Re: [PATCH v5 03/21] gpu: host1x: Show number of pending waiters in debugfs

Michał Mirosław
2021-04-08T04:26:33Z

On Thu, Apr 08, 2021 at 06:13:44AM +0200, Michał Mirosław wrote:
> On Fri, Apr 02, 2021 at 07:02:32PM +0300, Dmitry Osipenko wrote:
> > 02.04.2021 00:19, Michał Mirosław пишет:
> > > On Fri, Mar 26, 2021 at 04:34:13PM +0200, Mikko Perttunen wrote:
> > >> On 3/23/21 12:16 PM, Thierry Reding wrote:
> > >>> On Mon, Jan 11, 2021 at 03:00:01PM +0200, Mikko Perttunen wrote:
> > >>>> Show the number of pending waiters in the debugfs status file.
> > >>>> This is useful for testing to verify that waiters do not leak
> > >>>> or accumulate incorrectly.
> > >>>>
> > >>>> Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
> > >>>> ---
> > >>>> drivers/gpu/host1x/debug.c | 14 +++++++++++---
> > >>>> 1 file changed, 11 insertions(+), 3 deletions(-)
> > >>>>
> > >>>> diff --git a/drivers/gpu/host1x/debug.c b/drivers/gpu/host1x/debug.c
> > >>>> index 1b4997bda1c7..8a14880c61bb 100644
> > >>>> --- a/drivers/gpu/host1x/debug.c
> > >>>> +++ b/drivers/gpu/host1x/debug.c
> > >>>> @@ -69,6 +69,7 @@ static int show_channel(struct host1x_channel *ch, void *data, bool show_fifo)
> > >>>> static void show_syncpts(struct host1x *m, struct output *o)
> > >>>> {
> > >>>> + struct list_head *pos;
> > >>>> unsigned int i;
> > >>>> host1x_debug_output(o, "---- syncpts ----\n");
> > >>>> @@ -76,12 +77,19 @@ static void show_syncpts(struct host1x *m, struct output *o)
> > >>>> for (i = 0; i < host1x_syncpt_nb_pts(m); i++) {
> > >>>> u32 max = host1x_syncpt_read_max(m->syncpt + i);
> > >>>> u32 min = host1x_syncpt_load(m->syncpt + i);
> > >>>> + unsigned int waiters = 0;
> > >>>> - if (!min && !max)
> > >>>> + spin_lock(&m->syncpt[i].intr.lock);
> > >>>> + list_for_each(pos, &m->syncpt[i].intr.wait_head)
> > >>>> + waiters++;
> > >>>> + spin_unlock(&m->syncpt[i].intr.lock);
> > >>>
> > >>> Would it make sense to keep a running count so that we don't have to
> > >>> compute it here?
> > >>
> > >> Considering this is just a debug facility, I think I prefer not adding a new
> > >> field just for it.
> > >
> > > This looks like IRQ-disabled region, so unless only root can trigger
> > > this code, maybe the additional field could save a potential headache?
> > > How many waiters can there be in the worst case?
> >
> > The host1x's IRQ handler runs in a workqueue, so it should be okay.
>
> Why, then, this uses a spinlock (and it has 'intr' in its name)?

The critical sections are already O(n) in number of waiters, so this
patch doesn't make things worse as I previously thought. The questions
remain: What is the expected number and upper bound of workers?
Shouldn't this be a mutex instead?

Best Regards
Michał Mirosław