+
static void switched_from_fair(struct rq *rq, struct task_struct *p)
{
detach_task_cfs_rq(p);
@@ -15379,6 +15388,7 @@ DEFINE_SCHED_CLASS(fair) = {
.reweight_task = reweight_task_fair,
.prio_changed = prio_changed_fair,
.switching_from = switching_from_fair,
+ .switching_to = switching_to_fair,
.switched_from = switched_from_fair,
.switched_to = switched_to_fair,
--
2.53.0
^ permalink raw reply related [flat|nested] 4+ messages in thread* [PATCH v4 2/2] sched/fair: Keep load_avg in sync after reweighting
2026-06-24 9:16 ` [PATCH v4 0/2] sched/fair: Fix fair load state when switching to fair Zicheng Qu
2026-06-24 9:16 ` [PATCH v4 1/2] sched/fair: Rebuild load weight " Zicheng Qu
@ 2026-06-24 9:16 ` Zicheng Qu
2026-06-24 11:57 ` [PATCH v4 0/2] sched/fair: Fix fair load state when switching to fair Andrea Righi
2 siblings, 0 replies; 4+ messages in thread
From: Zicheng Qu @ 2026-06-24 9:16 UTC (permalink / raw)
To: sashiko-bot
Cc: sashiko-reviews, linux-kernel, sched-ext, peterz, arighi, brho,
bsegall, changwoo, dietmar.eggemann, haoluo, joshdon, juri.lelli,
kprateek.nayak, mgorman, mingo, rostedt, tj, vincent.guittot,
void, vschneid, tanghui20, zhangqiao22, quzicheng, quzicheng315
se->avg.load_avg is derived from se->avg.load_sum and se_weight().
When the fair load weight is rebuilt while switching back to fair,
load_avg must be rebuilt too before the entity is attached.
Factor the existing reweight_entity() calculation into a helper and use
it from switching_to_fair() as well.
Link: https://lore.kernel.org/all/20260530050609.0D0FB1F00893@smtp.kernel.org/
Signed-off-by: Zicheng Qu <quzicheng@huawei.com>
---
Changes in v4:
- New patch.
kernel/sched/fair.c | 14 +++++++++-----
1 file changed, 9 insertions(+), 5 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index edb11065a9fc..19270132a506 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4663,6 +4663,13 @@ rescale_entity(struct sched_entity *se, unsigned long weight, bool rel_vprot)
se->vprot = div64_long(se->vprot * old_weight, weight);
}
+static inline void update_load_avg_after_reweight(struct sched_entity *se)
+{
+ u32 divider = get_pelt_divider(&se->avg);
+
+ se->avg.load_avg = div_u64(se_weight(se) * se->avg.load_sum, divider);
+}
+
static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
unsigned long weight)
{
@@ -4692,11 +4699,7 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
rescale_entity(se, weight, rel_vprot);
update_load_set(&se->load, weight);
-
- do {
- u32 divider = get_pelt_divider(&se->avg);
- se->avg.load_avg = div_u64(se_weight(se) * se->avg.load_sum, divider);
- } while (0);
+ update_load_avg_after_reweight(se);
enqueue_load_avg(cfs_rq, se);
if (se->on_rq) {
@@ -15005,6 +15008,7 @@ static void switching_to_fair(struct rq *rq, struct task_struct *p)
* Rebuild it before the task is enqueued.
*/
set_load_weight(p, false);
+ update_load_avg_after_reweight(&p->se);
}
static void switched_from_fair(struct rq *rq, struct task_struct *p)
--
2.53.0
^ permalink raw reply related [flat|nested] 4+ messages in thread* Re: [PATCH v4 0/2] sched/fair: Fix fair load state when switching to fair
2026-06-24 9:16 ` [PATCH v4 0/2] sched/fair: Fix fair load state when switching to fair Zicheng Qu
2026-06-24 9:16 ` [PATCH v4 1/2] sched/fair: Rebuild load weight " Zicheng Qu
2026-06-24 9:16 ` [PATCH v4 2/2] sched/fair: Keep load_avg in sync after reweighting Zicheng Qu
@ 2026-06-24 11:57 ` Andrea Righi
2 siblings, 0 replies; 4+ messages in thread
From: Andrea Righi @ 2026-06-24 11:57 UTC (permalink / raw)
To: Zicheng Qu
Cc: sashiko-bot, sashiko-reviews, linux-kernel, sched-ext, peterz,
brho, bsegall, changwoo, dietmar.eggemann, haoluo, joshdon,
juri.lelli, kprateek.nayak, mgorman, mingo, rostedt, tj,
vincent.guittot, void, vschneid, tanghui20, zhangqiao22,
quzicheng315
Hi Zicheng,
On Wed, Jun 24, 2026 at 09:16:46AM +0000, Zicheng Qu wrote:
> Tasks outside fair may leave fair's sched_entity load state stale when
> they switch back to fair. Patch 1 rebuilds p->se.load from fair's
> switching_to hook, as in v3. Patch 2 keeps se->avg.load_avg in sync
> with the rebuilt load weight before the entity is attached back to fair.
Looks good to me.
Acked-by: Andrea Righi <arighi@nvidia.com>
Thanks,
-Andrea
>
> Changes in v4:
> - Add Tejun's Acked-by to patch 1.
> - Split the PELT load_avg synchronization into patch 2.
>
> Changes in v3:
> - Move the rebuild into fair's switching_to hook, as suggested by Peter.
> This lets fair prepare its own state before enqueue and avoids adding a
> sched_ext/fair-specific fixup to the generic sched_change_end() path.
>
> Changes in v2:
> - Move the fix from scx_root_disable() to the class switch path so it also
> covers partial-mode SCHED_EXT to SCHED_NORMAL transitions through
> sched_setscheduler(). Andrea identified this missing case in the v1
> discussion.
>
> Zicheng Qu (2):
> sched/fair: Rebuild load weight when switching to fair
> sched/fair: Keep load_avg in sync after reweighting
>
> kernel/sched/fair.c | 24 +++++++++++++++++++-----
> 1 file changed, 19 insertions(+), 5 deletions(-)
>
> --
> 2.53.0
^ permalink raw reply [flat|nested] 4+ messages in thread