FAILED: patch "[PATCH] sched_ext: Skip ops.set_weight() for disabled tasks" failed to apply to 7.1-stable tree

The patch below does not apply to the 7.1-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-7.1.y
git checkout FETCH_HEAD
git cherry-pick -x 0e2f4ab68a89fad42e0f5a9ff4b740738e7aa1d6
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026072131-citation-reshape-587c@gregkh' --subject-prefix 'PATCH 7.1.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 0e2f4ab68a89fad42e0f5a9ff4b740738e7aa1d6 Mon Sep 17 00:00:00 2001
From: Kuba Piecuch <[email protected]>
Date: Fri, 10 Jul 2026 14:43:41 +0000
Subject: [PATCH] sched_ext: Skip ops.set_weight() for disabled tasks

When switching a task's sched_class away from sched_ext, we get the following sequence of events in __sched_setscheduler():

sched_change_begin()
switched_from_scx()
scx_disable_task(p)
ops.disable(p)
__setscheduler_params()
set_load_weight()
reweight_task_scx(p)
ops.set_weight(p)
p->sched_class = next_class;
sched_change_end()
...

Notably, ops.set_weight() is called _after_ ops.disable(). This violates the expected semantics of the callbacks, the expectation being that ops.disable() can only be followed by ops.exit_task() or ops.enable().

Skipping the weight adjustment for disabled tasks should be harmless since the weight will be recalculated in scx_enable_task() if the task ever rejoins SCX.

index aeee44e016ab..e3fa7b2fac9d 100644
@@ -3967,6 +3967,17 @@ static void reweight_task_scx(struct rq *rq, struct task_struct *p,
 	if (task_dead_and_done(p))
 		return;
 
+	/*
+	 * When switching sched_class away from SCX, reweight_task_scx()
+	 * is called _after_ scx_disable_task(). Skip calling ops.set_weight()
+	 * since the BPF scheduler may have already forgotten the task in
+	 * ops.disable().
+	 * p->scx.weight will be recalculated in scx_enable_task() if the task
+	 * ever returns to SCX class.
+	 */
+	if (scx_get_task_state(p) != SCX_TASK_ENABLED)
+		return;
+
 	p->scx.weight = sched_weight_to_cgroup(scale_load_down(lw->weight));
 	if (SCX_HAS_OP(sch, set_weight))
 		SCX_CALL_OP_TASK(sch, set_weight, rq, p, p->scx.weight);

Hi Greg,

I'm unable to reproduce the failure using the steps provided. The patch applies cleanly. The code compiles and tests are passing.

FWIW, below is the patch after cherry-picking the commit:

Thanks,
Kuba

------------------------ 8< ----------------------------

From adc748e669cc1564255430299abf086b9ecd9b45 Mon Sep 17 00:00:00 2001
From: Kuba Piecuch <[email protected]>
Date: Fri, 10 Jul 2026 14:43:41 +0000
Subject: [PATCH] sched_ext: Skip ops.set_weight() for disabled tasks

When switching a task's sched_class away from sched_ext, we get the following sequence of events in __sched_setscheduler():

sched_change_begin()
switched_from_scx()
scx_disable_task(p)
ops.disable(p)
__setscheduler_params()
set_load_weight()
reweight_task_scx(p)
ops.set_weight(p)
p->sched_class = next_class;
sched_change_end()
...

Notably, ops.set_weight() is called _after_ ops.disable(). This violates the expected semantics of the callbacks, the expectation being that ops.disable() can only be followed by ops.exit_task() or ops.enable().

Skipping the weight adjustment for disabled tasks should be harmless since the weight will be recalculated in scx_enable_task() if the task ever rejoins SCX.

---
 kernel/sched/ext.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

index 5d2d19473a82..8429bebc21a1 100644
@@ -3904,6 +3904,17 @@ static void reweight_task_scx(struct rq *rq, struct task_struct *p,
        if (task_dead_and_done(p))
                return;
 
+       /*
+        * When switching sched_class away from SCX, reweight_task_scx()
+        * is called _after_ scx_disable_task(). Skip calling ops.set_weight()
+        * since the BPF scheduler may have already forgotten the task in
+        * ops.disable().
+        * p->scx.weight will be recalculated in scx_enable_task() if the task
+        * ever returns to SCX class.
+        */
+       if (scx_get_task_state(p) != SCX_TASK_ENABLED)
+               return;
+
        p->scx.weight = sched_weight_to_cgroup(scale_load_down(lw->weight));
        if (SCX_HAS_OP(sch, set_weight))
                SCX_CALL_OP_TASK(sch, set_weight, rq, p, p->scx.weight);

If you look, the original file was:
kernel/sched/ext/ext.c
your cherry-pick fixed that.

I'll take this, thanks.

greg k-h

Nope, the whitespace is corrupted, please send a good copy.

thanks,

greg k-h

When switching a task's sched_class away from sched_ext, we get the following sequence of events in __sched_setscheduler():

sched_change_begin()
switched_from_scx()
scx_disable_task(p)
ops.disable(p)
__setscheduler_params()
set_load_weight()
reweight_task_scx(p)
ops.set_weight(p)
p->sched_class = next_class;
sched_change_end()
...

Notably, ops.set_weight() is called _after_ ops.disable(). This violates the expected semantics of the callbacks, the expectation being that ops.disable() can only be followed by ops.exit_task() or ops.enable().

Skipping the weight adjustment for disabled tasks should be harmless since the weight will be recalculated in scx_enable_task() if the task ever rejoins SCX.

---
 kernel/sched/ext.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

index 5d2d19473a82..8429bebc21a1 100644
@@ -3904,6 +3904,17 @@ static void reweight_task_scx(struct rq *rq, struct task_struct *p,
 	if (task_dead_and_done(p))
 		return;
 
+	/*
+	 * When switching sched_class away from SCX, reweight_task_scx()
+	 * is called _after_ scx_disable_task(). Skip calling ops.set_weight()
+	 * since the BPF scheduler may have already forgotten the task in
+	 * ops.disable().
+	 * p->scx.weight will be recalculated in scx_enable_task() if the task
+	 * ever returns to SCX class.
+	 */
+	if (scx_get_task_state(p) != SCX_TASK_ENABLED)
+		return;
+
 	p->scx.weight = sched_weight_to_cgroup(scale_load_down(lw->weight));
 	if (SCX_HAS_OP(sch, set_weight))
 		SCX_CALL_OP_TASK(sch, set_weight, rq, p, p->scx.weight);

2.55.0.229.g6434b31f56-goog

Queued for 7.1, thanks.

Thanks,
Sasha