[Intel-xe] [RFC PATCH 00/10] Xe DRM scheduler and long running workload plans
Zeng, Oak
oak.zeng at intel.com
Fri Apr 7 00:20:35 UTC 2023
So this series basically go with option 2. The part that option2 makes me uncomfortable is, dma-fence doesn't work for long running workload, why we generate it in the first place? As long as dma-fence is generated, it will become a source of confusion in the future. It doesn't matter how much you annotate it/document it. So if we decide to go with option2, the bottom line is, don't generate dma-fence for long running workload during job submission. This requires some rework in drm scheduler.
The cleanest solution to me is option3. Dma-fence is a very old technology. When it was created, no gpu support page fault. Obviously this is not a good technology for modern gpu with page fault support. I think the best way is to create a new scheduler and dependency tracking mechanism works for both page fault enabled and page fault disabled context. I think this matches what Christian said below. Maybe nobody think this is easy?
Thanks,
Oak
> -----Original Message-----
> From: Brost, Matthew <matthew.brost at intel.com>
> Sent: April 5, 2023 2:53 PM
> To: Zeng, Oak <oak.zeng at intel.com>
> Cc: Christian König <christian.koenig at amd.com>; Vetter, Daniel
> <daniel.vetter at intel.com>; Thomas Hellström
> <thomas.hellstrom at linux.intel.com>; dri-devel at lists.freedesktop.org; intel-
> xe at lists.freedesktop.org; robdclark at chromium.org; airlied at linux.ie;
> lina at asahilina.net; boris.brezillon at collabora.com; faith.ekstrand at collabora.com
> Subject: Re: [RFC PATCH 00/10] Xe DRM scheduler and long running workload
> plans
>
> On Wed, Apr 05, 2023 at 12:06:53PM -0600, Zeng, Oak wrote:
> > Hi,
> >
> > Using dma-fence for completion/dependency tracking for long-run
> workload(more precisely on-demand paging/page fault enabled workload) can
> cause deadlock. This seems the significant issue here. Other issues such as the
> drm scheduler completion order implication etc are minors which can be solve
> inside the framework of drm scheduler. We need to evaluate below paths:
> >
> > 1) still use drm scheduler for job submission, and use dma-fence for job
> completion waiting/dependency tracking. This is solution proposed in this series.
> Annotate dma-fence for long-run workload: user can still wait dma-fence for job
> completion but can't wait dma-fence while holding any memory management
> locks. We still use dma-fence for dependency tracking. But it is just very easily
> run into deadlock when on-demand paging is in the picture. The annotation helps
> us to detect deadlock but not solve deadlock problems. Seems *not* a complete
> solution: It is almost impossible to completely avoid dependency deadlock in
> complex runtime environment
> >
>
> No one can wait on LR fence, so it is impossible to deadlock. The
> annotations enforce this. Literally this is only for flow controling the
> ring / hold pending jobs in in the DRM schedule list.
>
> > 2) Still use drm scheduler but not use dma-fence for completion signaling
> and dependency tracking. This way we still get some free functions (reset, err
> handling ring flow control as Matt said)from drm scheduler, but push the
> dependency/completion tracking completely to user space using techniques such
> as user space fence. User space doesn't have chance to wait fence while holding
> a kernel memory management lock, thus the dma-fence deadlock issue is solved.
> >
>
> We use user space fence for syncs.
>
> > 3) Completely discard drm scheduler and dma-fence for long-run
> workload. Use user queue/doorbell for super fast submission, directly interact
> with fw scheduler. Use user fence for completion/dependency tracking.
> >
>
> This is a hard no from me, I want 1 submission path in Xe. Either we use
> the DRM scheduler or we don't.
>
> Matt
>
> > Thanks,
> > Oak
> >
> > > -----Original Message-----
> > > From: Christian König <christian.koenig at amd.com>
> > > Sent: April 5, 2023 3:30 AM
> > > To: Brost, Matthew <matthew.brost at intel.com>; Zeng, Oak
> > > <oak.zeng at intel.com>
> > > Cc: dri-devel at lists.freedesktop.org; intel-xe at lists.freedesktop.org;
> > > robdclark at chromium.org; thomas.hellstrom at linux.intel.com;
> airlied at linux.ie;
> > > lina at asahilina.net; boris.brezillon at collabora.com;
> faith.ekstrand at collabora.com
> > > Subject: Re: [RFC PATCH 00/10] Xe DRM scheduler and long running workload
> > > plans
> > >
> > > Am 04.04.23 um 20:08 schrieb Matthew Brost:
> > > > On Tue, Apr 04, 2023 at 12:02:03PM -0600, Zeng, Oak wrote:
> > > >> Hi Matt, Thomas,
> > > >>
> > > >> Some very bold out of box thinking in this area:
> > > >>
> > > >> 1. so you want to use drm scheduler and dma-fence for long running
> workload.
> > > Why you want to do this in the first place? What is the benefit? Drm scheduler
> is
> > > pretty much a software scheduler. Modern gpu has scheduler built at fw/hw
> > > level, as you said below for intel this is Guc. Can xe driver just directly submit
> job
> > > to Guc, bypassing drm scheduler?
> > > >>
> > > > If we did that now we have 2 paths for dependency track, flow controling
> > > > the ring, resets / error handling / backend submission implementations.
> > > > We don't want this.
> > >
> > > Well exactly that's the point: Why?
> > >
> > > As far as I can see that are two completely distinct use cases, so you
> > > absolutely do want two completely distinct implementations for this.
> > >
> > > >> 2. using dma-fence for long run workload: I am well aware that page fault
> (and
> > > the consequent memory allocation/lock acquiring to fix the fault) can cause
> > > deadlock for a dma-fence wait. But I am not convinced that dma-fence can't
> be
> > > used purely because the nature of the workload that it runs very long
> (indefinite).
> > > I did a math: the dma_fence_wait_timeout function's third param is the
> timeout
> > > which is a signed long type. If HZ is 1000, this is about 23 days. If 23 days is not
> long
> > > enough, can we just change the timeout parameter to signed 64 bits so it is
> much
> > > longer than our life time...
> > > >>
> > > >> So I mainly argue we can't use dma-fence for long-run workload is not
> > > because the workload runs very long, rather because of the fact that we use
> > > page fault for long-run workload. If we enable page fault for short-run
> workload,
> > > we can't use dma-fence either. Page fault is the key thing here.
> > > >>
> > > >> Now since we use page fault which is *fundamentally* controversial with
> > > dma-fence design, why now just introduce a independent concept such as
> user-
> > > fence instead of extending existing dma-fence?
> > > >>
> > > >> I like unified design. If drm scheduler, dma-fence can be extended to work
> for
> > > everything, it is beautiful. But seems we have some fundamental problem
> here.
> > > >>
> > > > Thomas's patches turn a dma-fence into KMD sync point (e.g. we just use
> > > > the signal / CB infrastructure) and enforce we don't use use these
> > > > dma-fences from the scheduler in memory reclaim paths or export these to
> > > > user space or other drivers. Think of this mode as SW only fence.
> > >
> > > Yeah and I truly think this is an really bad idea.
> > >
> > > The signal/CB infrastructure in the dma_fence turned out to be the
> > > absolutely nightmare I initially predicted. Sorry to say that, but in
> > > this case the "I've told you so" is appropriate in my opinion.
> > >
> > > If we need infrastructure for long running dependency tracking we should
> > > encapsulate that in a new framework and not try to mangle the existing
> > > code for something it was never intended for.
> > >
> > > Christian.
> > >
> > > >
> > > > Matt
> > > >
> > > >> Thanks,
> > > >> Oak
> > > >>
> > > >>> -----Original Message-----
> > > >>> From: dri-devel <dri-devel-bounces at lists.freedesktop.org> On Behalf Of
> > > >>> Matthew Brost
> > > >>> Sent: April 3, 2023 8:22 PM
> > > >>> To: dri-devel at lists.freedesktop.org; intel-xe at lists.freedesktop.org
> > > >>> Cc: robdclark at chromium.org; thomas.hellstrom at linux.intel.com;
> > > airlied at linux.ie;
> > > >>> lina at asahilina.net; boris.brezillon at collabora.com; Brost, Matthew
> > > >>> <matthew.brost at intel.com>; christian.koenig at amd.com;
> > > >>> faith.ekstrand at collabora.com
> > > >>> Subject: [RFC PATCH 00/10] Xe DRM scheduler and long running workload
> > > plans
> > > >>>
> > > >>> Hello,
> > > >>>
> > > >>> As a prerequisite to merging the new Intel Xe DRM driver [1] [2], we
> > > >>> have been asked to merge our common DRM scheduler patches first as
> well
> > > >>> as develop a common solution for long running workloads with the DRM
> > > >>> scheduler. This RFC series is our first attempt at doing this. We
> > > >>> welcome any and all feedback.
> > > >>>
> > > >>> This can we thought of as 4 parts detailed below.
> > > >>>
> > > >>> - DRM scheduler changes for 1 to 1 relationship between scheduler and
> > > >>> entity (patches 1-3)
> > > >>>
> > > >>> In Xe all of the scheduling of jobs is done by a firmware scheduler (the
> > > >>> GuC) which is a new paradigm WRT to the DRM scheduler and presents
> > > >>> severals problems as the DRM was originally designed to schedule jobs
> on
> > > >>> hardware queues. The main problem being that DRM scheduler expects
> the
> > > >>> submission order of jobs to be the completion order of jobs even across
> > > >>> multiple entities. This assumption falls apart with a firmware scheduler
> > > >>> as a firmware scheduler has no concept of jobs and jobs can complete
> out
> > > >>> of order. A novel solution for was originally thought of by Faith during
> > > >>> the initial prototype of Xe, create a 1 to 1 relationship between scheduler
> > > >>> and entity. I believe the AGX driver [3] is using this approach and
> > > >>> Boris may use approach as well for the Mali driver [4].
> > > >>>
> > > >>> To support a 1 to 1 relationship we move the main execution function
> > > >>> from a kthread to a work queue and add a new scheduling mode which
> > > >>> bypasses code in the DRM which isn't needed in a 1 to 1 relationship.
> > > >>> The new scheduling mode should unify all drivers usage with a 1 to 1
> > > >>> relationship and can be thought of as using scheduler as a dependency /
> > > >>> infligt job tracker rather than a true scheduler.
> > > >>>
> > > >>> - Generic messaging interface for DRM scheduler
> > > >>>
> > > >>> Idea is to be able to communicate to the submission backend with in
> band
> > > >>> (relative to main execution function) messages. Messages are backend
> > > >>> defined and flexable enough for any use case. In Xe we use these
> > > >>> messages to clean up entites, set properties for entites, and suspend /
> > > >>> resume execution of an entity [5]. I suspect other driver can leverage
> > > >>> this messaging concept too as it a convenient way to avoid races in the
> > > >>> backend.
> > > >>>
> > > >>> - Support for using TDR for all error paths of a scheduler / entity
> > > >>>
> > > >>> Fix a few races / bugs, add function to dynamically set the TDR timeout.
> > > >>>
> > > >>> - Annotate dma-fences for long running workloads.
> > > >>>
> > > >>> The idea here is to use dma-fences only as sync points within the
> > > >>> scheduler and never export them for long running workloads. By
> > > >>> annotating these fences as long running we ensure that these dma-
> fences
> > > >>> are never used in a way that breaks the dma-fence rules. A benefit of
> > > >>> thus approach is the scheduler can still safely flow control the
> > > >>> execution ring buffer via the job limit without breaking the dma-fence
> > > >>> rules.
> > > >>>
> > > >>> Again this a first draft and looking forward to feedback.
> > > >>>
> > > >>> Enjoy - Matt
> > > >>>
> > > >>> [1] https://gitlab.freedesktop.org/drm/xe/kernel
> > > >>> [2] https://patchwork.freedesktop.org/series/112188/
> > > >>> [3] https://patchwork.freedesktop.org/series/114772/
> > > >>> [4]
> https://patchwork.freedesktop.org/patch/515854/?series=112188&rev=1
> > > >>> [5] https://gitlab.freedesktop.org/drm/xe/kernel/-/blob/drm-xe-
> > > >>> next/drivers/gpu/drm/xe/xe_guc_submit.c#L1031
> > > >>>
> > > >>> Matthew Brost (8):
> > > >>> drm/sched: Convert drm scheduler to use a work queue rather than
> > > >>> kthread
> > > >>> drm/sched: Move schedule policy to scheduler / entity
> > > >>> drm/sched: Add DRM_SCHED_POLICY_SINGLE_ENTITY scheduling
> policy
> > > >>> drm/sched: Add generic scheduler message interface
> > > >>> drm/sched: Start run wq before TDR in drm_sched_start
> > > >>> drm/sched: Submit job before starting TDR
> > > >>> drm/sched: Add helper to set TDR timeout
> > > >>> drm/syncobj: Warn on long running dma-fences
> > > >>>
> > > >>> Thomas Hellström (2):
> > > >>> dma-buf/dma-fence: Introduce long-running completion fences
> > > >>> drm/sched: Support long-running sched entities
> > > >>>
> > > >>> drivers/dma-buf/dma-fence.c | 142 +++++++---
> > > >>> drivers/dma-buf/dma-resv.c | 5 +
> > > >>> drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c | 14 +-
> > > >>> drivers/gpu/drm/amd/amdgpu/amdgpu_device.c | 15 +-
> > > >>> drivers/gpu/drm/drm_syncobj.c | 5 +-
> > > >>> drivers/gpu/drm/etnaviv/etnaviv_sched.c | 5 +-
> > > >>> drivers/gpu/drm/lima/lima_sched.c | 5 +-
> > > >>> drivers/gpu/drm/msm/adreno/adreno_device.c | 6 +-
> > > >>> drivers/gpu/drm/msm/msm_ringbuffer.c | 5 +-
> > > >>> drivers/gpu/drm/panfrost/panfrost_job.c | 5 +-
> > > >>> drivers/gpu/drm/scheduler/sched_entity.c | 127 +++++++--
> > > >>> drivers/gpu/drm/scheduler/sched_fence.c | 6 +-
> > > >>> drivers/gpu/drm/scheduler/sched_main.c | 278 +++++++++++++++--
> ---
> > > >>> drivers/gpu/drm/v3d/v3d_sched.c | 25 +-
> > > >>> include/drm/gpu_scheduler.h | 130 +++++++--
> > > >>> include/linux/dma-fence.h | 60 ++++-
> > > >>> 16 files changed, 649 insertions(+), 184 deletions(-)
> > > >>>
> > > >>> --
> > > >>> 2.34.1
> >
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