Re: [PATCH 3/3] drm/scheduler: Clean up jobs when the scheduler is torn down.

[Matthew Brost]

On Mon, Jul 17, 2023 at 01:40:38PM -0400, Luben Tuikov wrote:
> 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.
> 

Xe doesn't need this as our reference counting scheme doesn't allow
drm_sched_fini to be called when jobs are pending. If we want to
teardown a drm_sched we set TDR timeout to zero and all pending jobs
gets cleaned up that way, the ref of sched will go to zero, and
drm_sched_fini is called. The caveat here being I think we need a worker
to call drm_sched_fini as the last ref to scheduler might be dropped
from within scheduler main thread.

That being said, I doubt this patch breaks anything in Xe so do not a
real strong opinion on this.

Matt

> > 
> >>
> >>> 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
> > 
>