Re: [RFC] drm/i915: Add sync framework support to execbuff IOCTL

John Harrison
2015-07-06T14:26:14Z

On 06/07/2015 14:59, Daniel Vetter wrote:
> On Mon, Jul 06, 2015 at 01:58:25PM +0100, John Harrison wrote:
>> On 06/07/2015 10:29, Daniel Vetter wrote:
>>> On Fri, Jul 03, 2015 at 12:17:33PM +0100, Tvrtko Ursulin wrote:
>>>> On 07/02/2015 04:55 PM, Chris Wilson wrote:
>>>>> It would be nice if we could reuse one seqno both for internal/external
>>>>> fences. If you need to expose a fence ordering within a timeline that is
>>>>> based on the creation stamp rather than execution stamp, it seems like
>>>>> we could just add such a stamp when creating the sync_pt and not worry
>>>>> about its relationship to the execution seqno.
>>>>>
>>>>> Doing so does expose that requests are reordered to userspace since the
>>>>> signalling timeline is not the same as userspace's ordered timeline. Not
>>>>> sure if that is a problem or not.
>>>>>
>>>>> Afaict the sync uapi is based on waiting for all of a set of fences to
>>>>> retire. It doesn't seem to rely on fence ordering (that is knowing that
>>>>> fence A will signal before fence B so it need only wait on fence B).
>>>>>
>>>>> Here's hoping that we can have both simplicity and efficiency...
>>>> Jumping in with not even perfect understanding of everything here - but
>>>> timeline business has always been confusing me. There is nothing in the
>>>> uapi which needs it afaics and iirc there was some discussion at the time
>>>> Jesse floated his patches that it can be removed. Based on that when I
>>>> squashed his patches and ported them on top of John's request to fence
>>>> conversion it ended up something like the below (manually edited a bit to
>>>> be less noisy and some prep patches omitted):
>>>>
>>>> This implements the ioctl based uapi and indeed seqnos are not actually
>>>> used in waits. So is this insufficient for some reason? (Other that it
>>>> does not implement the input fence side of things.)
>>> Yeah android syncpt on top of struct fence embedded int i915 request is
>>> what I'd have expected.
>> The thing I'm not happy with in that plan is that it leaves the kernel
>> driver at the mercy of user land applications. If we return a fence object
>> to user land via a file descriptor (or indeed any other mechanism) then that
>> fence object must be locked until user land closes the file. If the fence
>> object is the one embedded within our request structure then that means user
>> land is effectively locking our request structure. Given that more and more
>> stuff is being attached to the request, that could be a fair bit of memory
>> tied up that we can do nothing about. E.g. if a rogue/buggy application
>> requests a fence be returned for every batch buffer submitted but never
>> closes them. Whereas, if we go the route of a separate fence object
>> specifically for user land then they can leak them like a sieve and we won't
>> really care so much.
> Userspace can exhaust kernel allocations, that's nothing new. And if we
> keep it userspace simply needs to leak a few more fence fds than if
> there's a bit more data attached to it.
>
> The solution to this problem is to have a mem cgroup limit set. No need to
> complicate our kernel code.

There is still the extra complication that request unreferencing cannot
require any kind of mutex lock if we are allowing it to happen from
outside of the driver. That means the unreference callback must move the
request to a 'please clean me later' list, schedule a worker thread to
run, and thus do the clean up asynchronously.

>>>> diff --git a/drivers/gpu/drm/i915/Kconfig b/drivers/gpu/drm/i915/Kconfig
>>>> index 74acca9..07f6ad9 100644
>>>> --- a/drivers/gpu/drm/i915/Kconfig
>>>> +++ b/drivers/gpu/drm/i915/Kconfig
>>>> @@ -71,3 +71,17 @@ config DRM_I915_PRELIMINARY_HW_SUPPORT
>>>> option changes the default for that module option.
>>>> If in doubt, say "N".
>>>> +
>>>> +config DRM_I915_SYNC
>>>> + bool "Enable explicit sync support"
>>>> + depends on DRM_I915
>>>> + default y if STAGING
>>>> + depends on STAGING
>>>> + select ANDROID
>>>> + select SYNC
>>>> + help
>>> No Kconfig for userspace ABI please. Yes this means we need to destage
>>> android syncpts first.
>> There is already a CONFIG_SYNC flag that wraps up all the existing sync code
>> in the staging branch. There's not a lot we can do about that is there? We
>> have to at least wrap the sync specific code in the i915 driver with '#if
>> CONFIG_SYNC' otherwise it won't compile.
> User-settable CONFIG_SYNC is one of these bits we need to fix up when
> de-staging - it should be an internal variable which is selected by i915,
> like all the other optional kernel services we use.
>
>>> The problem I see there is that apparently google
>>> is still changing the uabi a lot, and that's a no-go for upstream. And it
>>> needs to be cleaned up to work more seamlessly with struct fence (i.e.
>>> anything that's missing there should be moved to struct fence, drivers
>>> should only use fd_to_fence and fenct_to_fd functions similar to dma-buf).
>> Are Google changing it or is it upstream that are changing it? The only
>> changes to android/staging/sync.c have been a few minor bug fixes and
>> Maarten Lankhorst's conversion to use struct fence which was back in July
>> last year.
> destaging android syncpt will probably require a few changes, but more so
> it will freeze the abi. If we do that effort and google ignores it it's
> fairly pointless (as long as android is the only serious user of syncpts).
>
>>> And we don't have anyone except android using syncpts, so a bit a trouble
>>> with finding userspace vehicles for this. We probably need agreement from
>>> google to be happy with a frozen abi for syncpts first ...
>>> -Daniel
>> I believe Jesse is wanting to use it for his work.
> Yes I know, but afaik it's also a long way off. Which means not useful as
> an open-source demonstration vehicle unfortunately.
> -Daniel