Re: [PATCH 1/2] tests/gem_eio: New ABI - no EIO even from wait_ioctl

[Daniel Vetter <daniel@ffwll.ch>]

On Tue, Dec 01, 2015 at 09:04:23AM +0000, Chris Wilson wrote:
> On Tue, Dec 01, 2015 at 09:28:08AM +0100, Daniel Vetter wrote:
> > On Mon, Nov 30, 2015 at 10:11:12AM +0000, Chris Wilson wrote:
> > > On Thu, Nov 26, 2015 at 12:34:34PM +0100, Daniel Vetter wrote:
> > > > So there's 3 competing proposals for what wait_ioctl should do wrt
> > > > -EIO:
> > > > 
> > > > - return -EIO when the gpu is wedged. Not terribly useful for
> > > >   userspace since it might race with a hang and then there's no
> > > >   guarantee that a subsequent execbuf won't end up in an -EIO.
> > > >   Terminally wedge really can only be reliably signalled at execbuf
> > > >   time, and userspace needs to cope with that (or decide not to
> > > >   bother).
> > > > 
> > > > - EIO for any obj that suffered from a reset. This means big internal
> > > >   reorginazation in the kernel since currently we track reset stats
> > > >   per-ctx and not on the obj. That's also what arb robustness wants.
> > > >   We could do this, but this feels like new ABI territory with the
> > > >   usual userspace requirements and high hurdles.
> > > > 
> > > > - No -EIO at all. Consistent with set_domain_ioctl and simplest to
> > > >   implement. Which is what this patch does.
> > > 
> > > Since no one else is weighing into the ABI discussion, I'm happy with
> > > losing EIO here. I thought it could be useful, but as no one is using or
> > > seems likely to start using it, begone.
> > > 
> > > > We can always opt to change this later on if there's a real need.
> > > > 
> > > > To make the test really exercise this do a full wedged gpu hang, to
> > > > make sure -EIO doesn't leak out at all.
> > > > 
> > > > Cc: Chris Wilson <chris@chris-wilson.co.uk>
> > > > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> > > > ---
> > > >  tests/gem_eio.c | 6 +++++-
> > > >  1 file changed, 5 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/tests/gem_eio.c b/tests/gem_eio.c
> > > > index a24c8f1c53b5..8345d1a7a429 100644
> > > > --- a/tests/gem_eio.c
> > > > +++ b/tests/gem_eio.c
> > > > @@ -161,10 +161,14 @@ static void test_wait(int fd)
> > > >  {
> > > >  	igt_hang_ring_t hang;
> > > >  
> > > > +	igt_require(i915_reset_control(false));
> > > 
> > > However, this is not required to test the ABI change above as the wait
> > > itself will still hang, whether or not it wedges the GPU.
> > 
> > Yes it's not strictly required, but without it the testcase is fairly
> > boring. If we move the check_wedge out of wait_request then a normail gpu
> > reset would always return 0 (after retrying a few times perhaps), so I
> > figured testing the wedged case is the only one that's worth it.
> 
> But wedging during the hang is also not interesting as we have no
> opportunity to see the reset failure in the test case. Putting the GPU
> into the wedged state before the wait, should be a trivial test that the
> object is idle after the reset.

Right now (with current kernels) we see an -EIO with this testcase instead
of 0 in the wait. Without disabling reset we see 0 both on fixed and
broken kernels. So I don't really see why not testing this case is a good
idea? It's the one we're currently failing at and leak -EIO to userspace.

Assuming ofc we still go with the "let's curb -EIO except for execbuf"
ABI.

> > Maybe we should dupe the subtests all and have wedged and non-wedged cases
> > for all of them? That would imo make more sense.
> 
> The others, what matters is how we handle the GPU being wedged before we
> queue an execbuf or throttling. In terms of testing no error is reported
> for the hanging case, we should add tests for set-domain (so that it is
> explicit and not reliant on implementation inside lib/), GTT faulting
> (that would need both wedged and hanging cases, but we are not likely to
> be able to cover all the cases like waiting on fence and other secondary
> waits) and throttle.

Yeah there's more gaps than just the one above.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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