[Intel-gfx] [PATCH 03/11] drm/i915/execlists: Suppress redundant preemption

[Chris Wilson]

Quoting Tvrtko Ursulin (2019-03-01 11:31:26)
> 
> ping on below
> 
> On 28/02/2019 13:11, Tvrtko Ursulin wrote:
> > 
> > On 26/02/2019 10:23, Chris Wilson wrote:
> >> 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.
> >>
> >> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> >> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> >> ---
> >>   drivers/gpu/drm/i915/intel_lrc.c | 38 ++++++++++++++++++++++++++++----
> >>   1 file changed, 34 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/i915/intel_lrc.c 
> >> b/drivers/gpu/drm/i915/intel_lrc.c
> >> index 0e20f3bc8210..dba19baf6808 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 +414,15 @@ __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.
> >> +     *
> >> +     * 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
> > 
> > After the previous patch wait priority is non-preemptible by definition 
> > making this suggestion preemption boost is making it so not accurate.
> > 
> >> +     * 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)) {
> > 
> > What is the importance of the has_started check? Hasn't the active 
> > request been running by definition?

No. Semaphores. This is all about defending against incorrect promotion
while a request is still spinning on its dependencies (or else we get
promoted above them and PI is broken).
-Chris