Re: [PATCH v2 08/10] sched/fair: Add newidle balance to pick_task_fair()

[Aaron Lu]

Hi Peter,

On Mon, May 11, 2026 at 01:31:12PM +0200, Peter Zijlstra wrote:
> With commit 50653216e4ff ("sched: Add support to pick functions to
> take rf") removing the balance callback, the pick_task() callback is
> in charge of newidle balancing.
> 
> This means pick_task_fair() should do so too. This hasn't been a
> problem in practise because pick_next_task_fair() is used. However,
> since we'll be removing that one shortly, make sure pick_next_task()
> is up to scratch.

While testing Prateek's throttle series, I noticed a panic issue when
coresched is enabled and bisected to this patch.

I fed the panic log and this patch to an agent and its analysis looks
correct to me(cpu56 and cpu57 are siblings in a VM):

       cpu57 (holds core-wide lock)

     pick_next_task() [core scheduling]
     for_each_cpu_wrap(i, smt_mask, 57):
       i=57: pick_task(rq_57)
             pick_task_fair(rq_57)
             -> picks task A
       rq_57->core_pick = task A
       // task_rq(A) == rq_57

       i=56: pick_task(rq_56)
             pick_task_fair(rq_56)
             cfs_rq->nr_queued == 0
             goto idle
             sched_balance_newidle(rq_56)
             raw_spin_rq_unlock(rq_56)
             // core-wide lock released
             newidle_balance() pulls
               task A: rq_57 -> rq_56
             // task_rq(A) == rq_56 now
             raw_spin_rq_lock(rq_56)
             // core-wide lock re-acquired
             return > 0
             goto again
             pick_task_fair(rq_56)
             -> picks task A
       rq_56->core_pick = task A

     // first loop done
     // rq_57->core_pick is still task A (set before lock release)
     // but task_rq(A) == rq_56 now
     next = rq_57->core_pick  // = task A

     put_prev_set_next_task(rq_57, prev, task A)
     __set_next_task_fair(rq_57, task A)
     hrtick_start_fair(rq_57, task A)
     WARN_ON_ONCE(task_rq(task A) != rq_57)
     // task_rq(A) == rq_56

I applied below diff and the problem is gone:

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 5f48af700fd44..942a543af3e54 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9897,6 +9897,9 @@ static struct task_struct *pick_task_fair(struct rq *rq, struct rq_flags *rf)
 	return p;
 
 idle:
+	if (sched_core_enabled(rq))
+		return NULL;
+
 	new_tasks = sched_balance_newidle(rq, rf);
 	if (new_tasks < 0)
 		return RETRY_TASK;

Full dmesg of the panic is attached for your reference.

Attachment: dmesg_b3a2dfa8b42
Description: Binary data