On Tue, Aug 10, 2021 at 11:23:39AM +0200, Daniel Vetter wrote:
> On Mon, Aug 09, 2021 at 07:13:11PM +0000, Matthew Brost wrote:
> > On Mon, Aug 09, 2021 at 06:36:44PM +0200, Daniel Vetter wrote:
> > > On Tue, Aug 03, 2021 at 03:29:22PM -0700, Matthew Brost wrote:
> > > > Display the workqueue status in debugfs for GuC contexts that are in
> > > > parent-child relationship.
> > > >
> > > > Signed-off-by: Matthew Brost <matthew.brost@xxxxxxxxx>
> > > > ---
> > > > .../gpu/drm/i915/gt/uc/intel_guc_submission.c | 56 +++++++++++++------
> > > > 1 file changed, 39 insertions(+), 17 deletions(-)
> > > >
> > > > diff --git a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> > > > index 30df1c8db491..44a7582c9aed 100644
> > > > --- a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> > > > +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> > > > @@ -4527,31 +4527,53 @@ void intel_guc_submission_print_info(struct intel_guc *guc,
> > > > gse_log_submission_info(guc->gse[i], p, i);
> > > > }
> > > >
> > > > +static inline void guc_log_context(struct drm_printer *p,
> > > > + struct intel_context *ce)
> > > > +{
> > > > + drm_printf(p, "GuC lrc descriptor %u:\n", ce->guc_id);
> > > > + drm_printf(p, "\tHW Context Desc: 0x%08x\n", ce->lrc.lrca);
> > > > + drm_printf(p, "\t\tLRC Head: Internal %u, Memory %u\n",
> > > > + ce->ring->head,
> > > > + ce->lrc_reg_state[CTX_RING_HEAD]);
> > > > + drm_printf(p, "\t\tLRC Tail: Internal %u, Memory %u\n",
> > > > + ce->ring->tail,
> > > > + ce->lrc_reg_state[CTX_RING_TAIL]);
> > > > + drm_printf(p, "\t\tContext Pin Count: %u\n",
> > > > + atomic_read(&ce->pin_count));
> > > > + drm_printf(p, "\t\tGuC ID Ref Count: %u\n",
> > > > + atomic_read(&ce->guc_id_ref));
> > > > + drm_printf(p, "\t\tNumber Requests Not Ready: %u\n",
> > > > + atomic_read(&ce->guc_num_rq_not_ready));
> > > > + drm_printf(p, "\t\tSchedule State: 0x%x, 0x%x\n\n",
> > > > + ce->guc_state.sched_state,
> > > > + atomic_read(&ce->guc_sched_state_no_lock));
> > >
> > > It's all debugfs, but I think proper locking even there is good. It at
> > > least reduces the confusion when the locking scheme is largely
> > > undocumented. Also given how much we have rcu for everything would be good
> > > to double-check all pointer dererences are properly protected.
> > >
> >
> > Not sure if I 100% follow this but I don't think any of the pointers
> > dref here are RCU protected. Certainly none of the GuC ones are.
> >
> > Will double before the next respin though.
> >
> > > > +}
> > > > +
> > > > void intel_guc_submission_print_context_info(struct intel_guc *guc,
> > > > struct drm_printer *p)
> > > > {
> > > > struct intel_context *ce;
> > > > unsigned long index;
> > > > xa_for_each(&guc->context_lookup, index, ce) {
> > >
> > > xa_for_each doesn't provide any guarantees, so doesn't protect against
> > > concurrent removeal or anything like that. We need to do better than that.
> >
> > https://elixir.bootlin.com/linux/latest/source/include/linux/xarray.h#L498
> > 'It is safe to modify the array during the iteration.'
>
> The xarray. Not the thing you're dereferencing, because the xarray only
> stores pointers, not your data structure. So yeah correct statement is
> that it doesn't provide you any guarantees beyond "the iterator wont be
> confused if the xarray itself is modified during iteration". Which isn't
> what you need here, you need a lot more.
Or spelled out: The pointer you get could become immediately meaningless,
before you can look at it, due to a concurrent removal/release. All the
xa_for_each guarantees you is that on the next round you get the next
pointer, until you got them all (plus/minus concurrent changes). But that
next pointer could have become meaningless right away too.
So you need your own locking to make use of these pointers you got and
make sure they're not immediately meaningless before your loop body even
started.
One of the reasons why I think this is so important is that debugfs files
nest a lot of loops fairly often, so are good cheat-sheet for the locking
if it happens to be undocumented (which also shouldn't be the case). Ofc
if there's no locking in debugfs, no cheat-sheet :-)
Cheers, Daniel
> -Daniel
>
> >
> > Matt
> >
> > > -Daniel
> > >
> > > > - drm_printf(p, "GuC lrc descriptor %u:\n", ce->guc_id);
> > > > - drm_printf(p, "\tHW Context Desc: 0x%08x\n", ce->lrc.lrca);
> > > > - drm_printf(p, "\t\tLRC Head: Internal %u, Memory %u\n",
> > > > - ce->ring->head,
> > > > - ce->lrc_reg_state[CTX_RING_HEAD]);
> > > > - drm_printf(p, "\t\tLRC Tail: Internal %u, Memory %u\n",
> > > > - ce->ring->tail,
> > > > - ce->lrc_reg_state[CTX_RING_TAIL]);
> > > > - drm_printf(p, "\t\tContext Pin Count: %u\n",
> > > > - atomic_read(&ce->pin_count));
> > > > - drm_printf(p, "\t\tGuC ID Ref Count: %u\n",
> > > > - atomic_read(&ce->guc_id_ref));
> > > > - drm_printf(p, "\t\tNumber Requests Not Ready: %u\n",
> > > > - atomic_read(&ce->guc_num_rq_not_ready));
> > > > - drm_printf(p, "\t\tSchedule State: 0x%x, 0x%x\n\n",
> > > > - ce->guc_state.sched_state,
> > > > - atomic_read(&ce->guc_sched_state_no_lock));
> > > > + GEM_BUG_ON(intel_context_is_child(ce));
> > > >
> > > > + guc_log_context(p, ce);
> > > > guc_log_context_priority(p, ce);
> > > > +
> > > > + if (intel_context_is_parent(ce)) {
> > > > + struct guc_process_desc *desc = __get_process_desc(ce);
> > > > + struct intel_context *child;
> > > > +
> > > > + drm_printf(p, "\t\tWQI Head: %u\n",
> > > > + READ_ONCE(desc->head));
> > > > + drm_printf(p, "\t\tWQI Tail: %u\n",
> > > > + READ_ONCE(desc->tail));
> > > > + drm_printf(p, "\t\tWQI Status: %u\n\n",
> > > > + READ_ONCE(desc->wq_status));
> > > > +
> > > > + for_each_child(ce, child)
> > > > + guc_log_context(p, child);
> > > > + }
> > > > }
> > > > }
> > > >
> > > > --
> > > > 2.28.0
> > > >
> > >
> > > --
> > > Daniel Vetter
> > > Software Engineer, Intel Corporation
> > > http://blog.ffwll.ch
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
