Re: [PATCH v5 2/3] drm/sched: Adjust outdated docu for run_job()

[Matthew Brost <matthew.brost@intel.com>]

On Mon, Feb 24, 2025 at 03:43:49PM +0100, Danilo Krummrich wrote:
> On Mon, Feb 24, 2025 at 10:29:26AM -0300, Maíra Canal wrote:
> > On 20/02/25 12:28, Philipp Stanner wrote:
> > > On Thu, 2025-02-20 at 10:28 -0300, Maíra Canal wrote:
> > > > Would it be possible to add a comment that `run_job()` must check if
> > > > `s_fence->finished.error` is different than 0? If you increase the
> > > > karma
> > > > of a job and don't check for `s_fence->finished.error`, you might run
> > > > a
> > > > cancelled job.
> > > 
> > > s_fence->finished is only signaled and its error set once the hardware
> > > fence got signaled; or when the entity is killed.
> > 
> > If you have a timeout, increase the karma of that job with
> > `drm_sched_increase_karma()` and call `drm_sched_resubmit_jobs()`, the
> > latter will flag an error in the dma fence. If you don't check for it in
> > `run_job()`, you will run the guilty job again.
> 
> Considering that drm_sched_resubmit_jobs() is deprecated I don't think we need
> to add this hint to the documentation; the drivers that are still using the API
> hopefully got it right.
> 
> > I'm still talking about `drm_sched_resubmit_jobs()`, because I'm
> > currently fixing an issue in V3D with the GPU reset and we still use
> > `drm_sched_resubmit_jobs()`. I read the documentation of `run_job()` and
> > `timeout_job()` and the information I commented here (which was crucial
> > to fix the bug) wasn't available there.
> 
> Well, hopefully... :-)
> 
> > 
> > `drm_sched_resubmit_jobs()` was deprecated in 2022, but Xe introduced a
> > new use in 2023
> 
> Yeah, that's a bit odd, since Xe relies on a firmware scheduler and uses a 1:1
> scheduler - entity setup. I'm a bit surprised Xe does use this function.
> 

To clarify Xe's usage. We use this function to resubmit jobs after
device reset for queues which had nothing to do with the device reset.
In practice, a device should never occur as we have per-queue resets in
our harwdare. If a per-queue reset occurs, we ban the queue rather than
doing a resubmit.

Matt  

> > for example. The commit that deprecated it just
> > mentions AMD's case, but do we know if the function works as expected
> > for the other users?
> 
> I read the comment [1] you're referring to differently. It says that
> "Re-submitting jobs was a concept AMD came up as cheap way to implement recovery
> after a job timeout".
> 
> It further explains that "there are many problem with the dma_fence
> implementation and requirements. Either the implementation is risking deadlocks
> with core memory management or violating documented implementation details of
> the dma_fence object", which doesn't give any hint to me that the conceptual
> issues are limited to amdgpu.
> 
> > For V3D, it does. Also, we need to make it clear which
> > are the dma fence requirements that the functions violates.
> 
> This I fully agree with, unfortunately the comment does not explain what's the
> issue at all.
> 
> While I do think I have a vague idea of what's the potential issue with this
> approach, I think it would be way better to get Christian, as the expert for DMA
> fence rules to comment on this.
> 
> @Christian: Can you please shed some light on this?
> 
> > 
> > If we shouldn't use `drm_sched_resubmit_jobs()`, would it be possible to
> > provide a common interface for job resubmission?
> 
> I wonder why this question did not come up when drm_sched_resubmit_jobs() was
> deprecated two years ago, did it?
> 
> Anyway, let's shed some light on the difficulties with drm_sched_resubmit_jobs()
> and then we can figure out how we can do better.
> 
> I think it would also be interesting to know how amdgpu handles job from
> unrelated entities being discarded by not re-submitting them when a job from
> another entitiy hangs the HW ring.
> 
> [1] https://patchwork.freedesktop.org/patch/msgid/20221109095010.141189-5-christian.koenig@amd.com