Re: [RFC 2/4] drm/i915/guc: Keep the previous context pinned until the next one has been completed

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 08/04/16 15:37, Chris Wilson wrote:
> On Fri, Apr 08, 2016 at 02:54:56PM +0100, Tvrtko Ursulin wrote:
>> From: Tvrtko Ursulin <tvrtko@ursulin.net>
>>
>> In GuC mode submitting requests is only putting them into the
>> GuC queue with the actual submission to hardware following at
>> some future point. This makes the per engine last context
>> tracking insufficient for closing the premature context
>> unpin race.
>>
>> Instead we need to make requests track and pin the previous
>> context on the same engine, and only unpin them when they
>> themselves are retired. This will ensure contexts remain pinned
>> in all cases until the are fully saved by the GPU.
>>
>> v2: Only track contexts.
>>
>> Signed-off-by: Tvrtko Ursulin <tvrtko@ursulin.net>
>> Cc: Alex Dai <yu.dai@intel.com>
>> Cc: Dave Gordon <david.s.gordon@intel.com>
>> Cc: Nick Hoath <nicholas.hoath@intel.com>
>> Cc: Chris Wilson <chris@chris-wilson.co.uk>
>> ---
>>   drivers/gpu/drm/i915/i915_drv.h  |  4 +++-
>>   drivers/gpu/drm/i915/i915_gem.c  |  5 +++++
>>   drivers/gpu/drm/i915/intel_lrc.c | 17 +++++++++++++++++
>>   3 files changed, 25 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
>> index fa199b7e51b5..216b2297ae3e 100644
>> --- a/drivers/gpu/drm/i915/i915_drv.h
>> +++ b/drivers/gpu/drm/i915/i915_drv.h
>> @@ -2283,6 +2283,9 @@ struct drm_i915_gem_request {
>>   	struct intel_context *ctx;
>>   	struct intel_ringbuffer *ringbuf;
>>
>> +	/** Context preceding this one on the same engine. Can be NULL. */
>> +	struct intel_context *prev_ctx;
>> +
>>   	/** Batch buffer related to this request if any (used for
>>   	    error state dump only) */
>>   	struct drm_i915_gem_object *batch_obj;
>> @@ -2318,7 +2321,6 @@ struct drm_i915_gem_request {
>>
>>   	/** Execlists no. of times this request has been sent to the ELSP */
>>   	int elsp_submitted;
>> -
>>   };
>>
>>   struct drm_i915_gem_request * __must_check
>> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
>> index c1df696f9002..118911ce7231 100644
>> --- a/drivers/gpu/drm/i915/i915_gem.c
>> +++ b/drivers/gpu/drm/i915/i915_gem.c
>> @@ -1445,6 +1445,11 @@ static void i915_gem_request_retire(struct drm_i915_gem_request *request)
>>   	 */
>>   	request->ringbuf->last_retired_head = request->postfix;
>>
>> +	if (request->prev_ctx) {
>> +		intel_lr_context_unpin(request->prev_ctx, request->engine);
>> +		request->prev_ctx = NULL;
>> +	}
>> +
>>   	__i915_gem_request_release(request);
>>   }
>>
>> diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
>> index 31445aa3429b..3d08dac0a9b0 100644
>> --- a/drivers/gpu/drm/i915/intel_lrc.c
>> +++ b/drivers/gpu/drm/i915/intel_lrc.c
>> @@ -798,6 +798,23 @@ intel_logical_ring_advance_and_submit(struct drm_i915_gem_request *request)
>>   	if (intel_engine_stopped(engine))
>>   		return 0;
>>
>> +	/*
>> +	 * Pin the previous context on this engine to ensure it is not
>> +	 * prematurely unpinned in GuC mode.
>> +	 * Previous context will be unpinned when this request is retired,
>> +	 * ensuring the GPU has switched out from the previous context and into
>> +	 * a new context at that point.
>> +	 */
>> +	if (engine->last_context) {
>> +		if (engine->last_context != request->i915->kernel_context) {
>> +			intel_lr_context_pin(engine->last_context, engine);
>> +			request->prev_ctx = engine->last_context;
>> +		}
>> +	}
>> +
>> +	/*
>> +	 * Track and pin the last context submitted on an engine.
>> +	 */
>>   	if (engine->last_context != request->ctx) {
>>   		if (engine->last_context)
>>   			intel_lr_context_unpin(engine->last_context, engine);
>
> We can use pinned_context to reduce the code complexity, not make it
> worse.
>
> If you first apply the execlists default context fix, adding
> pinned_context is a reduction is code.

Okay I'll pull that patch in this series.

Regards,

Tvrtko


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