Re: [Intel-gfx] [PATCH 0/3] drm/i915: Handle hanging during nonblocking modeset correctly.

[Maarten Lankhorst <maarten.lankhorst@linux.intel.com>]

Hey,

Op 31-01-17 om 08:46 schreef Daniel Vetter:
> On Mon, Jan 30, 2017 at 03:42:17PM +0100, Maarten Lankhorst wrote:
>> Op 30-01-17 om 09:17 schreef Daniel Vetter:
>>> On Fri, Jan 27, 2017 at 03:08:45PM +0000, Chris Wilson wrote:
>>>> On Fri, Jan 27, 2017 at 03:58:08PM +0100, Daniel Vetter wrote:
>>>>> On Fri, Jan 27, 2017 at 02:31:55PM +0000, Chris Wilson wrote:
>>>>>> On Fri, Jan 27, 2017 at 03:21:29PM +0100, Daniel Vetter wrote:
>>>>>>> On Fri, Jan 27, 2017 at 09:30:50AM +0000, Chris Wilson wrote:
>>>>>>>> On Thu, Jan 26, 2017 at 04:59:21PM +0100, Maarten Lankhorst wrote:
>>>>>>>>> When writing some testcases for nonblocking modesets. I found out that the
>>>>>>>>> infinite wait on the old fb was causing issues.
>>>>>>>> The crux of the issue here is the locked wait for old dependencies and
>>>>>>>> the inability to inject the intel_prepare_reset disabling of all planes.
>>>>>>>> There are a couple of locked waits on struct_mutex within the modeset
>>>>>>>> locks for intel_overlay and if we happen to be using the display plane
>>>>>>>> for the first time.
>>>>>>>>
>>>>>>>> The first I suggested solving using fences to track dependencies and
>>>>>>>> keep the order between atomic states. Cancelling the outstanding
>>>>>>>> modesets, replacing with a disable and then on restore jumping to the
>>>>>>>> final state look doable. It also requires avoiding the struct_mutex for
>>>>>>>> disabling, which is quite easy. To avoid the wait under struct_mutex,
>>>>>>>> we've talked about switching to mmio, but for starters we could move the
>>>>>>>> wait from inside intel_overlay into the fence for the atomic operation.
>>>>>>>> (But's that a little more surgery than we would like for intel_overlay I
>>>>>>>> guess - dig out Ville's patches for overlay planes?) And to prevent the
>>>>>>>> wait under struct_mutex for pin_to_display_plane, my plane is to move
>>>>>>>> that to an async fenced operation that is then naturally waited upon by
>>>>>>>> the atomic modeset.
>>>>>>> A bit more a hack, but a different idea, and I think hack for gen234.0 is
>>>>>>> ok:
>>>>>>>
>>>>>>> We complete all the requests before we start the hw reset with fence.error
>>>>>>> = -EIO. But we do this only when we need to get at the display locks. A
>>>>>>> slightly more elegant solution would be to trylock modeset locks, and if
>>>>>>> one of them fails (and only then) complete all requests with -EIO to get
>>>>>>> the concurrent modeset to proceed before we reset the hardware. That's
>>>>>>> essentially the logic we had before all the reworks, and it worked. But I
>>>>>>> didn't look at how scary that all would be to make it work again ...
>>>>>> The modeset lock may not just be waiting on our requests (even on pnv we
>>>>>> can expect that there are already users celebrating that pnv+nouveau
>>>>>> finally works ;) and that the display is not the only user/observer of
>>>>>> those requests. Using the requests to break the modeset lock just feels
>>>>>> like the wrong approach.
>>>>> It's a cycle, and we need to break it somewhere. Another option might be
>>>>> to break the cycle the same way we do it for gem locks: Wake up everyone
>>>>> and restart the modeset ioctl. Since the trouble only happens for
>>>>> synchronous modesets where we hold the locks while waiting for fences, we
>>>>> can also break out of that and restart. And I also don't think that would
>>>>> leak to other drivers, after all our gem locking restart dances also don't
>>>>> leak to other drivers - it's just our own driver's lock which are affected
>>>>> by these special wakupe semantics.
>>>> It's a queue of nonblocking modesets that we need to worry about, afaik.
>>>> Moving the wait for blocking modeset outside of modeset lock is easily
>>>> achievable (and avoiding the other waits under both the modeset + 
>>>> struct_mutex I have at least an idea for). So the challenge is how to
>>>> inject all-planes-off for gen3 and then allow the queue to continue again
>>>> afterwards.
>>> Hm right, I missed the nonblocking updates which don't take locks. But
>>> assuming we do the display reset for gpu resets as a full modeset (i.e.
>>> going through ->atomic_commit) it should still work out correctly:
>>>
>>> Starting state: gpu is hung, nonblocking modeset waiting for some requests
>>> to complete.
>> Missing one evil detail here, else things would have moved forward..
>>
>> A unrelated thread performs a blocking commit, and holds all locks until the nonblocking modeset completes.
> And where is the problem in that? If we first set all fences to -EIO, and
> then try to grab locks, that other thread will be able to complete. After
> all this scheme worked before we reworked the reset logic completely.
True, but we probably still want to cap the timeout (patch 2) to prevent a deadlock when a fence on another driver misbehaves.

And if we have a timeout, then things will move forward eventually even if we wait for locks, though it might still be a good idea to complete everything with -EIO first
to make it happen faster. :)

~Maarten
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