Re: [RFC 00/11] TDR/watchdog timeout support for gen8

Daniel Vetter
2015-06-23T11:35:43Z

On Tue, Jun 23, 2015 at 11:47:16AM +0100, Tomas Elf wrote:
> On 23/06/2015 11:05, Daniel Vetter wrote:
> >Your patches don't apply cleanly any more and I can't find a suitable
> >baseline where they would. But I'd like to see it all in context to check
> >a few things.
> >
> >Can you pls push a git branch with these somewhere?
> >
>
> Here's the baseline for my local tree:
> cd07637 - drm-intel-nightly: 2015y-04m-13d-09h-46m-59s UTC integration
> manifest <daniel.vetter@ffwll.ch>

I don't have that baseline around here (any more at least). Happens
regularly with rebasing trees.

> I haven't updated it in a while obviously since I thought that could wait
> until we'd worked our way through the RFC series and I could get to work on
> the first real patch series.
>
> Is it possible for you to set up a local tree of your own with my baseline
> and my RFC patches on top or would you prefer it if I push my branch to
> drm-private?

So yeah I need your branch ;-)
-Danil
>
> Thanks,
> Tomas
>
> >Thanks, Daniel
> >
> >On Mon, Jun 08, 2015 at 06:03:18PM +0100, Tomas Elf wrote:
> >>This patch series introduces the following features:
> >>
> >>* Feature 1: TDR (Timeout Detection and Recovery) for gen8 execlist mode.
> >>
> >>TDR is an umbrella term for anything that goes into detecting and recovering
> >>from GPU hangs and is a term more widely used outside of the upstream driver.
> >>This feature introduces an extensible framework that currently supports gen8
> >>but that can be easily extended to support gen7 as well (which is already the
> >>case in GMIN but unfortunately in a not quite upstreamable form). The code
> >>contained in this submission represents the essentials of what is currently in
> >>GMIN merged with what is currently in upstream (as of the time when this work
> >>commenced a few months back).
> >>
> >>This feature adds a new hang recovery path alongside the legacy GPU reset path,
> >>which takes care of engine recovery only. Aside from adding support for
> >>per-engine recovery this feature also introduces rules for how to promote a
> >>potential per-engine reset to a legacy, full GPU reset.
> >>
> >>The hang checker now integrates with the error handler in a slightly different
> >>way in that it allows hang recovery on multiple engines at the same time by
> >>passing an engine flag mask to the error handler where flags representing all
> >>of the hung engines are set. This allows us to schedule hang recovery once for
> >>all currently hung engines instead of one hang recovery per detected engine
> >>hang. Previously, when only full GPU reset was supported this was all the same
> >>since it wouldn't matter if one or four engines were hung at any given point
> >>since it would all amount to the same thing - the GPU getting reset. As it
> >>stands now the behaviour is different depending on which engine is hung since
> >>each engine is reset separately from all the other engines, therefore we have
> >>to think about this in terms of scheduling cost and recovery latency. (see
> >>open question below)
> >>
> >>OPEN QUESTIONS:
> >>
> >> 1. Do we want to investigate the possibility of per-engine hang
> >> detection? In the current upstream driver there is only one work queue
> >> that handles the hang checker and everything from initial hang
> >> detection to final hang recovery runs in this thread. This makes sense
> >> if you're only supporting one form of hang recovery - using full GPU
> >> reset and nothing tied to any particular engine. However, as part
> >> of this patch series we're changing that by introducing per-engine
> >> hang recovery. It could make sense to introduce multiple work
> >> queues - one per engine - to run multiple hang checking threads in
> >> parallel.
> >>
> >> This would potentially allow savings in terms of recovery latency since
> >> we don't have to scan all engines every time the hang checker is
> >> scheduled and the error handler does not have to scan all engines every
> >> time it is scheduled. Instead, we could implement one work queue per
> >> engine that would invoke the hang checker that only checks _that_
> >> particular engine and then the error handler is invoked for _that_
> >> particular engine. If one engine has hung the latency for getting to
> >> the hang recovery path for that particular engine would be (Time For
> >> Hang Checking One Engine) + (Time For Error Handling One Engine) rather
> >> than the time it takes to do hang checking for all engines + the time
> >> it takes to do error handling for all engines that have been detected
> >> as hung (which in the worst case would be all engines). There would
> >> potentially be as many hang checker and error handling threads going on
> >> concurrently as there are engines in the hardware but they would all be
> >> running in parallel without any significant locking. The first time
> >> where any thread needs exclusive access to the driver is at the point
> >> of the actual hang recovery but the time it takes to get there would
> >> theoretically be lower and the time it actually takes to do per-engine
> >> hang recovery is quite a lot lower than the time it takes to actually
> >> detect a hang reliably.
> >>
> >> How much we would save by such a change still needs to be analysed and
> >> compared against the current single-thread model but it makes sense
> >> from a theoretical design point of view.
> >>
> >> 2. How does per-engine reset integrate with the public reset stats
> >> IOCTL? These stats are used for the GL robustness interface and
> >> currently these tests are failing when running per-engine hang recovery
> >> since we treat per-engine recovery differently from full GPU recovery,
> >> which is nothing that userland knows anything about. When userland
> >> expects to hang the hardware it expects the reset stat interface to
> >> reflect this, which is something that has changed as part of this code
> >> submission. There's more than one way to solve this. Here are two options:
> >>
> >> 1. Expose per-engine reset statistics and set contexts as
> >> guilty the same way for per-engine reset as for full GPU
> >> resets.
> >>
> >> That would make this change to the hang recovery mechanism
> >> transparent to userland but it would change the semantics since
> >> an active context in the reset stats no longer implies that the
> >> GPU was fully reset.
> >>
> >> 2. Add a new set of statistics for per-engine reset (one group
> >> of statistics for each engine) to reflect the extended
> >> capabilities that per-engine hang recovery offers.
> >>
> >> Would that be breaking the ABI?
> >>
> >> ... Or are there any other way of doing this?
> >>
> >>* Feature 2: Watchdog Timeout (a.k.a "media engine reset") for gen8.
> >>
> >>This feature allows userland applications to control whether or not individual
> >>batch buffers should have a first-level, fine-grained, hardware-based hang
> >>detection mechanism on top of the ordinary, software-based periodic hang
> >>checker that is already in the driver. The advantage over relying solely on the
> >>current software-based hang checker is that the watchdog timeout mechanism is
> >>about 1000x quicker and more precise. Since it's not a full driver-level hang
> >>detection mechanism but only targetting one individual batch buffer at a time
> >>it can afford to be that quick without risking an increase in false positive
> >>hang detection.
> >>
> >>This feature includes the following changes:
> >>
> >>a) Watchdog timeout interrupt service routine for handling watchdog interrupts
> >>and connecting these to per-engine hang recovery.
> >>
> >>b) Injection of watchdog timer enablement/cancellation instructions
> >>before/after the batch buffer start instruction in the ring buffer so that
> >>watchdog timeout is connected to the submission of an individual batch buffer.
> >>
> >>c) Extension of the DRM batch buffer interface, exposing the watchdog timeout
> >>feature to userland. We've got two open source groups in VPG currently in the
> >>process of integrating support for this feature, which should make it
> >>principally possible to upstream this extension.
> >>
> >>There is currently full watchdog timeout support for gen7 in GMIN and it is
> >>quite similar to the gen8 implementation so there is nothing obvious that
> >>prevents us from upstreaming that code along with the gen8 code. However,
> >>watchdog timeout is fully dependent on the per-engine hang recovery path and
> >>that is not part of this code submission for gen7. Therefore watchdog timeout
> >>support for gen7 has been excluded until per-engine hang recovery support for
> >>gen7 has landed upstream.
> >>
> >>As part of this submission we've had to reinstate the work queue that was
> >>previously in place between the error handler and the hang recovery path. The
> >>reason for this is that the per-engine recovery path is called directly from
> >>the interrupt handler in the case of watchdog timeout. In that situation
> >>there's no way of grabbing the struct_mutex, which is a requirement for the
> >>hang recovery path. Therefore, by reinstating the work queue we provide a
> >>unified execution context for the hang recovery code that allows the hang
> >>recovery code to grab whatever locks it needs without sacrificing interrupt
> >>latency too much or sleeping indefinitely in hard interrupt context.
> >>
> >>* Feature 3. Context Submission Status Consistency checking
> >>
> >>Something that becomes apparent when you run long-duration operations tests
> >>with concurrent rendering processes with intermittently injected hangs is that
> >>it seems like the GPU forgets to send context completion interrupts to the
> >>driver under some circumstances. What this means is that the driver sometimes
> >>gets stuck on a context that never seems to finish, all the while the hardware
> >>has completed and is waiting for more work.
> >>
> >>The problem with this is that the per-engine hang recovery path relies on
> >>context resubmission to kick off the hardware again following an engine reset.
> >>This can only be done safely if the hardware and driver share the same opinion
> >>about the current state. Therefore we've extended the periodic hang checker to
> >>check for context submission state inconsistencies aside from the hang checking
> >>it already does.
> >>
> >>If such a state is detected it is assumed (based on experience) that a context
> >>completion interrupt has been lost somehow. If this state persists for some
> >>time an attempt to correct it is made by faking the presumably lost context
> >>completion interrupt by manually calling the execlist interrupt handler, which
> >>is normally called from the main interrupt handler cued by a received context
> >>event interrupt. Just because an interrupt goes missing does not mean that the
> >>context status buffer (CSB) does not get appropriately updated by the hardware,
> >>which means that we can expect to find all the recent changes to the context
> >>states for each engine captured there. If there are outstanding context status
> >>changes in store there then the faked context event interrupt will allow the
> >>interrupt handler to act on them. In the case of lost context completion
> >>interrupts this will prompt the driver to remove the already completed context
> >>from the execlist queue and move on to the next pending piece of work and
> >>thereby eliminating the inconsistency.
> >>
> >>* Feature 4. Debugfs extensions for per-engine hang recovery and TDR/watchdog trace
> >>points.
> >>
> >>
> >>Tomas Elf (11):
> >> drm/i915: Early exit from semaphore_waits_for for execlist mode.
> >> drm/i915: Introduce uevent for full GPU reset.
> >> drm/i915: Add reset stats entry point for per-engine reset.
> >> drm/i915: Adding TDR / per-engine reset support for gen8.
> >> drm/i915: Extending i915_gem_check_wedge to check engine reset in
> >> progress
> >> drm/i915: Disable warnings for TDR interruptions in the display
> >> driver.
> >> drm/i915: Reinstate hang recovery work queue.
> >> drm/i915: Watchdog timeout support for gen8.
> >> drm/i915: Fake lost context interrupts through forced CSB check.
> >> drm/i915: Debugfs interface for per-engine hang recovery.
> >> drm/i915: TDR/watchdog trace points.
> >>
> >> drivers/gpu/drm/i915/i915_debugfs.c | 146 +++++-
> >> drivers/gpu/drm/i915/i915_dma.c | 79 +++
> >> drivers/gpu/drm/i915/i915_drv.c | 201 ++++++++
> >> drivers/gpu/drm/i915/i915_drv.h | 91 +++-
> >> drivers/gpu/drm/i915/i915_gem.c | 93 +++-
> >> drivers/gpu/drm/i915/i915_gpu_error.c | 2 +-
> >> drivers/gpu/drm/i915/i915_irq.c | 378 ++++++++++++--
> >> drivers/gpu/drm/i915/i915_params.c | 10 +
> >> drivers/gpu/drm/i915/i915_reg.h | 13 +
> >> drivers/gpu/drm/i915/i915_trace.h | 298 +++++++++++
> >> drivers/gpu/drm/i915/intel_display.c | 16 +-
> >> drivers/gpu/drm/i915/intel_lrc.c | 858 ++++++++++++++++++++++++++++++-
> >> drivers/gpu/drm/i915/intel_lrc.h | 16 +-
> >> drivers/gpu/drm/i915/intel_lrc_tdr.h | 40 ++
> >> drivers/gpu/drm/i915/intel_ringbuffer.c | 87 +++-
> >> drivers/gpu/drm/i915/intel_ringbuffer.h | 109 ++++
> >> drivers/gpu/drm/i915/intel_uncore.c | 241 ++++++++-
> >> include/uapi/drm/i915_drm.h | 5 +-
> >> 18 files changed, 2589 insertions(+), 94 deletions(-)
> >> create mode 100644 drivers/gpu/drm/i915/intel_lrc_tdr.h
> >>
> >>--
> >>1.7.9.5
> >>
> >>_______________________________________________
> >>Intel-gfx mailing list
> >>Intel-gfx@lists.freedesktop.org
> >>http://lists.freedesktop.org/mailman/listinfo/intel-gfx
> >
>

--
Daniel Vetter
Software Engineer, Intel Corporation