Re: [PATCH v2 06/11] drm/panthor: Prepare the scheduler logic for FW events in IRQ context

[Boris Brezillon <boris.brezillon@collabora.com>]

On Tue, 19 May 2026 10:16:26 -0700
Chia-I Wu <olvaffe@gmail.com> wrote:

> On Tue, May 19, 2026 at 12:53 AM Boris Brezillon
> <boris.brezillon@collabora.com> wrote:
> >
> > On Mon, 18 May 2026 16:33:20 -0700
> > Chia-I Wu <olvaffe@gmail.com> wrote:
> >
> >  
> > > > >
> > > > >  
> > > > > >  
> > > > > > >  
> > > > > > > >         if (!ptdev->scheduler)
> > > > > > > >                 return;
> > > > > > > >
> > > > > > > > -       atomic_or(events, &ptdev->scheduler->fw_events);
> > > > > > > > -       sched_queue_work(ptdev->scheduler, fw_events);
> > > > > > > > +       guard(spinlock_irqsave)(&ptdev->scheduler->events_lock);
> > > > > > > > +
> > > > > > > > +       if (events & JOB_INT_GLOBAL_IF) {
> > > > > > > > +               sched_process_global_irq_locked(ptdev);
> > > > > > > > +               events &= ~JOB_INT_GLOBAL_IF;
> > > > > > > > +       }
> > > > > > > > +
> > > > > > > > +       while (events) {
> > > > > > > > +               u32 csg_id = ffs(events) - 1;
> > > > > > > > +
> > > > > > > > +               sched_process_csg_irq_locked(ptdev, csg_id);
> > > > > > > > +               events &= ~BIT(csg_id);
> > > > > > > > +       }  
> > > > > > > This handles all fw events in the irq context. Are there concerns that
> > > > > > > it may take too long? I might be wrong, but it seems possible to
> > > > > > > handle only CSG_SYNC_UPDATE and defer the rest as before.  
> > > > > >
> > > > > > I started with just the SYNC_UPDATE processing done in the hard-irq
> > > > > > context, but after auditing the other stuff done in the handler, I
> > > > > > realized it's basically just deferring all actual processing to work
> > > > > > items. Yes, there's the overhead of demuxing the events from the
> > > > > > ack/req regs, but part of this is already done to get to SYNC_UPDATE
> > > > > > anyway, so at this point we're probably better off demuxing everything
> > > > > > and scheduling works for all kind of events.
> > > > > >
> > > > > > I also compared the perfs between the two approaches (though I didn't
> > > > > > do as much testing as I did with the new version, so I might have
> > > > > > missed something), and it didn't seem to matter at all, because the
> > > > > > interrupts we receive the most are SYNC_UPDATE and IDLE events, and
> > > > > > those are at the same level.  
> > > > > Looking at ftrace irq events, when there is one active csg,
> > > > > panthor-job takes 6us (median) / 17us (95%) / 27us (slowest).
> > > > >
> > > > > I don't have a good sense if that's considered normal in hardirq. But
> > > > > if that is ever an issue, and if the majority of the time is spent in
> > > > > CSG_SYNC_UPDATE anyway, we can always revert the last patch to move
> > > > > processing to threaded handler.  
> > > >
> > > > Actually, the threaded -> hard transition (patch 9) is where the perf
> > > > gain is.  
> > > hardirq is even more timely for sure. For our use case, the threaded
> > > handler is RT and is also good enough.  
> >
> > Yeah, true. I forgot you were forcing RT priority on threaded handlers.
> > Anyway, let's stick to hardirqs for now, and revisit it if it proves to
> > be too much work done in irq context.  
> Just want to clarify that irq_thread calls sched_set_fifo to make the
> task RT. The behavior is universal and is not specific to any
> downstream kernel.

Hm, interesting. In my testing, any of the changes before patch 9
didn't make a huge difference in term of perf, patch 9 is where the perf
gains happen. For the record, patch 6 is where we get rid of the
threaded -> work round-trip for job completion/fence signaling, and it
didn't seem to reflect in the benchmark results, but I'll do another
round of tests before posting v3, just to confirm.