Re: [PATCH v5] sched_ext: Fix lock imbalance in dispatch_to_local_dsq()

[Tejun Heo <tj@kernel.org>]

On Mon, Jan 27, 2025 at 11:06:16PM +0100, Andrea Righi wrote:
> While performing the rq locking dance in dispatch_to_local_dsq(), we may
> trigger the following lock imbalance condition, in particular when
> multiple tasks are rapidly changing CPU affinity (i.e., running a
> `stress-ng --race-sched 0`):
> 
> [   13.413579] =====================================
> [   13.413660] WARNING: bad unlock balance detected!
> [   13.413729] 6.13.0-virtme #15 Not tainted
> [   13.413792] -------------------------------------
> [   13.413859] kworker/1:1/80 is trying to release lock (&rq->__lock) at:
> [   13.413954] [<ffffffff873c6c48>] dispatch_to_local_dsq+0x108/0x1a0
> [   13.414111] but there are no more locks to release!
> [   13.414176]
> [   13.414176] other info that might help us debug this:
> [   13.414258] 1 lock held by kworker/1:1/80:
> [   13.414318]  #0: ffff8b66feb41698 (&rq->__lock){-.-.}-{2:2}, at: raw_spin_rq_lock_nested+0x20/0x90
> [   13.414612]
> [   13.414612] stack backtrace:
> [   13.415255] CPU: 1 UID: 0 PID: 80 Comm: kworker/1:1 Not tainted 6.13.0-virtme #15
> [   13.415505] Workqueue:  0x0 (events)
> [   13.415567] Sched_ext: dsp_local_on (enabled+all), task: runnable_at=-2ms
> [   13.415570] Call Trace:
> [   13.415700]  <TASK>
> [   13.415744]  dump_stack_lvl+0x78/0xe0
> [   13.415806]  ? dispatch_to_local_dsq+0x108/0x1a0
> [   13.415884]  print_unlock_imbalance_bug+0x11b/0x130
> [   13.415965]  ? dispatch_to_local_dsq+0x108/0x1a0
> [   13.416226]  lock_release+0x231/0x2c0
> [   13.416326]  _raw_spin_unlock+0x1b/0x40
> [   13.416422]  dispatch_to_local_dsq+0x108/0x1a0
> [   13.416554]  flush_dispatch_buf+0x199/0x1d0
> [   13.416652]  balance_one+0x194/0x370
> [   13.416751]  balance_scx+0x61/0x1e0
> [   13.416848]  prev_balance+0x43/0xb0
> [   13.416947]  __pick_next_task+0x6b/0x1b0
> [   13.417052]  __schedule+0x20d/0x1740
> 
> This happens because dispatch_to_local_dsq() is racing with
> dispatch_dequeue() and, when the latter wins, we incorrectly assume that
> the task has been moved to dst_rq.
> 
> Fix by properly tracking the currently locked rq.
> 
> Fixes: 4d3ca89bdd31 ("sched_ext: Refactor consume_remote_task()")
> Signed-off-by: Andrea Righi <arighi@nvidia.com>

Applied to sched_ext/for-6.14-fixes.

Thanks.

-- 
tejun