Re: [PATCH RFC 11/18] drm/scheduler: Clean up jobs when the scheduler is torn down
From: Christian König
Date: Thu Mar 09 2023 - 03:43:30 EST
Am 08.03.23 um 20:37 schrieb Asahi Lina:
On 09/03/2023 03.12, Christian König wrote:
Am 08.03.23 um 18:32 schrieb Asahi Lina:
[SNIP]
Yes but... none of this cleans up jobs that are already submitted by the
scheduler and in its pending list, with registered completion callbacks,
which were already popped off of the entities.
*That* is the problem this patch fixes!
Ah! Yes that makes more sense now.
We could add a warning when users of this API doesn't do this
correctly, but cleaning up incorrect API use is clearly something we
don't want here.
It is the job of the Rust abstractions to make incorrect API use that
leads to memory unsafety impossible. So even if you don't want that in
C, it's my job to do that for Rust... and right now, I just can't
because drm_sched doesn't provide an API that can be safely wrapped
without weird bits of babysitting functionality on top (like tracking
jobs outside or awkwardly making jobs hold a reference to the scheduler
and defer dropping it to another thread).
Yeah, that was discussed before but rejected.
The argument was that upper layer needs to wait for the hw to become
idle before the scheduler can be destroyed anyway.
Unfortunately, that's not a requirement you can encode in the Rust type
system easily as far as I know, and Rust safety rules mean we need to
make it safe even if the upper layer doesn't do this... (or else we have
to mark the entire drm_sched abstraction unsafe, but that would be a pity).
Yeah, that should really not be something we should do.
But you could make the scheduler depend on your fw context object, don't
you?
Detaching the scheduler from the underlying hw fences is certainly
possible, but we removed that functionality because some people people
tried to force push some Windows recovery module into Linux. We are in
the process of reverting that and cleaning things up once more, but that
will take a while.
Instead of detaching you could also block for the hw to become idle, but
if you do that synchronous on process termination you run into trouble
as well.
I know it's a different way of thinking, but it has pretty clear
benefits since with Rust you can actually guarantee that things are safe
overall by just auditing explicitly unsafe code. If we just mark all of
drm_sched unsafe, that means we now need to audit all details about how
the driver uses it for safety. It makes more sense to just make the
abstraction safe, which is much easier to audit.
I'm pretty familiar with that approach.
Right now, it is not possible to create a safe Rust abstraction for
drm_sched without doing something like duplicating all job tracking in
the abstraction, or the above backreference + deferred cleanup mess, or
something equally silly. So let's just fix the C side please ^^
Nope, as far as I can see this is just not correctly tearing down the
objects in the right order.
There's no API to clean up in-flight jobs in a drm_sched at all.
Destroying an entity won't do it. So there is no reasonable way to do
this at all...
Yes, this was removed.
So you are trying to do something which is not supposed to work in the
first place.
I need to make things that aren't supposed to work impossible to do in
the first place, or at least fail gracefully instead of just oopsing
like drm_sched does today...
If you're convinced there's a way to do this, can you tell me exactly
what code sequence I need to run to safely shut down a scheduler
assuming all entities are already destroyed? You can't ask me for a list
of pending jobs (the scheduler knows this, it doesn't make any sense to
duplicate that outside), and you can't ask me to just not do this until
all jobs complete execution (because then we either end up with the
messy deadlock situation I described if I take a reference, or more
duplicative in-flight job count tracking and blocking in the free path
of the Rust abstraction, which doesn't make any sense either).
Good question. We don't have anybody upstream which uses the scheduler
lifetime like this.
Essentially the job list in the scheduler is something we wanted to
remove because it causes tons of race conditions during hw recovery.
When you tear down the firmware queue how do you handle already
submitted jobs there?
The firmware queue is itself reference counted and any firmware queue
that has acquired an event notification resource (that is, which is busy
with running or upcoming jobs) hands off a reference to itself into the
event subsystem, so it can get notified of job completions by the
firmware. Then once it becomes idle it unregisters itself, and at that
point if it has no owning userspace queue, that would be the last
reference and it gets dropped. So we don't tear down firmware queues
until they are idle.
And could those fw queue not reference the scheduler?
(There is a subtle deadlock break in the event module to make this work
out, where we clone a reference to the queue and drop the event
subsystem lock before signaling it of completions, so it can call back
in and take the lock as it unregisters itself if needed. Then the actual
teardown happens when the signaling is complete and that reference clone
is the last one to get dropped.)
If a queue is idle at the firmware level but has upcoming jobs queued in
drm_sched, when those get deleted as part of an explicit drm_sched
teardown (free_job()) the queue notices it lost its upcoming jobs and
relinquishes the event resource if there are no running jobs. I'm not
even sure exactly what order this all happens in in practice (it depends
on structure field order in Rust!), but it doesn't really matter because
either way everything gets cleaned up one way or another.
I actually don't know of any way to actively abort jobs on the firmware,
so this is pretty much the only option I have. I've even seen
long-running compute jobs on macOS run to completion even if you kill
the submitting process, so there might be no way to do this at all.
Though in practice since we unmap everything from the VM anyway when the
userspace stuff gets torn down, almost any normal GPU work is going to
immediately fault at that point (macOS doesn't do this because macOS
effectively does implicit sync with BO tracking at the kernel level...).
Oh, that is an interesting information. How does macOS do explicit sync
then or isn't that supported at all?
By the way, I don't really use the hardware recovery stuff right now.
I'm not even sure if there is a sensible way I could use it, since as I
said we can't exactly abort jobs. I know there are ways to lock up the
firmware/GPU, but so far those have all been things the kernel driver
can prevent, and I'm not even sure if there is any way to recover from
that anyway. The firmware itself has its own timeouts and recovery for
"normal" problems. From the point of view of the driver and everything
above it, in-flight commands during a GPU fault or timeout are just
marked complete by the firmware, after a firmware recovery cycle where
the driver gets notified of the problem (that's when we mark the
commands failed so we can propagate the error).
Yeah, that's exactly what we are telling our fw people for years that we
need this as well.
There is no re-submission or anything, userspace just gets told of the problem but
the queue survives.
In the future it might be possible to re-submit innocent commands
Long story short: Don't do this! This is what the Windows drivers have
been doing and it creates tons of problems.
Just signal the problem back to userspace and let the user space driver
decide what to do.
The background is that most graphics applications (games etc..) then
rather start on the next frame instead of submitting the current one
again while compute applications make sure that the abort and tell the
user that the calculations might be corrupted and need to be redone.
Regards,
Christian.
(it is possible for a GPU fault to break another
process running concurrently, and this is a problem macOS has too...),
which is still not perfect due to side effects but might work most of
the time, but that depends on the "command patching" stuff I mentioned,
and I'm still not even sure if it will be possible to do safely. There's
a lot of subtlety around what we can and can't do during a firmware
recovery cycle that I haven't even started to investigate yet (the
answer could be "nothing" even).
~~ Lina