Re: [PATCH v4 3/7] drm/i915: Allow error capture without a request

["Ceraolo Spurio, Daniele" <daniele.ceraolospurio@xxxxxxxxx>]

On 1/20/2023 3:28 PM, John.C.Harrison@xxxxxxxxx wrote:
> From: John Harrison <John.C.Harrison@xxxxxxxxx>
>
> There was a report of error captures occurring without any hung
> context being indicated despite the capture being initiated by a 'hung
> context notification' from GuC. The problem was not reproducible.
> However, it is possible to happen if the context in question has no
> active requests. For example, if the hang was in the context switch
> itself then the breadcrumb write would have occurred and the KMD would
> see an idle context.
>
> In the interests of attempting to provide as much information as
> possible about a hang, it seems wise to include the engine info
> regardless of whether a request was found or not. As opposed to just
> prentending there was no hang at all.
>
> So update the error capture code to always record engine information
> if a context is given. Which means updating record_context() to take a
> context instead of a request (which it only ever used to find the
> context anyway). And split the request agnostic parts of
> intel_engine_coredump_add_request() out into a seaprate function.
>
> v2: Remove a duplicate 'if' statement (Umesh) and fix a put of a null
> pointer.
> v3: Tidy up request locking code flow (Tvrtko)
> v4: Pull in improved info message from next patch and fix up potential
> leak of GuC register state (Daniele)

In the very unlikely case that the capture fails, we're leaving the data 
inside the GuC buffer. This is not new with this patch and not a bug 
(that buffer is a ring and the stale data will be overwritten if it gets 
full), but maybe something that can be improved as a follow-up.

Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@xxxxxxxxx>

Daniele

> Signed-off-by: John Harrison <John.C.Harrison@xxxxxxxxx>
> Reviewed-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@xxxxxxxxx>
> Acked-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> ---
>   drivers/gpu/drm/i915/i915_gpu_error.c | 74 ++++++++++++++++++---------
>   1 file changed, 50 insertions(+), 24 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_gpu_error.c b/drivers/gpu/drm/i915/i915_gpu_error.c
> index b20bd6365615b..225f1b11a6b93 100644
> --- a/drivers/gpu/drm/i915/i915_gpu_error.c
> +++ b/drivers/gpu/drm/i915/i915_gpu_error.c
> @@ -1370,14 +1370,14 @@ static void engine_record_execlists(struct intel_engine_coredump *ee)
>   }
>   
>   static bool record_context(struct i915_gem_context_coredump *e,
> -			   const struct i915_request *rq)
> +			   struct intel_context *ce)
>   {
>   	struct i915_gem_context *ctx;
>   	struct task_struct *task;
>   	bool simulated;
>   
>   	rcu_read_lock();
> -	ctx = rcu_dereference(rq->context->gem_context);
> +	ctx = rcu_dereference(ce->gem_context);
>   	if (ctx && !kref_get_unless_zero(&ctx->ref))
>   		ctx = NULL;
>   	rcu_read_unlock();
> @@ -1396,8 +1396,8 @@ static bool record_context(struct i915_gem_context_coredump *e,
>   	e->guilty = atomic_read(&ctx->guilty_count);
>   	e->active = atomic_read(&ctx->active_count);
>   
> -	e->total_runtime = intel_context_get_total_runtime_ns(rq->context);
> -	e->avg_runtime = intel_context_get_avg_runtime_ns(rq->context);
> +	e->total_runtime = intel_context_get_total_runtime_ns(ce);
> +	e->avg_runtime = intel_context_get_avg_runtime_ns(ce);
>   
>   	simulated = i915_gem_context_no_error_capture(ctx);
>   
> @@ -1532,15 +1532,37 @@ intel_engine_coredump_alloc(struct intel_engine_cs *engine, gfp_t gfp, u32 dump_
>   	return ee;
>   }
>   
> +static struct intel_engine_capture_vma *
> +engine_coredump_add_context(struct intel_engine_coredump *ee,
> +			    struct intel_context *ce,
> +			    gfp_t gfp)
> +{
> +	struct intel_engine_capture_vma *vma = NULL;
> +
> +	ee->simulated |= record_context(&ee->context, ce);
> +	if (ee->simulated)
> +		return NULL;
> +
> +	/*
> +	 * We need to copy these to an anonymous buffer
> +	 * as the simplest method to avoid being overwritten
> +	 * by userspace.
> +	 */
> +	vma = capture_vma(vma, ce->ring->vma, "ring", gfp);
> +	vma = capture_vma(vma, ce->state, "HW context", gfp);
> +
> +	return vma;
> +}
> +
>   struct intel_engine_capture_vma *
>   intel_engine_coredump_add_request(struct intel_engine_coredump *ee,
>   				  struct i915_request *rq,
>   				  gfp_t gfp)
>   {
> -	struct intel_engine_capture_vma *vma = NULL;
> +	struct intel_engine_capture_vma *vma;
>   
> -	ee->simulated |= record_context(&ee->context, rq);
> -	if (ee->simulated)
> +	vma = engine_coredump_add_context(ee, rq->context, gfp);
> +	if (!vma)
>   		return NULL;
>   
>   	/*
> @@ -1550,8 +1572,6 @@ intel_engine_coredump_add_request(struct intel_engine_coredump *ee,
>   	 */
>   	vma = capture_vma_snapshot(vma, rq->batch_res, gfp, "batch");
>   	vma = capture_user(vma, rq, gfp);
> -	vma = capture_vma(vma, rq->ring->vma, "ring", gfp);
> -	vma = capture_vma(vma, rq->context->state, "HW context", gfp);
>   
>   	ee->rq_head = rq->head;
>   	ee->rq_post = rq->postfix;
> @@ -1604,25 +1624,31 @@ capture_engine(struct intel_engine_cs *engine,
>   		return NULL;
>   
>   	intel_engine_get_hung_entity(engine, &ce, &rq);
> -	if (!rq || !i915_request_started(rq))
> -		goto no_request_capture;
> +	if (rq && !i915_request_started(rq)) {
> +		drm_info(&engine->gt->i915->drm, "Got hung context on %s with active request %lld:%lld [0x%04X] not yet started\n",
> +			 engine->name, rq->fence.context, rq->fence.seqno, ce->guc_id.id);
> +		i915_request_put(rq);
> +		rq = NULL;
> +	}
>   
> -	capture = intel_engine_coredump_add_request(ee, rq, ATOMIC_MAYFAIL);
> -	if (!capture)
> -		goto no_request_capture;
> -	if (dump_flags & CORE_DUMP_FLAG_IS_GUC_CAPTURE)
> -		intel_guc_capture_get_matching_node(engine->gt, ee, ce);
> +	if (rq) {
> +		capture = intel_engine_coredump_add_request(ee, rq, ATOMIC_MAYFAIL);
> +		i915_request_put(rq);
> +	} else if (ce) {
> +		capture = engine_coredump_add_context(ee, ce, ATOMIC_MAYFAIL);
> +	}
>   
> -	intel_engine_coredump_add_vma(ee, capture, compress);
> -	i915_request_put(rq);
> +	if (capture) {
> +		intel_engine_coredump_add_vma(ee, capture, compress);
>   
> -	return ee;
> +		if (dump_flags & CORE_DUMP_FLAG_IS_GUC_CAPTURE)
> +			intel_guc_capture_get_matching_node(engine->gt, ee, ce);
> +	} else {
> +		kfree(ee);
> +		ee = NULL;
> +	}
>   
> -no_request_capture:
> -	if (rq)
> -		i915_request_put(rq);
> -	kfree(ee);
> -	return NULL;
> +	return ee;
>   }
>   
>   static void