On 2023-07-16 03:51, Asahi Lina wrote:
> 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.
>
>>> 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.
Not sure about other drivers--they can speak for themselves and the CC list
should include them--please use "dim add-missing-cc" and make sure
that the Git commit description contains the Cc tags--then git send-email
will populate the SMTP CC. Feel free to add more Cc tags on top of that.
>
>>
>>> 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.
It's worth posting it as a stand-alone patch. Please make sure to add Cc tags
into the commit description--use "dim add-missing-cc", perhaps also
git-blame and git-log might help with additional Cc. "scripts/get_maintainer.pl"
for files unaffected by this commit. (dim add-missing-cc uses get_maintainer.pl
for affected files.)
Feel free to post it stand-alone and we'll let the natural review process take over. :-)
>
>>
>>> +
>>> + 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
As a general rule of thumb, in my writing emails I try to avoid using
"you" and "I" as much as possible--it sets this divisive stage, and it
can get misrepresented, especially in email.
As is the case in research literature, if I absolutely have to use a pronoun--which
rarely happens, I always use "we", and this is the most number of "I"-s I've used
in a long while.
> 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 ref-count definitely helps here.
> 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?
Christian is correct in that we don't want to hang upstream control
to the whims of a low-level device driver.
> 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
Long time ago, in a galaxy far far away, this was needed in order
to prevent device write-DMA into non-existing (random) memory. As
this is not the case anymore, go with Christian's comment.
> 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.
Perhaps in the future, as more complex workloads are deferred to this
hardware and driver, a real-time requirement might be needed for this
"tend to die pretty quickly", that that there's some guarantee of
work resuming in some finite time.
--
Regards,
Luben
>
> ~~ Lina
>
