Re: [tip:sched/core] [sched/fair] 79104becf4: BUG:kernel_NULL_pointer_dereference,address

[John Stultz]

On Thu, Nov 6, 2025 at 2:54 AM Fernand Sieber <sieberf@xxxxxxxxxx> wrote:
> > But yeah, if the task holding your resource is doing yield() you're
> > 'sad'. Basically a sched-fair yield() means: I've no fucking clue what
> > I'm doing and lets hope we can make progress a little later.
> >
> > And it gets worse in the context of PI/proxy, because in that case your
> > fair task can deadlock the system through sheer incompetence.
> >
> > Anyway, consider the PI case, we bump a fair task to FIFO and then
> > yield() would do the FIFO yield -- with all the possible problems.
> >
> > And we want the same for proxy, if the boosting context is FIFO, we want
> > a FIFO yield.
>
> Alright, makes sense. Based on your suggestion I've submitted a patch:
> lore.kernel.org/lkml/20251106104022.195157-1-sieberf@xxxxxxxxxx/T/#u
>
> This is essentially the same as your suggestion + handling of yield to and
> scx.
>
> Also confirmed no crashes with this patch + v3 of the vruntime forfeit patch
> https://lore.kernel.org/lkml/20250918150528.292620-1-sieberf@xxxxxxxxxx/
>

Fernand, Thank you so much for root causing this, and helping validate
the fix works!
I apologise for missing this case in my testing.  I'll try to see if I
can include the trinity test in my regular stress testing.

Do let me know if you do see any other proxy related issues!

thanks
-john