Re: [RFC 2/2] drm/panthor: Use separate workqueue for DRM scheduler

[Tvrtko Ursulin]

On 22/05/2026 17:08, Boris Brezillon wrote:
On Fri, 22 May 2026 12:38:17 +0100
Tvrtko Ursulin <tvrtko.ursu...@igalia.com> wrote:

Currently an unordered workqueue is used for the DRM scheduler which means
its concurrency is externally managed, and given there is one scheduler
instance per userspace queue, that means workqueue management logic is
within its rights to spawn many kernel threads to submit their respective
jobs.

Problem there is that all run job callbacks are serialized on the device
global mutex, making the potential thread storm just causing lock
contention.

Yeah, so initially we were not supposed to take the lock over the whole
run_job() function. We should normally be able to queue things to the
ring buffer, and only briefly take the lock to check if the context is
still resident and kick the group scheduler if it's not. I agree that
in practice it turned in a huge synchronization point. I guess we should
consider turning that mutex into a rwsem that's taken in write mode in
the tick path, and read-mode elsewhere.

There is some software state modified too, so I am not sure how easy or 
hard it would be to make run job only hold the read lock?

If we add a separate ordered workqueue for the DRM scheduler integration
we can avoid this problem, since the ordered property directly expresses
the nature of the submission backend implementation.

I don't see alloc_ordered_workqueue() being used for the sched_wq
workqueue, is that intended (according to this comment, you seem to
want an ordered wq).

Yeah, it seems I mistyped the wq allocation below.

And considering the other user of this workqueue, the free job callback,
which is not globally serialized in this manner so could be thought to
potentially regress with this change, it should not be the case since
commit
a58f317c1ca0 ("drm/sched: Free all finished jobs at once")
made the DRM scheduler handle the cleanup of finished jobs more promptly.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursu...@igalia.com>
Cc: Boris Brezillon <boris.brezil...@collabora.com>
Cc: Liviu Dudau <liviu.du...@arm.com>
Cc: Steven Price <steven.pr...@arm.com>
---
  drivers/gpu/drm/panthor/panthor_sched.c | 27 ++++++++++++++++---------
  1 file changed, 17 insertions(+), 10 deletions(-)

diff --git a/drivers/gpu/drm/panthor/panthor_sched.c 
b/drivers/gpu/drm/panthor/panthor_sched.c
index 2bee1c92fb9e..cc6b3e2b015a 100644
--- a/drivers/gpu/drm/panthor/panthor_sched.c
+++ b/drivers/gpu/drm/panthor/panthor_sched.c
@@ -147,13 +147,11 @@ struct panthor_scheduler {
        struct panthor_device *ptdev;
  
  	/**
-        * @wq: Workqueue used by our internal scheduler logic and
-        * drm_gpu_scheduler.
+        * @wq: Workqueue used by our internal scheduler logic.
         *
         * Used for the scheduler tick, group update or other kind of FW
         * event processing that can't be handled in the threaded interrupt
-        * path. Also passed to the drm_gpu_scheduler instances embedded
-        * in panthor_queue.
+        * path.
         */
        struct workqueue_struct *wq;
  
@@ -166,6 +164,14 @@ struct panthor_scheduler {
         */
        struct workqueue_struct *heap_alloc_wq;
  
+	/**
+        * @sched_wq: Workqueue used for the DRM scheduler.
+        *
+        * Workqueue used for drm_gpu_scheduler instances embedded in
+        * panthor_queue.
+        */
+       struct workqueue_struct *sched_wq;
+
        /** @tick_work: Work executed on a scheduling tick. */
        struct delayed_work tick_work;
  
@@ -3488,7 +3494,7 @@ group_create_queue(struct panthor_group *group,
  {
        struct drm_sched_init_args sched_args = {
                .ops = &panthor_queue_sched_ops,
-               .submit_wq = group->ptdev->scheduler->wq,
+               .submit_wq = group->ptdev->scheduler->sched_wq,
                /*
                 * The credit limit argument tells us the total number of
                 * instructions across all CS slots in the ringbuffer, with
@@ -4078,6 +4084,9 @@ static void panthor_sched_fini(struct drm_device *ddev, 
void *res)
        if (sched->heap_alloc_wq)
                destroy_workqueue(sched->heap_alloc_wq);
  
+	if (sched->sched_wq)
+               destroy_workqueue(sched->sched_wq);
+
        for (prio = PANTHOR_CSG_PRIORITY_COUNT - 1; prio >= 0; prio--) {
                drm_WARN_ON(ddev, !list_empty(&sched->groups.runnable[prio]));
                drm_WARN_ON(ddev, !list_empty(&sched->groups.idle[prio]));
@@ -4167,13 +4176,11 @@ int panthor_sched_init(struct panthor_device *ptdev)
         * FW is smart enough to fall back on other methods if the kernel can't
         * allocate memory, and fail the tiling job if none of these
         * countermeasures worked.
-        *
-        * Set WQ_MEM_RECLAIM on sched->wq to unblock the situation when the
-        * system is running out of memory.
         */
        sched->heap_alloc_wq = alloc_workqueue("panthor-heap-alloc", 
WQ_UNBOUND, 0);
-       sched->wq = alloc_workqueue("panthor-csf-sched", WQ_MEM_RECLAIM | 
WQ_UNBOUND, 0);
-       if (!sched->wq || !sched->heap_alloc_wq) {
+       sched->wq = alloc_workqueue("panthor-csf-sched", WQ_UNBOUND, 0);
+       sched->sched_wq = alloc_workqueue("panthor-drm-sched", WQ_MEM_RECLAIM, 
0);

The other one also needs MEM_RECLAIM, because you need work items
queued to sched->wq to run to guarantee forward progress, and you need
to guarantee forward progress to reclaim GPU mem.

Ack, I had a feeling that might be the case.

I will respin next week or so. Or if you tell me the global lock can be 
easily dropped from .run_job I can drop and forget about it. Wider 
context is that I am experimenting with kthread_worker conversion and 
trying to polish a 
somewhat-broken-but-showing-great-latency-improvements branch. For that 
I can kind of take either one global worker, or one worker per client 
route for the RFC, no big deal either way for the prototype.

Regards,

Tvrtko


+       if (!sched->wq || !sched->heap_alloc_wq || !sched->sched_wq) {
                panthor_sched_fini(&ptdev->base, sched);
                drm_err(&ptdev->base, "Failed to allocate the workqueues");
                return -ENOMEM;