Quoting Chris Wilson (2019-01-23 12:36:02)
> 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.
>
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> ---
> drivers/gpu/drm/i915/intel_lrc.c | 29 +++++++++++++++++++++++++----
> 1 file changed, 25 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> index d9d744f6ab2c..74726f647e47 100644
> --- a/drivers/gpu/drm/i915/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/intel_lrc.c
> @@ -163,6 +163,8 @@
> #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);
> @@ -181,13 +183,31 @@ static inline int rq_prio(const struct i915_request *rq)
> return rq->sched.attr.priority;
> }
>
> +static inline int active_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 preferrable if we didn't
> + * let it be gazumped in the first place!
> + *
> + * See __unwind_incomplete_requests()
> + */
> + if (i915_request_started(rq))
> + prio |= ACTIVE_PRIORITY;
Hmm, actually we are put to the tail of that priolist so we don't get
rerun ahead of the preemptee if we end up at the same priority.
ACTIVE_PRIORITY - 1 would seem to be the right compromise.
-Chris
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