On 2023-07-19 04:45, Christian König wrote:
> Am 16.07.23 um 09:51 schrieb Asahi Lina:
>> On 15/07/2023 16.14, Luben Tuikov wrote:
>>> On 2023-07-14 04:21, Asahi Lina wrote:
>>>> drm_sched_fini() currently leaves any pending jobs dangling, which
>>>> causes segfaults and other badness when job completion fences are
>>>> signaled after the scheduler is torn down.
>>>
>>> If there are pending jobs, ideally we want to call into the driver,
>>> so that it can release resources it may be holding for those.
>>> The idea behind "pending" is that they are pending in the hardware
>>> and we don't know their state until signalled/the callback called.
>>> (Or unless the device is reset and we get a notification of that fact.)
>>
>> That's what the job->free_job() callback does, then the driver is free
>> to do whatever it wants with those jobs. A driver could opt to
>> synchronously kill those jobs (if it can) or account for them
>> separately/asynchronously.
>>
>> What this patch basically says is that if you destroy a scheduler with
>> pending jobs, it immediately considers them terminated with an error,
>> and returns ownership back to the driver for freeing. Then the driver
>> can decide how to handle the rest and whatever the underlying hardware
>> state is.
>
> Yeah, and exactly that is absolutely *not* a good idea. Keep in mind
> that memory management depends on all this stuff and signal a dma_fence
> always requires that the hw give a go for that.
>
> If you want to cleanup a scheduler with pending jobs what needs to
> happen instead is that the driver cancels the processing and signals the
> hw fence.
>
>>
>>>> Explicitly detach all jobs from their completion callbacks and free
>>>> them. This makes it possible to write a sensible safe abstraction for
>>>> drm_sched, without having to externally duplicate the tracking of
>>>> in-flight jobs.
>>>>
>>>> This shouldn't regress any existing drivers, since calling
>>>> drm_sched_fini() with any pending jobs is broken and this change should
>>>> be a no-op if there are no pending jobs.
>>>
>>> While this statement is true on its own, it kind of contradicts
>>> the premise of the first paragraph.
>>
>> I mean right *now* it's broken, before this patch. I'm trying to make
>> it safe, but it shouldn't regress any exiting drivers since if they
>> trigger this code path they are broken today.
>
> Yes and exactly that's intentional.
>
> What you can do is to issue a *big* warning here when there are still
> pending unsignaled hw fences when the driver calls drm_sched_fini().
>
> But setting the scheduler fence to signaled without getting a signaled
> state from the hw fence is a complete NO-GO.
Okay, so we have the requirement (how). If we can also get a reason behind
it (why), perhaps we can add the requirement and the reason as a lucid comment
to drm_sched_fini() to come with this patch when reworked, so that future
drivers whether they be in Rust or C, can take note.
Perhaps this will also help future development in DRM itself.
--
Regards,
Luben
>
> Regards,
> Christian.
>
>>
>>>
>>>> Signed-off-by: Asahi Lina <lina@xxxxxxxxxxxxx>
>>>> ---
>>>> drivers/gpu/drm/scheduler/sched_main.c | 32
>>>> ++++++++++++++++++++++++++++++--
>>>> 1 file changed, 30 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c
>>>> b/drivers/gpu/drm/scheduler/sched_main.c
>>>> index 1f3bc3606239..a4da4aac0efd 100644
>>>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>>>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>>>> @@ -1186,10 +1186,38 @@ EXPORT_SYMBOL(drm_sched_init);
>>>> void drm_sched_fini(struct drm_gpu_scheduler *sched)
>>>> {
>>>> struct drm_sched_entity *s_entity;
>>>> + struct drm_sched_job *s_job, *tmp;
>>>> int i;
>>>> - if (sched->thread)
>>>> - kthread_stop(sched->thread);
>>>> + if (!sched->thread)
>>>> + return;
>>>> +
>>>> + /*
>>>> + * Stop the scheduler, detaching all jobs from their hardware
>>>> callbacks
>>>> + * and cleaning up complete jobs.
>>>> + */
>>>> + drm_sched_stop(sched, NULL);
>>>> +
>>>> + /*
>>>> + * Iterate through the pending job list and free all jobs.
>>>> + * This assumes the driver has either guaranteed jobs are
>>>> already stopped, or that
>>>> + * otherwise it is responsible for keeping any necessary data
>>>> structures for
>>>> + * in-progress jobs alive even when the free_job() callback is
>>>> called early (e.g. by
>>>> + * putting them in its own queue or doing its own refcounting).
>>>> + */
>>>> + list_for_each_entry_safe(s_job, tmp, &sched->pending_list, list) {
>>>> + spin_lock(&sched->job_list_lock);
>>>> + list_del_init(&s_job->list);
>>>> + spin_unlock(&sched->job_list_lock);
>>>> +
>>>> + dma_fence_set_error(&s_job->s_fence->finished, -ESRCH);
>>>> + drm_sched_fence_finished(s_job->s_fence);
>>>
>>> I'd imagine it's better to rebase this on top of drm-misc-next where
>>> drm_sched_fence_finished() takes one more parameter--the error.
>>
>> Ah, sure! I can do that.
>>
>>>
>>>> +
>>>> + WARN_ON(s_job->s_fence->parent);
>>>> + sched->ops->free_job(s_job);
>>>> + }
>>>> +
>>>> + kthread_stop(sched->thread);
>>>> for (i = DRM_SCHED_PRIORITY_COUNT - 1; i >=
>>>> DRM_SCHED_PRIORITY_MIN; i--) {
>>>> struct drm_sched_rq *rq = &sched->sched_rq[i];
>>>>
>>>
>>> Conceptually I don't mind this patch--I see what it is trying to
>>> achieve,
>>> but technically, we want the driver to detect GPU removal and return
>>> shared
>>> resources back, such as "jobs", which DRM is also aware of.
>>
>> I think you missed the context of why I'm doing this, so in short: my
>> use case (like Xe's) involves using a separate drm_sched instance *per
>> file* since these queues are scheduled directly by the firmware. So
>> this isn't about GPU removal, but rather about a GPU context going
>> away while jobs are in flight (e.g. the process got killed). We want
>> that to quickly kill the "DRM view" of the world, including signaling
>> all the fences with an error and freeing resources like the scheduler
>> itself.
>>
>> In the case of this particular GPU, there is no known way to actively
>> and synchronously abort GPU jobs, so we need to let them run to
>> completion (or failure), but we don't want that to block process
>> cleanup and freeing a bunch of high-level resources. The driver is
>> architected roughly along the lines of a firmware abstraction layer
>> that maps to the firmware shared memory structures, and then a layer
>> on top that implements the DRM view. When a process gets killed, the
>> DRM side (which includes the scheduler, etc.) gets torn down
>> immediately, and it makes sense to handle this cleanup inside
>> drm_sched since it already has a view into what jobs are in flight.
>> Otherwise, I would have to duplicate job tracking in the driver
>> (actually worse: in the Rust abstraction for safety), which doesn't
>> make much sense.
>>
>> But what I *do* have in the driver is tracking of the firmware
>> structures. So when the drm_sched gets torn down and all the jobs
>> killed, the underlying firmware jobs do run to completion, and the
>> resources they use are all cleaned up after that (it's all reference
>> counted). The primitive involved here is that in-flight firmware jobs
>> are assigned an event completion slot, and that keeps a reference to
>> them from a global array until the events fire and all the jobs are
>> known to have completed. This keeps things memory-safe, since we
>> absolutely cannot free/destroy firmware structures while they are in
>> use (otherwise the firmware crashes, which is fatal on these GPUs -
>> requires a full system reboot to recover).
>>
>> In practice, with the VM map model that we use, what ends up happening
>> when a process gets killed is that all the user objects for in-flight
>> jobs get unmapped, which usually causes the GPU hardware (not
>> firmware) to fault. This then triggers early termination of jobs
>> anyway via the firmware fault recovery flow. But even that takes some
>> short amount of time, and by then all the drm_sched stuff is long gone
>> and we're just dealing with the in-flight firmware stuff.
>>
>>> In the case where we're initiating the tear, we should notify the
>>> driver that
>>> we're about to forget jobs (resources), so that it knows to return
>>> them back
>>> or that it shouldn't notify us for them (since we've notified we're
>>> forgetting them.)
>>
>> That contradicts Christian's comment. I tried to document that (after
>> this patch) the scheduler no longer cares about hw fences and whether
>> they are signaled or not after it's destroyed, and I got a strongly
>> worded NAK for it. Sooo... which is it? Is it okay for drivers not to
>> signal the hw fence after a scheduler teardown, or not?
>>
>> But really, I don't see a use case for an explicit "about to forget
>> job" callback. The job free callback already serves the purpose of
>> telling the driver to clean up resources associated with a job. If it
>> wants to synchronously abort things there, it could easily take over
>> its own fence signaling and do something with the underlying stuff if
>> the fence is not signaled yet.
>>
>> In my case, since the driver is written in Rust and free_job() just
>> maps to the destructor (Drop impl), that just ends up freeing a bunch
>> of memory and other objects, and I don't particularly care about the
>> state of the firmware side any more after that. The flow is the same
>> whether it was a successful job completion, a failure, or an early
>> destruction due to the drm_sched getting torn down.
>>
>>> (Note also that in this latter case, traditionally, the device would
>>> be reset,
>>> so that we can guarantee that it has forgotten all shared resources
>>> which
>>> we are to tear up. This is somewhat more complicated with GPUs, thus
>>> the method
>>> pointed out above.)
>>
>> Yeah, in the firmware scheduling case we can't do this at all unless
>> the firmware has an explicit teardown/forget op (which I'm not aware
>> of) and a full GPU reset isn't something we can do either. Hence we
>> just let the underlying jobs complete. In practice they tend to die
>> pretty quickly anyway once all the buffers are unmapped.
>>
>> ~~ Lina
>>
>
