Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> writes:
> In the unlikely case (thank you CI!), we may find ourselves wanting to
> issue a preemption but having no runnable requests left. In this case,
> we set the semaphore before computing the preemption and so must unset
> it before forgetting (or else we leave the machine busywaiting until the
> next request comes along and so likely hang).
>
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> ---
> drivers/gpu/drm/i915/gt/intel_lrc.c | 9 ++++++++-
> 1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
> index c8a0c9b32764..efccc31887de 100644
> --- a/drivers/gpu/drm/i915/gt/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
> @@ -233,13 +233,18 @@ static inline u32 intel_hws_preempt_address(struct intel_engine_cs *engine)
> static inline void
> ring_set_paused(const struct intel_engine_cs *engine, int state)
> {
> + u32 *sema = &engine->status_page.addr[I915_GEM_HWS_PREEMPT];
> +
> + if (*sema == state)
> + return;
> +
So you want to avoid useless wmb, as I don't see other
benefit. Makes this look suspiciously racy but seems
to be just my usual paranoia.
> /*
> * We inspect HWS_PREEMPT with a semaphore inside
> * engine->emit_fini_breadcrumb. If the dword is true,
> * the ring is paused as the semaphore will busywait
> * until the dword is false.
> */
> - engine->status_page.addr[I915_GEM_HWS_PREEMPT] = state;
> + *sema = state;
> wmb();
> }
>
> @@ -1243,6 +1248,8 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
> *port = execlists_schedule_in(last, port - execlists->pending);
> memset(port + 1, 0, (last_port - port) * sizeof(*port));
> execlists_submit_ports(engine);
> + } else {
> + ring_set_paused(engine, 0);
This looks like a right thing to do. But why did we end up
figuring things out wrong in need_preempt()?
One would think that if there were nothing to preempt into,
we would never set the pause in the first place.
Also the preempt to idle cycle mention in effective_prio()
seems to be off. Could be that someone forgot to
point that out when he did review preempt-to-busy.
-Mika
> }
> }
>
> --
> 2.20.1
>
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