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

Mika Kuoppala
2015-07-03T11:15:42Z

Tomas Elf <tomas.elf@intel.com> writes:

> 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.
>

Have you considered promoting from failed full gpu reset to pci level reset?
Would require some amount of rethinking on init as it would kill the
display also. On lunch Ville threw idea of pushing the device to
d3(?) state to kill the power. That would get rid of the most stubborn
state (wrt skl gpu reset problems).

-Mika

> 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
>
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