Quoting Tvrtko Ursulin (2020-03-27 15:59:45)
>
> On 27/03/2020 11:26, Chris Wilson wrote:
> > In what seems remarkably similar to the w/a required to not reload an
> > idle context with HEAD==TAIL, it appears we must prevent the HW from
> > switching to an idle context in ELSP[1], while simultaneously trying to
> > preempt the HW to run another context and a continuation of the idle
> > context (which is no longer idle).
> >
> > process_csb: vecs0: cs-irq head=0, tail=1
> > process_csb: vecs0: csb[1]: status=0x00000882:0x00000020
> > trace_ports: vecs0: preempted { 8c0:30!, 0:0 }
> > trace_ports: vecs0: promote { 8b2:32!, 8c0:30 }
> > trace_ports: vecs0: submit { 8b8:32, 8c0:32 }
> > process_csb: vecs0: cs-irq head=1, tail=2
> > process_csb: vecs0: csb[2]: status=0x00000814:0x00000040
> > trace_ports: vecs0: completed { 8b2:32!, 8c0:30 }
> > process_csb: vecs0: cs-irq head=2, tail=5
> > process_csb: vecs0: csb[3]: status=0x00000812:0x00000020
> > trace_ports: vecs0: preempted { 8c0:30!, 0:0 }
> > trace_ports: vecs0: promote { 8b8:32!, 8c0:32 }
> > process_csb: vecs0: csb[4]: status=0x00000814:0x00000060
> > trace_ports: vecs0: completed { 8b8:32!, 8c0:32 }
> > process_csb: vecs0: csb[5]: status=0x00000818:0x00000020
> > trace_ports: vecs0: completed { 8c0:32, 0:0 }
> > process_csb: vecs0: ring:{start:0x00021000, head:03f8, tail:03f8, ctl:00000000, mode:00000200}
> > process_csb: vecs0: rq:{start:00021000, head:03c0, tail:0400, seqno:8c0:32, hwsp:30},
> > process_csb: vecs0: ctx:{start:00021000, head:03f8, tail:03f8},
> > process_csb: GEM_BUG_ON("context completed before request")
> >
> > Fortunately, we just so happen to have a semaphore in place to prevent
> > the ring HEAD from proceeding past the end of a request that we can use
> > to fix the HEAD in position as we reprogram ELSP.
> >
> > Closes: https://gitlab.freedesktop.org/drm/intel/issues/1501
> > Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> > Cc: Mika Kuoppala <mika.kuoppala@xxxxxxxxxxxxxxx>
> > ---
> > drivers/gpu/drm/i915/gt/intel_gt_irq.c | 2 +
> > drivers/gpu/drm/i915/gt/intel_lrc.c | 72 +++++++++++++-------------
> > 2 files changed, 38 insertions(+), 36 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/gt/intel_gt_irq.c b/drivers/gpu/drm/i915/gt/intel_gt_irq.c
> > index f0e7fd95165a..be3817d99908 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_gt_irq.c
> > +++ b/drivers/gpu/drm/i915/gt/intel_gt_irq.c
> > @@ -24,6 +24,8 @@ cs_irq_handler(struct intel_engine_cs *engine, u32 iir)
> > {
> > bool tasklet = false;
> >
> > + ENGINE_TRACE(engine, "iir: %04x\n", iir);
> > +
> > if (unlikely(iir & GT_CS_MASTER_ERROR_INTERRUPT)) {
> > u32 eir;
> >
> > diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
> > index b12355048501..53650b452bc9 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_lrc.c
> > +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
> > @@ -1854,7 +1854,7 @@ static inline void clear_ports(struct i915_request **ports, int count)
> > memset_p((void **)ports, NULL, count);
> > }
> >
> > -static void execlists_dequeue(struct intel_engine_cs *engine)
> > +static bool execlists_dequeue(struct intel_engine_cs *engine)
> > {
> > struct intel_engine_execlists * const execlists = &engine->execlists;
> > struct i915_request **port = execlists->pending;
> > @@ -1928,13 +1928,6 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
> > execlists->queue_priority_hint);
> > record_preemption(execlists);
> >
> > - /*
> > - * Don't let the RING_HEAD advance past the breadcrumb
> > - * as we unwind (and until we resubmit) so that we do
> > - * not accidentally tell it to go backwards.
> > - */
> > - ring_set_paused(engine, 1);
> > -
> > /*
> > * Note that we have not stopped the GPU at this point,
> > * so we are unwinding the incomplete requests as they
> > @@ -1954,7 +1947,6 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
> > last->sched.attr.priority,
> > execlists->queue_priority_hint);
> >
> > - ring_set_paused(engine, 1);
> > defer_active(engine);
> >
> > /*
> > @@ -1988,7 +1980,7 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
> > * of timeslices, our queue might be.
> > */
> > start_timeslice(engine);
> > - return;
> > + return false;
> > }
> > }
> > }
> > @@ -2021,9 +2013,10 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
> > }
> >
> > if (last && !can_merge_rq(last, rq)) {
> > + /* leave this for another sibling */
> > spin_unlock(&ve->base.active.lock);
> > start_timeslice(engine);
> > - return; /* leave this for another sibling */
> > + return false;
> > }
> >
> > ENGINE_TRACE(engine,
> > @@ -2193,32 +2186,31 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
> > * interrupt for secondary ports).
> > */
> > execlists->queue_priority_hint = queue_prio(execlists);
> > + if (!submit)
> > + return false;
> >
> > - if (submit) {
> > - *port = execlists_schedule_in(last, port - execlists->pending);
> > - execlists->switch_priority_hint =
> > - switch_prio(engine, *execlists->pending);
> > + *port = execlists_schedule_in(last, port - execlists->pending);
> > + execlists->switch_priority_hint =
> > + switch_prio(engine, *execlists->pending);
> >
> > - /*
> > - * Skip if we ended up with exactly the same set of requests,
> > - * e.g. trying to timeslice a pair of ordered contexts
> > - */
> > - if (!memcmp(active, execlists->pending,
> > - (port - execlists->pending + 1) * sizeof(*port))) {
> > - do
> > - execlists_schedule_out(fetch_and_zero(port));
> > - while (port-- != execlists->pending);
> > -
> > - goto skip_submit;
> > - }
> > - clear_ports(port + 1, last_port - port);
> > + /*
> > + * Skip if we ended up with exactly the same set of requests,
> > + * e.g. trying to timeslice a pair of ordered contexts
> > + */
> > + if (!memcmp(active, execlists->pending,
> > + (port - execlists->pending + 1) * sizeof(*port))) {
> > + do
> > + execlists_schedule_out(fetch_and_zero(port));
> > + while (port-- != execlists->pending);
> >
> > - execlists_submit_ports(engine);
> > - set_preempt_timeout(engine, *active);
> > - } else {
> > -skip_submit:
> > - ring_set_paused(engine, 0);
> > + return false;
> > }
> > + clear_ports(port + 1, last_port - port);
> > +
> > + execlists_submit_ports(engine);
> > + set_preempt_timeout(engine, *active);
> > + tasklet_hi_schedule(&execlists->tasklet); /* lost interrupt */
> > + return true;
> > }
> >
> > static void
> > @@ -2478,7 +2470,16 @@ static void __execlists_submission_tasklet(struct intel_engine_cs *const engine)
> > lockdep_assert_held(&engine->active.lock);
> > if (!READ_ONCE(engine->execlists.pending[0])) {
> > rcu_read_lock(); /* protect peeking at execlists->active */
> > - execlists_dequeue(engine);
> > +
> > + /*
> > + * Don't let the RING_HEAD advance past the breadcrumb
> > + * as we unwind (and until we resubmit) so that we do
> > + * not accidentally tell it to go backwards.
> > + */
> > + ring_set_paused(engine, 1);
> > + if (!execlists_dequeue(engine))
> > + ring_set_paused(engine, 0);
> > +
> > rcu_read_unlock();
> > }
> > }
> > @@ -2816,8 +2817,7 @@ static void execlists_reset(struct intel_engine_cs *engine, const char *msg)
> > ring_set_paused(engine, 1); /* Freeze the current request in place */
> > if (execlists_capture(engine))
> > intel_engine_reset(engine, msg);
> > - else
> > - ring_set_paused(engine, 0);
> > + ring_set_paused(engine, 0);
> >
> > tasklet_enable(&engine->execlists.tasklet);
> > clear_and_wake_up_bit(bit, lock);
> >
>
> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
The problem appears to be that if we lite-restore into a context inside
the semaphore, it doesn't yield an arbitration point and we do not raise
a CSB event.
Out of the frying pan and into the fire.
-Chris
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