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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 <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190301170901.8340-1-chris@chris-wilson.co.uk
hifive-unleashed-5.2
Chris Wilson 2019-03-01 17:08:58 +00:00
parent 34ae8455f4
commit 1e3f697e47
1 changed files with 40 additions and 4 deletions
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44
drivers/gpu/drm/i915/intel_lrc.c
View File

@ -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));