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

[Chia-I Wu]

On Tue, May 19, 2026 at 1:45 PM Chia-I Wu <olvaffe@xxxxxxxxx> wrote:
>
> On Tue, May 19, 2026 at 11:26 AM Boris Brezillon
> <boris.brezillon@xxxxxxxxxxxxx> wrote:
> >
> > On Tue, 19 May 2026 10:16:26 -0700
> > Chia-I Wu <olvaffe@xxxxxxxxx> wrote:
> >
> > > On Tue, May 19, 2026 at 12:53 AM Boris Brezillon
> > > <boris.brezillon@xxxxxxxxxxxxx> wrote:
> > > >
> > > > On Mon, 18 May 2026 16:33:20 -0700
> > > > Chia-I Wu <olvaffe@xxxxxxxxx> 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.
> We care the most about signaling latency for this series. I collected
> some numbers with baseline, with this series, and with patch 9
> reverted at https://gitlab.freedesktop.org/panfrost/linux/-/work_items/85#note_3481308.
> Reposting the numbers here for reference
>
> |                    | baseline | entire series | patch 9 reverted |
> | -                  | -        | -             | -                |
> | frag job median    | 2.8ms    | 2.2ms         | 2.2ms            |
> | frag job 95%       | 4.5ms    | 2.8ms         | 2.8ms            |
> | frag job 99%       | 4.9ms    | 2.8ms         | 2.8ms            |
> | panthor-job median | 0.8us    | 6.2us         | 0.9us            |
> | panthor-job 95%    | 1.5us    | 16.6us        | 1.5us            |
> | panthor-job 99%    | 1.6us    | 28.0us        | 1.8us            |

panthor-job rows are the durations of the raw irq handlers, collected
from irq/irq_handler_{entry,exit}.

frag job rows are the durations from frag jobs, collected from
gpu_scheduler/drm_sched_job_{run,done}.

The fence signaling paths of them are

 - baseline: raw handler -> rt threaded handler -> wq job -> wq job ->
fence signal
 - entire series: raw handler -> fence signal
 - patch 9 reverted: raw handler -> rt threaded handler -> fence signal

This is a synthetic test with a busy userspace. The higher durations
and variances of the baseline main come from the scheduling latencies
of wq jobs.