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

[Michel Thierry <michel.thierry@intel.com>]

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@intel.com>
>> Signed-off-by: Ian Lister <ian.lister@intel.com>
>> Signed-off-by: Arun Siluvery <arun.siluvery@linux.intel.com>
>> Signed-off-by: Michel Thierry <michel.thierry@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?

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