Re: [PATCH v2] drm/i915: Speed up idle detection by kicking the tasklets

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

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

> Quoting Mika Kuoppala (2018-05-07 09:34:24)
>> Chris Wilson <chris@chris-wilson.co.uk> writes:
>> 
>> > We rely on ksoftirqd to run in a timely fashion in order to drain the
>> > execlists queue. Quite frequently, it does not. In some cases we may see
>> > latencies of over 200ms triggering our idle timeouts and forcing us to
>> > declare the driver wedged!
>> >
>> > Thus we can speed up idle detection by bypassing ksoftirqd in these
>> > cases and flush our tasklet to confirm if we are indeed still waiting
>> > for the ELSP to drain.
>> >
>> > v2: Put the execlists.first check back; it is required for handling
>> > reset!
>> >
>> > References: https://bugs.freedesktop.org/show_bug.cgi?id=106373
>> > 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/intel_engine_cs.c | 15 ++++++++++++---
>> >  1 file changed, 12 insertions(+), 3 deletions(-)
>> >
>> > diff --git a/drivers/gpu/drm/i915/intel_engine_cs.c b/drivers/gpu/drm/i915/intel_engine_cs.c
>> > index 70325e0824e3..a3111511ea1d 100644
>> > --- a/drivers/gpu/drm/i915/intel_engine_cs.c
>> > +++ b/drivers/gpu/drm/i915/intel_engine_cs.c
>> > @@ -945,10 +945,19 @@ bool intel_engine_is_idle(struct intel_engine_cs *engine)
>> >               return true;
>> >  
>> >       /* Waiting to drain ELSP? */
>> > -     if (READ_ONCE(engine->execlists.active))
>> > -             return false;
>> > +     if (READ_ONCE(engine->execlists.active)) {
>> > +             struct intel_engine_execlists *execlists = &engine->execlists;
>> > +
>> > +             if (tasklet_trylock(&execlists->tasklet)) {
>> 
>> Now that we have the lock, sample active again to catch
>> the late tasklet run and skip running if so?
>
> It becomes a nop in the submission tasklet, it's not dangerous. So it
> comes down to what looks less of a wart!

The nice side effect of this kick is that now the hangcheck also won't
fall a victim.

Should we have a test which adds random and long tasklet delays?

Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
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