From e1fee72c2ea2e9c0c6e6743d32a6832f21337d6c Mon Sep 17 00:00:00 2001 From: Oscar Mateo Date: Thu, 24 Jul 2014 17:04:40 +0100 Subject: [PATCH] drm/i915/bdw: Avoid non-lite-restore preemptions MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit 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 Reviewed-by: Damien Lespiau [danvet: Align function parameters ...] Signed-off-by: Daniel Vetter --- drivers/gpu/drm/i915/intel_lrc.c | 29 +++++++++++++++++++++++++---- drivers/gpu/drm/i915/intel_lrc.h | 2 ++ 2 files changed, 27 insertions(+), 4 deletions(-) diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c index 22f6a7c0cb18..0f1b6b2b0f0e 100644 --- a/drivers/gpu/drm/i915/intel_lrc.c +++ b/drivers/gpu/drm/i915/intel_lrc.c @@ -268,6 +268,7 @@ 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; @@ -277,9 +278,15 @@ static void execlists_context_unqueue(struct intel_engine_cs *ring) } } + WARN_ON(req1 && req1->elsp_submitted); + WARN_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, @@ -298,9 +305,14 @@ 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; + } } } @@ -333,7 +345,16 @@ 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 a3f135cf439e..331c6c2ba376 100644 --- a/drivers/gpu/drm/i915/intel_lrc.h +++ b/drivers/gpu/drm/i915/intel_lrc.h @@ -66,6 +66,8 @@ 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); -- 2.30.2