[Mesa-dev] [RFC] i965: alternative to memctx for cleaning up nir variants

Rob Clark robdclark at gmail.com
Mon Dec 28 12:31:50 PST 2015

On Mon, Dec 28, 2015 at 1:58 PM, Connor Abbott <cwabbott0 at gmail.com> wrote:
> On Mon, Dec 28, 2015 at 1:35 PM, Rob Clark <robdclark at gmail.com> wrote:
>> On Mon, Dec 28, 2015 at 12:37 PM, Connor Abbott <cwabbott0 at gmail.com> wrote:
>>> On Mon, Dec 28, 2015 at 10:13 AM, Rob Clark <robdclark at gmail.com> wrote:
>>>> On Tue, Dec 22, 2015 at 10:11 PM, Connor Abbott <cwabbott0 at gmail.com> wrote:
>>>>> On Tue, Dec 22, 2015 at 9:55 PM, Rob Clark <robdclark at gmail.com> wrote:
>>>>>> On Tue, Dec 22, 2015 at 9:47 PM, Connor Abbott <cwabbott0 at gmail.com> wrote:
>>>>>>> On Tue, Dec 22, 2015 at 9:02 PM, Rob Clark <robdclark at gmail.com> wrote:
>>>>>>>> On Mon, Dec 21, 2015 at 1:48 PM, Jason Ekstrand <jason at jlekstrand.net> wrote:
>>>>>>>>> I think two different concepts of ownership are getting conflated here:
>>>>>>>>> Right/responsibility to delete and right to modify.
>>>>>>>>> The way I understand it, gallium, as it stands, gives neither to the driver.
>>>>>>>>> A back-end using NIR requires the right to modify but who deletes it doesn't
>>>>>>>>> ultimately matter.  I think it's dangerous to pass one of these rights to
>>>>>>>>> the driver and not the other but we need to think about both.
>>>>>>>> yeah, uneasy about driver modifying the IR if the state tracker is
>>>>>>>> still going to potentially spin off variants of the IR.. that sounds
>>>>>>>> like madness.
>>>>>>>> The refcnt'ing I proposed does deal w/ right to modify vs delete via
>>>>>>>> nir_shader(_is)_mutable() which returns something that is guaranteed
>>>>>>>> to be safe to modify (ie. has only a single reference)
>>>>>>>>> What I'm trying to say is that we have two options here:
>>>>>>>>> 1) gallium passes IR to the back-end that it is free to modify and is
>>>>>>>>> required to delete when it's done.
>>>>>>>> with refcnt'ing, s/delete/unref/
>>>>>>>> The idea is, the st transfers ownership of the reference it passes to
>>>>>>>> the driver.  If the st wants to hang on to a reference itself, it must
>>>>>>>> increment the refcnt before passing to the driver (backend).
>>>>>>>> Without refcnt'ing, I suppose we could (since we don't have to follow
>>>>>>>> TGSI semantics), just decree that the driver always takes ownership of
>>>>>>>> the copy passed in, and if the st wants to hang on to a copy too, then
>>>>>>>> it must clone.  I suppose this would work well enough for
>>>>>>>> freedreno/vc4, which both end up generating variants later.  It does
>>>>>>>> force an extra clone for drivers that immediately translate into their
>>>>>>>> own backend IR and don't need to keep the NIR around, for example.
>>>>>>>> Maybe that is not worth caring about (since at this point it is
>>>>>>>> hypothetical).
>>>>>>> While always cloning does have this disadvantage, I don't think it's
>>>>>>> really relevant here. Even if the driver throws away the NIR
>>>>>>> immediately after consuming it, almost invariably it's going to want
>>>>>>> to modify  it. The generic NIR passed in by the state tracker (other
>>>>>>> IR -> NIR + some optimizations) is almost never going to be the same
>>>>>>> as the NIR after going through driver-specific lowering passes, which
>>>>>>> means that drivers are never going to want a read-only version of the
>>>>>>> IR. In light of that, I think making the driver own the IR passed in
>>>>>>> seems like the most sensible thing.
>>>>>> well, unless the driver is already doing it's own lowering in it's own
>>>>>> native IR..
>>>>> Well, if you're not doing any lowering in NIR, then you aren't really
>>>>> taking any advantage of it. I can't see a plausible scenario where all
>>>>> the lowering is done in the driver's own IR -- and as soon as you do
>>>>> anything in NIR, you need the driver-owns-IR semantics.
>>>> When it comes to shader variants, I have a mix, with some things
>>>> lowered in nir and others just handled in backend..
>>>> The re-work / cleanup that I have had on a branch for a while now
>>>> (since it is currently blocked on refcnt'ing) does a first round of
>>>> variant-key independent NIR lowering/opt passes.  And then at draw
>>>> time, if the variant key has anything that is lowered in nir, I do a
>>>> second round.
>>>>>> Maybe it is too much of a hypothetical.. I still think refcnt'ing
>>>>>> gives some nice flexibility to deal with various scenarios, and having
>>>>>> to call nir_shader_unref() isn't so much of a burden.
>>>>> Still, I can't see how this flexibility is at all useful, and it seems
>>>>> like overkill since the driver will always want a mutable version of
>>>>> the IR anyways.
>>>> Well, due to the structure I mentioned above, at draw time when I need
>>>> to generate a variant with nothing lowered in NIR, I simply incr the
>>>> refcnt on the IR which has already gone through first round of NIR
>>>> passes, and pass that in to my back end compiler.  At the end, once
>>>> the shader binary is generated, I can unconditionally unref the
>>>> nir_shader without having to care.
>>>> Without refcnt'ing I'd either have to generate a pointless clone or
>>>> keep track that the nir_shader should not actually be free'd.  Not
>>>> impossible, just a bit more ugly.
>>> Assuming you do all your variant management in your driver's IR, then
>>> you don't need to do anything. If you do some variant management in
>>> NIR, then in the function where you do the NIR-based lowering you can
>>> check if you need to do any lowering based on the shader key, clone
>>> first, and give the NIR->ir3 function the cloned IR and then free it.
>>> It might be a "bit more ugly," but it's really not that much different
>>> from the refcounting, and when the extra shader gets created/freed is
>>> made explicit.
>> I'd have to re-arrange things some compared to how the backend works
>> now..  not impossible, but annoying.  Also, I could more easily unref
>> the NIR once I've converted into ir3 rather than waiting until after
>> the backend passes.
> That's just a matter of how you structure things... if you do the
> NIR-based lowering in the same function as converting to ir3, which
> seems like a good idea anyways, then it should be possible to unref
> before you run your backend passes. But I doubt the difference matters
> anyways. Our experience with i965 is that holding a lot of shaders at
> once can eat into memory, but memory consumption while actually doing
> work hardly matters (I guess it could if it's totally out of control,
> but that's not here...).

well, at a minimum, not having refcnt'ing is constraining the way the
code is structured..

>>>> (The gallium glsl_to_nir stuff is also currently using refcnt'ing,
>>>> although at least for freedreno/ir3 it isn't strictly needed.. I could
>>>> just unconditionally clone in the state tracker.  That said, I'm still
>>>> of the opinion that refcnt'ing could be useful to some other driver
>>>> someday)
>>> "It could be useful to some driver someday" isn't a good argument for
>>> adding stuff today. We've already had enough examples of things in NIR
>>> that we added because we thought it was useful, but turned out not to
>>> be.
>> well, it is useful to one driver today, as I explained..  so it is
>> more a matter of "It could be also useful to some other driver...".
>> Otherwise I would agree with you.
> But even for you, this is still overkill. Reference counting is most
> useful when there are potentially many different owners of a resource,
> which isn't the case here. If we add reference counting now, we'd be
> adding an essentially useless feature, and the fact that you used it
> doesn't change the fact that it's unnecessary.

well, I guess it depends on how you define "owners".. I mean, just
because something is one driver, doesn't mean things can't be
structured/modularized to separate concerns.  Reference counting gives
some nice flexibility to avoid copies for expensive data structures
and lets you deal w/ (sub)modules owning references rather than the
entire data structure.

Not having reference counting constrains the way the code is
structured, and (still, IMHO) for no particularly good reason.


>> BR,
>> -R
>>>> BR,
>>>> -R
>>>>>> BR,
>>>>>> -R
>>>>>>>> (I guess nouveau is the one driver, that if it ever consumed NIR,
>>>>>>>> would translate immediately into it's own backend IR?)
>>>>>>>>> 2) gallium passes read-only IR to the back-end and it always makes a copy.
>>>>>>>> This is how it is w/ TGSI, but I think with NIR we are free to make a
>>>>>>>> clean break.  And we *definitely* want to avoid
>>>>>>>> the-driver-always-copies semantics..
>>>>>>>>> It sounds like, from what Marek is saying, that gallium is currently doing
>>>>>>>>> (2) and changing it to (1) would be painful.  I think reference counting is
>>>>>>>>> more like an awkward option 1.5 than option 3.  Reference counting would
>>>>>>>>> mean that gallium passes a reference to the driver which it is expected to
>>>>>>>>> unref but may keep a second reference if it wants to keep the driver from
>>>>>>>>> modifying it.  Then the driver may or may not make a copy based on the
>>>>>>>>> number of references.  Why don't we just make it explicit and add a
>>>>>>>>> read-only bit and call it a day.
>>>>>>>>> One of the reasons I don't like passing a reference is that it effectively
>>>>>>>>> puts allocation and freeing in different components of the driver.
>>>>>>>> With refcnt'ing you should talk in terms of ref/unref rather than
>>>>>>>> allocate/free.. imho.  Although maybe that is what you meant.  (In
>>>>>>>> which case, yes, that was my idea, that passing in to driver transfers
>>>>>>>> ownership of the passed reference.)
>>>>>>>>> This
>>>>>>>>> means that if and driver doesn't care at all about the shader that gets
>>>>>>>>> passed in, it still has to under it to avoid a memory leak.  You can't have
>>>>>>>>> the driver take the reference because then, either it comes in with a
>>>>>>>>> recount of 0 and should have been deleted, or the "can I modify this" check
>>>>>>>>> becomes "recount <= 2" which makes no sense.
>>>>>>>> hmm, no, if ownership of the reference is transferred to the driver,
>>>>>>>> then it becomes "refcount == 1" (and refcount == 0 should be an
>>>>>>>> assert, because something has gone horribly wrong)
>>>>>>>> BR,
>>>>>>>> -R

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