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

Daniel Vetter
2015-07-06T15:09:16Z

On Mon, Jul 06, 2015 at 03:46:49PM +0100, Tvrtko Ursulin wrote:
>
> On 07/06/2015 03:26 PM, John Harrison wrote:
> >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.
>
> For this particular issue my solution was to extend the sync_fence
> constructor to take a mutex and store it inside the object. Then at
> destruction time, which happens at sync_fd->f_ops->release() time, it is
> just a matter of calling kref_put_mutex instead of kref_put.
>
> Seemed to work under some quick testing but that is as much as I did back
> then.

The problem is that it doesn't scale since everyone wants some other kind
of mutex to serialize the final kref_put. If something is supposed to be
cross-subsystem/driver (which is the case for fences) then we really can't
do that kind of leaky locking design. Imo we should have a kref_put_mutex
considered harmful sign somewhere ...

If you have weak references somewhere and need to prevent the object from
disappearing untimely while chasing that weak reference then imo the
better design pattern is to use kref_get_unless_zero. If you need the
serialization the mutex provides for some other reason (someone is only
hodling the mutex instead of grabbing a proper refernce when they really
should grab one) then your refcounting scheme probably needs another kind
of fixup patch.
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation