[Intel-gfx] [PATCH i-g-t 2/3] lib: Align ring measurement to timer

[Chris Wilson]

Quoting Antonio Argenziano (2018-05-30 18:30:36)
> 
> 
> On 30/05/18 03:33, Chris Wilson wrote:
> > After hitting the SIGINT from execbuf, wait until the next timer signal
> > before trying again. This aligns the start of the ioctl to the timer,
> > hopefully maximising the amount of time we have for processing before
> > the next signal -- trying to prevent the case where we are scheduled out
> > in the middle of processing and so hit the timer signal too early.
> > 
> > References: https://bugs.freedesktop.org/show_bug.cgi?id=106695
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> 
> Not sure I understand what is the sequence of events, is the problem we 
> get a signal in the middle of a 'good' execbuf and exit the while loop 
> prematurely? If so maybe we can also think of making the timer 'VIRTUAL' 
> so that it would decrement only when the process is executing.

If it's VIRTUAL it'll never fire when we wait for space (as being asleep
no user/sys time is consumed).

The only way I can explain 106695 would be with some very strange
scheduler behaviour, but even then it requires us to hit a path where we
actually check for a pending signal -- which should only happen when we
run out of ring space for this setup. Not even the device being wedged
(which it wasn't) would cause the ring to drain. Possibly going over 10s
and the cork being unplugged? Very stange.

> > ---
> >   lib/i915/gem_ring.c | 2 ++
> >   1 file changed, 2 insertions(+)
> > 
> > diff --git a/lib/i915/gem_ring.c b/lib/i915/gem_ring.c
> > index 7d64165eb..0c061000c 100644
> > --- a/lib/i915/gem_ring.c
> > +++ b/lib/i915/gem_ring.c
> > @@ -96,6 +96,8 @@ __gem_measure_ring_inflight(int fd, unsigned int engine, enum measure_ring_flags
> >               if (last == count)
> >                       break;
> >   
> > +             /* sleep until the next timer interrupt (woken on signal) */
> > +             pause();
> 
> Does it cause any (sensible) slowdown?

Adds at most one timer interval, 10us. Ok, at a push 2 timer intervals
if it takes longer than first to setup the sleep.
-Chris