Re: PREEMPT_RT vs i915

Ville Syrjälä
2025-07-10T15:04:45Z

On Thu, Jul 10, 2025 at 08:41:36AM +0200, Sebastian Andrzej Siewior wrote:
> On 2025-07-09 23:09:22 [+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.
>
> Well it works on my machine and this machine dose not pass the code
> paths that I patch.
>
> Every patch made was done because someone reported an error/ warning and
> confirmed afterwards that the patch fixes it for them and they can use
> the machine and don't observe 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.
>
> no, not just that. Making the lock raw led to latency spikes in simple
> spikes and I just disabled trace points. It could be worked around by
> taking the lock if the tracepoint is enabled and then invoking the
> tracepoint unconditionally and not taking the lock anymore. Steven made
> a suggestion a while ago how to put this in macro as far as I remember.

When this was last discussed I suggested that there should be a
versions of the tracepoint macros that do the sampling outside
the lock, but that wasn't deemed acceptable for whatever reason.
I don't even know why the current macros are doing the
sampling while holding the lock...

--
Ville Syrjälä
Intel