Re: [PATCH 1/4] drm/i915: Allow error capture without a request

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On 12/01/2023 20:40, John Harrison wrote:
On 1/12/2023 02:01, Tvrtko Ursulin wrote:
On 12/01/2023 02:53, 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 an engine 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.

Signed-off-by: John Harrison <John.C.Harrison@xxxxxxxxx>
Reviewed-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@xxxxxxxxx>
---
  drivers/gpu/drm/i915/i915_gpu_error.c | 61 +++++++++++++++++++--------
  1 file changed, 43 insertions(+), 18 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gpu_error.c b/drivers/gpu/drm/i915/i915_gpu_error.c
index 9d5d5a397b64e..bd2cf7d235df0 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;
@@ -1608,8 +1628,11 @@ capture_engine(struct intel_engine_cs *engine,
      if (ce) {
          intel_engine_clear_hung_context(engine);
          rq = intel_context_find_active_request(ce);
-        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 no active request!\n",

Suggest s/active/started/ since we have both i915_request_active and i915_request_started, so to align the terminology.
The message text was based on the intent of the activity not the naming of some internal helper function. Can change it if you really want but "with no started request" just reads like bad English to me. Plus it gets removed in the next patch anyway...



+                 engine->name);
+            rq = NULL;
+        }
      } else {
          /*
           * Getting here with GuC enabled means it is a forced error capture
@@ -1622,22 +1645,24 @@ capture_engine(struct intel_engine_cs *engine,
                             flags);
          }
      }
-    if (rq)
+    if (rq) {
          rq = i915_request_get_rcu(rq);
+        capture = intel_engine_coredump_add_request(ee, rq, ATOMIC_MAYFAIL);
+    } else if (ce) {
+        capture = engine_coredump_add_context(ee, ce, ATOMIC_MAYFAIL);
+    }
  -    if (!rq)
-        goto no_request_capture;
-
-    capture = intel_engine_coredump_add_request(ee, rq, ATOMIC_MAYFAIL);
      if (!capture) {
-        i915_request_put(rq);
+        if (rq)
+            i915_request_put(rq);
          goto no_request_capture;
      }
      if (dump_flags & CORE_DUMP_FLAG_IS_GUC_CAPTURE)
          intel_guc_capture_get_matching_node(engine->gt, ee, ce);

This step requires non-NULL ce, so if you move it under the "else if (ce)" above then I *think* exit from the function can be consolidated to just:

if (capture) {
    intel_engine_coredump_add_vma(ee, capture, compress);
    if (rq)
        i915_request_put(rq);
Is there any reason the rq ref needs to be held during the add_vma call? Can it now just be moved earlier to be:
     if (rq) {
         rq = i915_request_get_rcu(rq);
        capture = intel_engine_coredump_add_request(ee, rq, ATOMIC_MAYFAIL);
         i915_request_put(rq);
     }

The internals of the request object are only touched in the above _add_request() code. The later _add_vma() call fiddles around with vmas that pulled from the request but the capture_vma code inside _add_request() has already copied everything, hasn't it? Or rather, it has grabbed its own private vma resource locks. So there is no requirement to keep the request itself around still?

Don't know.. it is a question if changes from 60dc43d1190d ("drm/i915: Use struct vma_resource instead of struct vma_snapshot") removed the need for holding the rq reference that "long" I guess? Adding Thomas and Matt to perhaps comment.

Regards,

Tvrtko


John.


} else {
    kfree(ee);
    ee = NULL;
}

return ee;

No "if (rq) i915_request_put()" twice, and goto label can be completely removed.

Regards,

Tvrtko

        intel_engine_coredump_add_vma(ee, capture, compress);
-    i915_request_put(rq);
+    if (rq)
+        i915_request_put(rq);
        return ee;




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