sched_ext: Don't leave balance callbacks queued across a pick retry

The dispatch path can request deferred work to be run from a balance
callback and pick_task_scx() queues rq->scx.deferred_bal_cb right after
dispatching. However, a balance callback must be consumed before the rq
lock is released and pick_task_scx() is not necessarily the end of the
pick:

 - it returns RETRY_TASK when a higher priority class task has been
   enqueued while the dispatch had the rq lock released;

 - the dl_server pick can return NULL, in which case the lower classes
   are picked next.

In both cases the callback stays queued while the core keeps picking,
and the rq lock can be released again, either by prev_balance() or by
another dispatch. For example, with an RT task as prev:

  __schedule()
    __pick_next_task()
      pick_task_scx()
        dispatch_pick()
          maybe_queue_balance_callback()    <- deferred_bal_cb queued
        return RETRY_TASK
      prev_balance()
        balance_rt()
          pull_rt_task()
            double_lock_balance()           <- rq lock released

Any CPU acquiring the rq lock in that window finds a pending balance
callback and triggers the following warning in rq_pin_lock():

  WARNING: kernel/sched/sched.h:1919 at rq_lock+0x7f/0xe0, CPU#10: stress-ng-sleep/533
  CPU: 10 UID: 0 PID: 533 Comm: stress-ng-sleep Not tainted 7.2.0-rc6-virtme #61 PREEMPT(full)
  Sched_ext: qmap (enabled+all)
  Call Trace:
   <IRQ>
   try_to_wake_up+0x2c1/0x5c0
   hrtimer_wakeup+0x22/0x30
   __hrtimer_run_queues+0x1d5/0x350
   hrtimer_interrupt+0x106/0x1e0
   __sysvec_apic_timer_interrupt+0x6f/0x210
   sysvec_apic_timer_interrupt+0x71/0x90
   </IRQ>

This can be reproduced running scx_qmap -I together with a SCHED_FIFO
workload that frequently sleeps, e.g.:

  $ vng -v --cpus 16 --memory 4G -- \
      'tools/sched_ext/build/bin/scx_qmap -I >/dev/null & \
       stress-ng --sleep 4 --sched fifo --sched-prio 1 -t 30 -q'

Fix this by queueing the balance callbacks only when the pick is final.
Otherwise, defer the pending work to the irq_work path, which doesn't
depend on the current rq lock section, and leave the kick sync wait
pending for the next pick.

Fixes: 4c95380701f5 ("sched/ext: Fold balance_scx() into pick_task_scx()")
Cc: stable@vger.kernel.org # v6.19+
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

0 files changed, 0 insertions, 0 deletions