[Intel-gfx] [PATCH i-g-t 1/3] igt/gem_sync: Exercise sync after context switch

[Chris Wilson]

Quoting Antonio Argenziano (2018-08-15 18:20:10)
> 
> 
> On 15/08/18 03:26, Chris Wilson wrote:
> > Quoting Antonio Argenziano (2018-08-15 00:50:43)
> >>
> >>
> >> On 10/08/18 04:01, Chris Wilson wrote:
> >>> This exercises a special case that may be of interest, waiting for a
> >>> context that may be preempted in order to reduce the wait.
> >>>
> >>> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> >>> ---
> >>> +             cycles = 0;
> >>> +             elapsed = 0;
> >>> +             start = gettime();
> >>> +             do {
> >>> +                     do {
> >>> +                             double this;
> >>> +
> >>> +                             gem_execbuf(fd, &contexts[0].execbuf);
> >>> +                             gem_execbuf(fd, &contexts[1].execbuf);
> >>
> >> I'm not sure where the preemption, mentioned in the commit message, is
> >> coming in.
> > 
> > Internally. I've suggested that we reorder equivalent contexts in order
> > to satisfy client waits earlier. So having created two independent
> > request queues, userspace should be oblivious to the execution order.
> 
> But there isn't an assert because you don't want that to be part of the 
> contract between the driver and userspace, is that correct?

Correct. Userspace hasn't specified an order between the two contexts so
can't actually assert it happens in a particular order. We are free then
to do whatever we like, but that also means no assertion. Just the
figures look pretty and ofc we have to check that nothing actually
breaks.
-Chris