Re: [Intel-gfx] [CI] drm/i915/execlists: Prevent GPU death on ELSP[1] promotion to idle context

[Mika Kuoppala <mika.kuoppala@linux.intel.com>]

Chris Wilson <chris@chris-wilson.co.uk> writes:

> 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@chris-wilson.co.uk>
>> > Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>> > Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
>> > ---
>> >   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@intel.com>
>
> 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.

Trying to make senses of it all...

we do not raise == the hardware does not raise?

So if it is about lite restoring, we can't workaround by always making
sure elsp[1] head != tail?

-Mika


>
> Out of the frying pan and into the fire.
> -Chris
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