Re: sched: observed instability under stress in 6.12 and mainline

[Hao Jia <jiahao.kernel@gmail.com>]

On 2025/9/5 00:33, Chris Friesen wrote:
> Hi,
> 
> I'd like to draw the attention of the scheduler maintainers to a number 
> of kernel bugzilla reports submitted by a colleague a couple of weeks ago:
> 
> 6.12.18:
> https://bugzilla.kernel.org/show_bug.cgi?id=220447
> https://bugzilla.kernel.org/show_bug.cgi?id=220448
> 
> v6.16-rt3
> https://bugzilla.kernel.org/show_bug.cgi?id=220450
> https://bugzilla.kernel.org/show_bug.cgi?id=220449
> 
> There seems to be something wrong with either the logic or the locking. 
> In one case this resulted in a NULL pointer dereference in 
> pick_next_entity().  In another case it resulted in 
> BUG_ON(!rq->nr_running) in dequeue_top_rt_rq() and 
> SCHED_WARN_ON(!se->on_rq) in update_entity_lag().
> 
> My colleague suggests that the NULL pointer dereference may be due to 
> pick_eevdf() returning NULL in pick_next_entity().
> 
> I did some digging and found that 
> https://gitlab.com/linux-kernel/stable/-/commit/86b37810 would not have 
> been included in 6.12.18, but the equivalent fix should have been in the 
> 6.16 load.
> 
> We haven't yet bottomed out the root cause.
> 
> Any suggestions or assistance would be appreciated.
> 
> Thanks,
> Chris
> 
> 

Maybe this patch can be useful for your problem.
https://lore.kernel.org/all/tencent_3177343A3163451463643E434C61911B4208@qq.com/

If I understand correctly, we may dequeue_entity twice in 
rt_mutex_setprio()/__sched_setscheduler(). cfs_bandwidth may break the 
state of p->on_rq and se->on_rq.


Thanks,
Hao