From 1e3f697e47f61293351c72c35e3045b1774851c2 Mon Sep 17 00:00:00 2001 From: Chris Wilson Date: Fri, 1 Mar 2019 17:08:58 +0000 Subject: [PATCH] drm/i915/execlists: Suppress redundant preemption On unwinding the active request we give it a small (limited to internal priority levels) boost to prevent it from being gazumped a second time. However, this means that it can be promoted to above the request that triggered the preemption request, causing a preempt-to-idle cycle for no change. We can avoid this if we take the boost into account when checking if the preemption request is valid. v2: After preemption the active request will be after the preemptee if they end up with equal priority. v3: Tvrtko pointed out that this, the existing logic, makes I915_PRIORITY_WAIT non-preemptible. Document this interesting quirk! v4: Prove Tvrtko was right about WAIT being non-preemptible and test it. v5: Except not all priorities were made equal, and the WAIT not preempting is only if we start off as !NEWCLIENT. v6: More commentary after coming to an understanding about what I had forgotten to say. Signed-off-by: Chris Wilson Cc: Tvrtko Ursulin Reviewed-by: Tvrtko Ursulin Link: https://patchwork.freedesktop.org/patch/msgid/20190301170901.8340-1-chris@chris-wilson.co.uk --- drivers/gpu/drm/i915/intel_lrc.c | 44 +++++++++++++++++++++++++++++--- 1 file changed, 40 insertions(+), 4 deletions(-) diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c index 4f2187aa44e4..3fd0c45a2920 100644 --- a/drivers/gpu/drm/i915/intel_lrc.c +++ b/drivers/gpu/drm/i915/intel_lrc.c @@ -164,6 +164,8 @@ #define WA_TAIL_DWORDS 2 #define WA_TAIL_BYTES (sizeof(u32) * WA_TAIL_DWORDS) +#define ACTIVE_PRIORITY (I915_PRIORITY_NEWCLIENT) + static int execlists_context_deferred_alloc(struct i915_gem_context *ctx, struct intel_engine_cs *engine, struct intel_context *ce); @@ -190,8 +192,30 @@ static inline int rq_prio(const struct i915_request *rq) static int effective_prio(const struct i915_request *rq) { + int prio = rq_prio(rq); + + /* + * On unwinding the active request, we give it a priority bump + * equivalent to a freshly submitted request. This protects it from + * being gazumped again, but it would be preferable if we didn't + * let it be gazumped in the first place! + * + * See __unwind_incomplete_requests() + */ + if (~prio & ACTIVE_PRIORITY && __i915_request_has_started(rq)) { + /* + * After preemption, we insert the active request at the + * end of the new priority level. This means that we will be + * _lower_ priority than the preemptee all things equal (and + * so the preemption is valid), so adjust our comparison + * accordingly. + */ + prio |= ACTIVE_PRIORITY; + prio--; + } + /* Restrict mere WAIT boosts from triggering preemption */ - return rq_prio(rq) | __NO_PREEMPTION; + return prio | __NO_PREEMPTION; } static int queue_prio(const struct intel_engine_execlists *execlists) @@ -359,7 +383,7 @@ __unwind_incomplete_requests(struct intel_engine_cs *engine) { struct i915_request *rq, *rn, *active = NULL; struct list_head *uninitialized_var(pl); - int prio = I915_PRIORITY_INVALID | I915_PRIORITY_NEWCLIENT; + int prio = I915_PRIORITY_INVALID | ACTIVE_PRIORITY; lockdep_assert_held(&engine->timeline.lock); @@ -390,9 +414,21 @@ __unwind_incomplete_requests(struct intel_engine_cs *engine) * The active request is now effectively the start of a new client * stream, so give it the equivalent small priority bump to prevent * it being gazumped a second time by another peer. + * + * Note we have to be careful not to apply a priority boost to a request + * still spinning on its semaphores. If the request hasn't started, that + * means it is still waiting for its dependencies to be signaled, and + * if we apply a priority boost to this request, we will boost it past + * its signalers and so break PI. + * + * One consequence of this preemption boost is that we may jump + * over lesser priorities (such as I915_PRIORITY_WAIT), effectively + * making those priorities non-preemptible. They will be moved forward + * in the priority queue, but they will not gain immediate access to + * the GPU. */ - if (!(prio & I915_PRIORITY_NEWCLIENT)) { - prio |= I915_PRIORITY_NEWCLIENT; + if (~prio & ACTIVE_PRIORITY && __i915_request_has_started(active)) { + prio |= ACTIVE_PRIORITY; active->sched.attr.priority = prio; list_move_tail(&active->sched.link, i915_sched_lookup_priolist(engine, prio)); -- 2.30.2