Date:   Wed, 19 Jul 2023 10:45:01 +0200
From:   Christian König <christian.koenig@....com>
To:     Asahi Lina <lina@...hilina.net>,
        Luben Tuikov <luben.tuikov@....com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Sumit Semwal <sumit.semwal@...aro.org>
Cc:     Faith Ekstrand <faith.ekstrand@...labora.com>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        linux-media@...r.kernel.org, asahi@...ts.linux.dev
Subject: Re: [PATCH 3/3] drm/scheduler: Clean up jobs when the scheduler is
 torn down.

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.

Regards,
Christian.

>
>>
>>> Signed-off-by: Asahi Lina <lina@...hilina.net>
>>> ---
>>>   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
>

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