Re: PREEMPT_RT vs i915

Matthew Brost
2025-07-09T22:03:04Z

On Wed, Jul 09, 2025 at 11:09:22PM +0300, Ville Syrjälä wrote:
> On Wed, Jul 09, 2025 at 09:44:43PM +0200, Sebastian Andrzej Siewior wrote:
> > On 2025-07-09 20:30:26 [+0300], Ville Syrjälä wrote:
> > > >
> > > > It seems like the critical uncore lock is currently held in a lot of
> > > > places and potentially for a long time.
> > >
> > > It shouldn't be held for that long. I think it should just be
> > > a raw spinlock.
> >
> > What about I resubmit the series and we look again? I don't think the
> > lock should be made raw just to be done with it.
>
> Until someone actually does the work to confirm the thing is working
> reliably there's no point in posting anything.
>
> And IIRC the other remaining problem with RT was the spinlocks used
> inside tracepoints (which is uncore lock, and probably some vblank
> locks). So that too needs some kind of solution because it's going to
> very hard to debug the timing sensitive parts without the tracepoints.

A bit of a drive-by comment, but taking locks inside tracepoints seems
like a pretty horrible idea in general. We've managed to write an entire
driver (Xe) from scratch and bring it up without doing this. I'd be very
surprised if this is truly necessary in i915.

Matt

>
> --
> Ville Syrjälä
> Intel