[Intel-gfx] [RFC 1/3] drm/i915: Watchdog timeout: IRQ handler for gen8+

Thu Feb 23 21:49:44 UTC 2017

On Thu, Feb 23, 2017 at 01:21:03PM -0800, Michel Thierry wrote:
> 
> 
> On 23/02/17 12:57, Chris Wilson wrote:
> >On Thu, Feb 23, 2017 at 11:44:17AM -0800, Michel Thierry wrote:
> >>*** General ***
> >>
> >>Watchdog timeout (or "media engine reset") is a feature that allows
> >>userland applications to enable hang detection on individual batch buffers.
> >>The detection mechanism itself is mostly bound to the hardware and the only
> >>thing that the driver needs to do to support this form of hang detection
> >>is to implement the interrupt handling support as well as watchdog command
> >>emission before and after the emitted batch buffer start instruction in the
> >>ring buffer.
> >>
> >>The principle of the hang detection mechanism is as follows:
> >>
> >>1. Once the decision has been made to enable watchdog timeout for a
> >>particular batch buffer and the driver is in the process of emitting the
> >>batch buffer start instruction into the ring buffer it also emits a
> >>watchdog timer start instruction before and a watchdog timer cancellation
> >>instruction after the batch buffer start instruction in the ring buffer.
> >>
> >>2. Once the GPU execution reaches the watchdog timer start instruction
> >>the hardware watchdog counter is started by the hardware. The counter
> >>keeps counting until either reaching a previously configured threshold
> >>value or the timer cancellation instruction is executed.
> >>
> >>2a. If the counter reaches the threshold value the hardware fires a
> >>watchdog interrupt that is picked up by the watchdog interrupt handler.
> >>This means that a hang has been detected and the driver needs to deal with
> >>it the same way it would deal with a engine hang detected by the periodic
> >>hang checker. The only difference between the two is that we already blamed
> >>the active request (to ensure an engine reset).
> >>
> >>2b. If the batch buffer completes and the execution reaches the watchdog
> >>cancellation instruction before the watchdog counter reaches its
> >>threshold value the watchdog is cancelled and nothing more comes of it.
> >>No hang is detected.
> >>
> >>Note about future interaction with preemption: Preemption could happen
> >>in a command sequence prior to watchdog counter getting disabled,
> >>resulting in watchdog being triggered following preemption. The driver will
> >>need to explicitly disable the watchdog counter as part of the
> >>preemption sequence.
> >>
> >>*** This patch introduces: ***
> >>
> >>1. IRQ handler code for watchdog timeout allowing direct hang recovery
> >>based on hardware-driven hang detection, which then integrates directly
> >>with the hang recovery path. This is independent of having per-engine reset
> >>or just full gpu reset.
> >>
> >>2. Watchdog specific register information.
> >>
> >>Currently the render engine and all available media engines support
> >>watchdog timeout (VECS is only supported in GEN9). The specifications elude
> >>to the BCS engine being supported but that is currently not supported by
> >>this commit.
> >>
> >>Note that the value to stop the counter is different between render and
> >>non-render engines.
> >>
> >>Signed-off-by: Tomas Elf <tomas.elf at intel.com>
> >>Signed-off-by: Ian Lister <ian.lister at intel.com>
> >>Signed-off-by: Arun Siluvery <arun.siluvery at linux.intel.com>
> >>Signed-off-by: Michel Thierry <michel.thierry at intel.com>
> >>---
> >> drivers/gpu/drm/i915/i915_drv.h        |  4 ++++
> >> drivers/gpu/drm/i915/i915_irq.c        | 31 ++++++++++++++++++++++++++++++-
> >> drivers/gpu/drm/i915/i915_reg.h        |  6 ++++++
> >> drivers/gpu/drm/i915/intel_hangcheck.c | 13 +++++++++----
> >> drivers/gpu/drm/i915/intel_lrc.c       | 16 ++++++++++++++++
> >> 5 files changed, 65 insertions(+), 5 deletions(-)
> >>
> >>diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> >>index eed9ead1b592..0e4f4cc3c6de 100644
> >>--- a/drivers/gpu/drm/i915/i915_drv.h
> >>+++ b/drivers/gpu/drm/i915/i915_drv.h
> >>@@ -1568,6 +1568,9 @@ struct i915_gpu_error {
> >> 	 * recovery. All waiters on the reset_queue will be woken when
> >> 	 * that happens.
> >> 	 *
> >>+	 * When hw detects a hang before us, we can use I915_RESET_WATCHDOG to
> >>+	 * report the hang detection cause accurately.
> >>+	 *
> >> 	 * This counter is used by the wait_seqno code to notice that reset
> >> 	 * event happened and it needs to restart the entire ioctl (since most
> >> 	 * likely the seqno it waited for won't ever signal anytime soon).
> >>@@ -1580,6 +1583,7 @@ struct i915_gpu_error {
> >>
> >> 	unsigned long flags;
> >> #define I915_RESET_IN_PROGRESS	0
> >>+#define I915_RESET_WATCHDOG	2 /* looking at the future */
> >> #define I915_WEDGED		(BITS_PER_LONG - 1)
> >>
> >> 	/**
> >>diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
> >>index bc70e2c451b2..4ef73363bbe9 100644
> >>--- a/drivers/gpu/drm/i915/i915_irq.c
> >>+++ b/drivers/gpu/drm/i915/i915_irq.c
> >>@@ -1352,6 +1352,28 @@ gen8_cs_irq_handler(struct intel_engine_cs *engine, u32 iir, int test_shift)
> >> 		set_bit(ENGINE_IRQ_EXECLIST, &engine->irq_posted);
> >> 		tasklet_hi_schedule(&engine->irq_tasklet);
> >> 	}
> >>+
> >>+	if (iir & (GT_GEN8_WATCHDOG_INTERRUPT << test_shift)) {
> >>+		struct drm_i915_private *dev_priv = engine->i915;
> >>+		u32 watchdog_disable;
> >>+
> >>+		if (engine->id == RCS)
> >>+			watchdog_disable = GEN8_RCS_WATCHDOG_DISABLE;
> >>+		else
> >>+			watchdog_disable = GEN8_XCS_WATCHDOG_DISABLE;
> >>+
> >>+		/* Stop the counter to prevent further timeout interrupts */
> >>+		I915_WRITE_FW(RING_CNTR(engine->mmio_base), watchdog_disable);
> >
> >There's no guarrantee you hold forcewake, you need to use I915_WRITE.
> >Better yet would be to avoid having to wait for forcewake within the
> >hardirq handler.
> >
> >>+
> >>+		/* Make sure the active request will be marked as guilty */
> >>+		engine->hangcheck.stalled = true;
> >>+		engine->hangcheck.seqno = intel_engine_get_seqno(engine);
> >
> >Just set a flag saying the engine->hangcheck.watchdog = true. Don't
> >confuse us. engine->hangcheck.seqno does not give the guilty seqno!
> >
> >Also there is no guarrantee here that seqno is the guilty party. That's
> >a nasty bug. Servicing the interrupt will be running in parallel with
> >the GPU that may complete the request before we read the HWS.
> >
> >Please tell me we can use a PID along with the watchdog timer...
> 
> A 'watchdog' PID and 'running' PID in the HWSP would sound ok?

No, Another STORE_DWORD_IMM has the same asynchronicity issue as just
reading seqno. I take it there is no WATCHDOG_PID that is set when the
watchdog expires? Or we can't program the CS to stop when the watchdog
goes off?

The issue is that we may blame the following context (a completely
unrelated process) for the hang - dos ahoy.

Or we can do something like current hangcheck, program the watchdog to
fire twice before we declare a hang. And only reset if we see the same
seqno on both occasions.

> There's also the question if we want different thresholds per engine.

I suspect we do. But that can be extended through the same
context_set_param just by passing an array (size > 0) instead of a
single value.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre