[syzbot] [cgroups?] [mm?] WARNING in hrtick_start_fair

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: a87737435cfa Add linux-next specific files for 20260608
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=11715db6580000
kernel config: https://syzkaller.appspot.com/x/.config?x=da47745f686dc823
dashboard link: https://syzkaller.appspot.com/bug?extid=2cbf10efc23b22ff9c31
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=12df60ae580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=11edb0ae580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/85d19fe6bb4e/disk-a8773743.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/30c683ce26e1/vmlinux-a8773743.xz
kernel image: https://storage.googleapis.com/syzbot-assets/4db5027513d2/bzImage-a8773743.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+2cbf10...@syzkaller.appspotmail.com

------------[ cut here ]------------
task_rq(p) != rq
WARNING: kernel/sched/fair.c:7656 at hrtick_start_fair+0x196/0x1f0 kernel/sched/fair.c:7656, CPU#0: rcu_preempt/18
Modules linked in:
CPU: 0 UID: 0 PID: 18 Comm: rcu_preempt Not tainted syzkaller #0 PREEMPT_{RT,(full)}
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
RIP: 0010:hrtick_start_fair+0x196/0x1f0 kernel/sched/fair.c:7656
Code: 42 80 3c 20 00 74 08 4c 89 ff e8 85 e3 97 00 4d 39 37 0f 85 0c ff ff ff 48 89 df 5b 41 5c 41 5d 41 5e 41 5f e9 4b 65 fa ff 90 <0f> 0b 90 e9 d1 fe ff ff 44 89 f9 80 e1 07 80 c1 03 38 c1 0f 8c 82
RSP: 0018:ffffc900001777e0 EFLAGS: 00010087
RAX: ffff8880b873ba40 RBX: ffff8880b863ba40 RCX: ffffffff8197c7de
RDX: 0000000000000000 RSI: ffff88802c528000 RDI: ffff8880b863ba40
RBP: dffffc0000000000 R08: ffffffff8fcf0b0f R09: 1ffffffff1f9e161
R10: dffffc0000000000 R11: fffffbfff1f9e162 R12: dffffc0000000000
R13: 1ffff110170c78d6 R14: ffff88802c528000 R15: ffffffff8dc217d8
FS: 0000000000000000(0000) GS:ffff888125a76000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007eff95e6a540 CR3: 0000000028e30000 CR4: 00000000003526f0
Call Trace:
<TASK>
set_next_task_fair+0xa68/0xce0 kernel/sched/fair.c:15058
put_prev_set_next_task kernel/sched/sched.h:2770 [inline]
pick_next_task kernel/sched/core.c:6443 [inline]
__schedule+0x3e03/0x5550 kernel/sched/core.c:7144
__schedule_loop kernel/sched/core.c:7308 [inline]
schedule+0x164/0x360 kernel/sched/core.c:7323
schedule_timeout+0x158/0x2c0 kernel/time/sleep_timeout.c:99
rcu_gp_fqs_loop+0x312/0x11b0 kernel/rcu/tree.c:2123
rcu_gp_kthread+0x9e/0x2b0 kernel/rcu/tree.c:2325
kthread+0x388/0x470 kernel/kthread.c:436
ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
</TASK>


---
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***

Subject: [PATCH] sched/fair: Skip newidle balance for remote rqs
Author: jie....@intel.com

#syz test: https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git 49e02880ec0a8c378e811bc9d85da188d7c6204c

Commit b3a2dfa8b42e ("sched/fair: Add newidle balance to
pick_task_fair()") added sched_balance_newidle() to pick_task_fair().
With CONFIG_SCHED_CORE, the core-wide selection loop in pick_next_task()
calls pick_task() for each sibling rq. When a sibling has no CFS tasks,
pick_task_fair() enters newidle balancing which drops the shared
core-wide rq lock.

During this window, tasks already selected for earlier siblings can be
migrated away, making core_pick stale. This causes task_rq(p) != rq in
hrtick_start_fair().

Skip newidle balance when called for a remote rq. The sibling will do
its own newidle balance when it schedules itself.

Fixes: b3a2dfa8b42e ("sched/fair: Add newidle balance to pick_task_fair()")
Reported-by: syzbot+2cbf10...@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=2cbf10efc23b22ff9c31
Signed-off-by: Jie Wang <jie....@intel.com>
---
kernel/sched/fair.c | 7 +++++++
1 file changed, 7 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 1b23e73f48b0..5beb4e1b480b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9942,6 +9942,13 @@ struct task_struct *pick_task_fair(struct rq *rq, struct rq_flags *rf)
return p;

idle:
+ /*
+ * Skip newidle balance for remote rqs -- doing so would drop the
+ * core-wide rq lock, invalidating already-selected core_picks.
+ */
+ if (rq != this_rq())
+ return NULL;
+
new_tasks = sched_balance_newidle(rq, rf);
if (new_tasks < 0)
return RETRY_TASK;
--
2.43.0

[syzbot]

Hello,

syzbot has tested the proposed patch but the reproducer is still triggering an issue:
lost connection to test machine



Tested on:

commit: 49e02880 Add linux-next specific files for 20260609
git tree: https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git
console output: https://syzkaller.appspot.com/x/log.txt?x=12626156580000
kernel config: https://syzkaller.appspot.com/x/.config?x=c5df12af3478f8be

dashboard link: https://syzkaller.appspot.com/bug?extid=2cbf10efc23b22ff9c31
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8

patch: https://syzkaller.appspot.com/x/patch.diff?x=14f7e8ae580000