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sched: Add logic to zap balance callbacks if we pick again

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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.

Signed-off-by: John Stultz <jstultz@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: K Prateek Nayak <kprateek.nayak@amd.com>
Link: https://patch.msgid.link/20260324191337.1841376-9-jstultz@google.com
This commit is contained in:
John Stultz 2026-03-24 19:13:23 +00:00 committed by Peter Zijlstra
parent f9530b3183
commit 48fda62de6
1 changed files with 34 additions and 2 deletions
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36
kernel/sched/core.c
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@ -4917,6 +4917,34 @@ 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);
@ -6862,10 +6890,14 @@ 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;