[PATCH 32/43] drm/i915/bdw: Avoid non-lite-restore preemptions

Thomas Daniel
2014-07-24T16:05:25Z

From: Oscar Mateo <oscar.mateo@intel.com>

In the current Execlists feeding mechanism, full preemption is not
supported yet: only lite-restores are allowed (this is: the GPU
simply samples a new tail pointer for the context currently in
execution).

But we have identified an scenario in which a full preemption occurs:
1) We submit two contexts for execution (A & B).
2) The GPU finishes with the first one (A), switches to the second one
(B) and informs us.
3) We submit B again (hoping to cause a lite restore) together with C,
but in the time we spend writing to the ELSP, the GPU finishes B.
4) The GPU start executing B again (since we told it so).
5) We receive a B finished interrupt and, mistakenly, we submit C (again)
and D, causing a full preemption of B.

The race is avoided by keeping track of how many times a context has been
submitted to the hardware and by better discriminating the received context
switch interrupts: in the example, when we have submitted B twice, we won´t
submit C and D as soon as we receive the notification that B is completed
because we were expecting to get a LITE_RESTORE and we didn´t, so we know a
second completion will be received shortly.

Without this explicit checking, somehow, the batch buffer execution order
gets messed with. This can be verified with the IGT test I sent together with
the series. I don´t know the exact mechanism by which the pre-emption messes
with the execution order but, since other people is working on the Scheduler
+ Preemption on Execlists, I didn´t try to fix it. In these series, only Lite
Restores are supported (other kind of preemptions WARN).

v2: elsp_submitted belongs in the new intel_ctx_submit_request. Several
rebase changes.

v3: Clarify how the race is avoided, as requested by Daniel.

Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
---
drivers/gpu/drm/i915/intel_lrc.c | 28 ++++++++++++++++++++++++----
drivers/gpu/drm/i915/intel_lrc.h | 2 ++
2 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index 65f4f26..895dbfc 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -264,6 +264,7 @@ static void execlists_context_unqueue(struct intel_engine_cs *ring)
else if (req0->ctx == cursor->ctx) {
/* Same ctx: ignore first request, as second request
* will update tail past first request's workload */
+ cursor->elsp_submitted = req0->elsp_submitted;
list_del(&req0->execlist_link);
queue_work(dev_priv->wq, &req0->work);
req0 = cursor;
@@ -273,8 +274,14 @@ static void execlists_context_unqueue(struct intel_engine_cs *ring)
}
}

+ WARN_ON(req1 && req1->elsp_submitted);
+
BUG_ON(execlists_submit_context(ring, req0->ctx, req0->tail,
req1? req1->ctx : NULL, req1? req1->tail : 0));
+
+ req0->elsp_submitted++;
+ if (req1)
+ req1->elsp_submitted++;
}

static bool execlists_check_remove_request(struct intel_engine_cs *ring,
@@ -291,9 +298,13 @@ static bool execlists_check_remove_request(struct intel_engine_cs *ring,
struct drm_i915_gem_object *ctx_obj =
head_req->ctx->engine[ring->id].state;
if (intel_execlists_ctx_id(ctx_obj) == request_id) {
- list_del(&head_req->execlist_link);
- queue_work(dev_priv->wq, &head_req->work);
- return true;
+ WARN(head_req->elsp_submitted == 0,
+ "Never submitted head request\n");
+ if (--head_req->elsp_submitted <= 0) {
+ list_del(&head_req->execlist_link);
+ queue_work(dev_priv->wq, &head_req->work);
+ return true;
+ }
}
}

@@ -326,7 +337,16 @@ void intel_execlists_handle_ctx_events(struct intel_engine_cs *ring)
status_id = I915_READ(RING_CONTEXT_STATUS_BUF(ring) +
(read_pointer % 6) * 8 + 4);

- if (status & GEN8_CTX_STATUS_COMPLETE) {
+ if (status & GEN8_CTX_STATUS_PREEMPTED) {
+ if (status & GEN8_CTX_STATUS_LITE_RESTORE) {
+ if (execlists_check_remove_request(ring, status_id))
+ WARN(1, "Lite Restored request removed from queue\n");
+ } else
+ WARN(1, "Preemption without Lite Restore\n");
+ }
+
+ if ((status & GEN8_CTX_STATUS_ACTIVE_IDLE) ||
+ (status & GEN8_CTX_STATUS_ELEMENT_SWITCH)) {
if (execlists_check_remove_request(ring, status_id))
submit_contexts++;
}
diff --git a/drivers/gpu/drm/i915/intel_lrc.h b/drivers/gpu/drm/i915/intel_lrc.h
index 2e8929f..074b44f 100644
--- a/drivers/gpu/drm/i915/intel_lrc.h
+++ b/drivers/gpu/drm/i915/intel_lrc.h
@@ -67,6 +67,8 @@ struct intel_ctx_submit_request {

struct list_head execlist_link;
struct work_struct work;
+
+ int elsp_submitted;
};

void intel_execlists_handle_ctx_events(struct intel_engine_cs *ring);
--
1.7.9.5

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