[PATCH v26 08/10] sched: Add logic to zap balance callbacks if we pick again

John Stultz
2026-03-24T19:13:53Z

With proxy-exec, a task is selected to run via pick_next_task(),
and then if it is a mutex blocked task, we call find_proxy_task()
to find a runnable owner. If the runnable owner is on another
cpu, we will need to migrate the selected donor task away, after
which we will pick_again can call pick_next_task() to choose
something else.

However, in the first call to pick_next_task(), we may have
had a balance_callback setup by the class scheduler. After we
pick again, its possible pick_next_task_fair() will be called
which calls sched_balance_newidle() and sched_balance_rq().

This will throw a warning:
[ 8.796467] rq->balance_callback && rq->balance_callback != &balance_push_callback
[ 8.796467] WARNING: CPU: 32 PID: 458 at kernel/sched/sched.h:1750 sched_balance_rq+0xe92/0x1250
...
[ 8.796467] Call Trace:
[ 8.796467] <TASK>
[ 8.796467] ? __warn.cold+0xb2/0x14e
[ 8.796467] ? sched_balance_rq+0xe92/0x1250
[ 8.796467] ? report_bug+0x107/0x1a0
[ 8.796467] ? handle_bug+0x54/0x90
[ 8.796467] ? exc_invalid_op+0x17/0x70
[ 8.796467] ? asm_exc_invalid_op+0x1a/0x20
[ 8.796467] ? sched_balance_rq+0xe92/0x1250
[ 8.796467] sched_balance_newidle+0x295/0x820
[ 8.796467] pick_next_task_fair+0x51/0x3f0
[ 8.796467] __schedule+0x23a/0x14b0
[ 8.796467] ? lock_release+0x16d/0x2e0
[ 8.796467] schedule+0x3d/0x150
[ 8.796467] worker_thread+0xb5/0x350
[ 8.796467] ? __pfx_worker_thread+0x10/0x10
[ 8.796467] kthread+0xee/0x120
[ 8.796467] ? __pfx_kthread+0x10/0x10
[ 8.796467] ret_from_fork+0x31/0x50
[ 8.796467] ? __pfx_kthread+0x10/0x10
[ 8.796467] ret_from_fork_asm+0x1a/0x30
[ 8.796467] </TASK>

This is because if a RT task was originally picked, it will
setup the rq->balance_callback with push_rt_tasks() via
set_next_task_rt().

Once the task is migrated away and we pick again, we haven't
processed any balance callbacks, so rq->balance_callback is not
in the same state as it was the first time pick_next_task was
called.

To handle this, add a zap_balance_callbacks() helper function
which cleans up the balance callbacks without running them. This
should be ok, as we are effectively undoing the state set in
the first call to pick_next_task(), and when we pick again,
the new callback can be configured for the donor task actually
selected.

Reviewed-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: John Stultz <jstultz@google.com>
---
v20:
* Tweaked to avoid build issues with different configs
v22:
* Spelling fix suggested by K Prateek
* Collapsed the stub implementation to one line as suggested
by K Prateek
* Zap callbacks when we resched idle, as suggested by K Prateek
v24:
* Don't conditionalize function on CONFIG_SCHED_PROXY_EXEC as
the callers will be optimized out if that is unset, and the
dead function will be removed, as suggsted by K Prateek

Cc: Joel Fernandes <joelagnelf@nvidia.com>
Cc: Qais Yousef <qyousef@layalina.io>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Zimuzo Ezeozue <zezeozue@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Will Deacon <will@kernel.org>
Cc: Waiman Long <longman@redhat.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Metin Kaya <Metin.Kaya@arm.com>
Cc: Xuewen Yan <xuewen.yan94@gmail.com>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Suleiman Souhlal <suleiman@google.com>
Cc: kuyo chang <kuyo.chang@mediatek.com>
Cc: hupu <hupu.gm@gmail.com>
Cc: kernel-team@android.com
---
kernel/sched/core.c | 36 ++++++++++++++++++++++++++++++++++--
1 file changed, 34 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b316b6015ffea..4ed24ef590f73 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4920,6 +4920,34 @@ static inline void finish_task(struct task_struct *prev)
smp_store_release(&prev->on_cpu, 0);
}

+/*
+ * Only called from __schedule context
+ *
+ * There are some cases where we are going to re-do the action
+ * that added the balance callbacks. We may not be in a state
+ * where we can run them, so just zap them so they can be
+ * properly re-added on the next time around. This is similar
+ * handling to running the callbacks, except we just don't call
+ * them.
+ */
+static void zap_balance_callbacks(struct rq *rq)
+{
+ struct balance_callback *next, *head;
+ bool found = false;
+
+ lockdep_assert_rq_held(rq);
+
+ head = rq->balance_callback;
+ while (head) {
+ if (head == &balance_push_callback)
+ found = true;
+ next = head->next;
+ head->next = NULL;
+ head = next;
+ }
+ rq->balance_callback = found ? &balance_push_callback : NULL;
+}
+
static void do_balance_callbacks(struct rq *rq, struct balance_callback *head)
{
void (*func)(struct rq *rq);
@@ -6865,10 +6893,14 @@ static void __sched notrace __schedule(int sched_mode)
rq_set_donor(rq, next);
if (unlikely(next->blocked_on)) {
next = find_proxy_task(rq, next, &rf);
- if (!next)
+ if (!next) {
+ zap_balance_callbacks(rq);
goto pick_again;
- if (next == rq->idle)
+ }
+ if (next == rq->idle) {
+ zap_balance_callbacks(rq);
goto keep_resched;
+ }
}
if (rq->donor == prev_donor && prev != next) {
struct task_struct *donor = rq->donor;
--
2.53.0.1018.g2bb0e51243-goog