+Cc Matthew
On Wed, 2025-04-09 at 15:55 +0200, Christian König wrote:
> Am 09.04.25 um 12:28 schrieb Philipp Stanner:
> > On Fri, 2025-03-21 at 16:58 +0100, Christian König wrote:
> > > Sometimes drivers need to be able to submit multiple jobs which
> > > depend on
> > > each other to different schedulers at the same time, but using
> > > drm_sched_job_add_dependency() can't fail any more after the
> > > first
> > > job is
> > > initialized.
> > >
> > > This function preallocate memory for dependency slots so that no
> > > ENOMEM
> > > can come later while adding dependencies.
> > >
> > > v2: rework implementation an documentation
> > >
> > > Signed-off-by: Christian König <christian.koe...@amd.com>
> > > ---
> > > drivers/gpu/drm/scheduler/sched_main.c | 44
> > > ++++++++++++++++++++++++--
> > > include/drm/gpu_scheduler.h | 2 ++
> > > 2 files changed, 43 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/scheduler/sched_main.c
> > > b/drivers/gpu/drm/scheduler/sched_main.c
> > > index 4d4219fbe49d..ee3701f346b2 100644
> > > --- a/drivers/gpu/drm/scheduler/sched_main.c
> > > +++ b/drivers/gpu/drm/scheduler/sched_main.c
> > > @@ -852,6 +852,39 @@ void drm_sched_job_arm(struct drm_sched_job
> > > *job)
> > > }
> > > EXPORT_SYMBOL(drm_sched_job_arm);
> > >
> > > +/**
> > > + * drm_sched_job_prealloc_dependency_slots - avoid ENOMEM on
> > > adding
> > > dependencies
> > > + * @job: scheduler job where dependencies will be added
> > > + * @num_deps: number of dependencies to preallocate slots for
> > > + *
> > > + * Sometimes drivers need to be able to submit multiple jobs
> > > which
> > > depend on
> > > + * each other to different schedulers at the same time, but
> > > using
> > > + * drm_sched_job_add_dependency() can't fail any more after the
> > > first job is
> > > + * initialized.
> > > + *
> > > + * This function preallocate memory for dependency slots so that
> > > no
> > > ENOMEM can
> > > + * come later while adding dependencies.
> > > + *
> > > + * Return:
> > > + * 0 on success, or an error on failing to expand the array.
> > > + */
> > > +int drm_sched_job_prealloc_dependency_slots(struct drm_sched_job
> > > *job,
> > > + unsigned int
> > > num_deps)
> > > +{
> > > + u32 id = 0;
> > > + int ret;
> > > +
> > > + while (num_deps--) {
> > > + ret = xa_alloc(&job->dependencies, &id,
> > > XA_ZERO_ENTRY,
> > > + xa_limit_32b, GFP_KERNEL);
> > I've had some time to re-read the xarray documentation and I think
> > that
> > this is what xa_reserve() was born for. The Book of
> > Documentation/core-
> > api/xarray.rst sayeth:
> >
> > "Sometimes you need to ensure that a subsequent call to xa_store()
> > will not need to allocate memory. The xa_reserve() function
> > will store a reserved entry at the indicated index. Users of the
> > normal API will see this entry as containing ``NULL``."
> >
> > That's far better, this way we don't have to use that more or less
> > xarray-internal flag.
>
> Yeah I have seen that as well. The reason why I didn't followed this
> route was that I wasn't sure if I then need to check for NULL entries
> while iterating over the XA.
>
> Additional to that I couldn't figure out of hand how to determine a
> the next free index slot.
>
> Have you found any example how to use that? I mean the documentation
> could certainly be improved a bit.
Maybe Matthew can help us out here?
Matthew, what would be the idiomatic way to do this, and can we help
out with improving the Xarray's documentation?
Thx,
P.
>
> Regards,
> Christian.
>
> >
> >
> > > + if (ret != 0)
> > > + return ret;
> > > + }
> > > +
> > > + return 0;
> > > +}
> > > +EXPORT_SYMBOL(drm_sched_job_prealloc_dependency_slots);
> > > +
> > > /**
> > > * drm_sched_job_add_dependency - adds the fence as a job
> > > dependency
> > > * @job: scheduler job to add the dependencies to
> > > @@ -878,10 +911,15 @@ int drm_sched_job_add_dependency(struct
> > > drm_sched_job *job,
> > > * engines involved, rather than the number of BOs.
> > > */
> > > xa_for_each(&job->dependencies, index, entry) {
> > > - if (entry->context != fence->context)
> > > + if (xa_is_zero(entry)) {
> > > + /*
> > > + * Reserved entries must not alloc
> > > memory,
> > > but let's
> > > + * use GFP_ATOMIC just to be on the
> > > defensive side.
> > > + */
> > > + xa_store(&job->dependencies, index,
> > > fence,
> > > GFP_ATOMIC);
> > And regarding this – it can actually never happen, but you provide
> > ATOMIC just to be sure?
> >
> > I think it would be better if we'd just run into an obvious bug
> > here
> > instead, so like a deadlock with GFP_KERNEL.
> >
> > That's how we do it with pointers that cannot be NULL, too. If the
> > impossible were to happen and it were NULL, we'd crash.
> >
> > P.
> >
> > > + } else if (entry->context != fence->context) {
> > > continue;
> > > -
> > > - if (dma_fence_is_later(fence, entry)) {
> > > + } else if (dma_fence_is_later(fence, entry)) {
> > > dma_fence_put(entry);
> > > xa_store(&job->dependencies, index,
> > > fence,
> > > GFP_KERNEL);
> > > } else {
> > > diff --git a/include/drm/gpu_scheduler.h
> > > b/include/drm/gpu_scheduler.h
> > > index 1a7e377d4cbb..916e820b27ff 100644
> > > --- a/include/drm/gpu_scheduler.h
> > > +++ b/include/drm/gpu_scheduler.h
> > > @@ -632,6 +632,8 @@ int drm_sched_job_init(struct drm_sched_job
> > > *job,
> > > u32 credits, void *owner);
> > > void drm_sched_job_arm(struct drm_sched_job *job);
> > > void drm_sched_entity_push_job(struct drm_sched_job *sched_job);
> > > +int drm_sched_job_prealloc_dependency_slots(struct drm_sched_job
> > > *job,
> > > + unsigned int
> > > num_deps);
> > > int drm_sched_job_add_dependency(struct drm_sched_job *job,
> > > struct dma_fence *fence);
> > > int drm_sched_job_add_syncobj_dependency(struct drm_sched_job
> > > *job,
>
