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.
+
+ 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.
