Re: [FFmpeg-devel] [PATCH v2 02/27] avcodec/decode: Add new ProgressFrame API

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

Michael Niedermayer:
> On Mon, Apr 08, 2024 at 10:13:40PM +0200, Andreas Rheinhardt wrote:
>> Frame-threaded decoders with inter-frame dependencies
>> use the ThreadFrame API for syncing. It works as follows:
>>
>> During init each thread allocates an AVFrame for every
>> ThreadFrame.
>>
>> Thread A reads the header of its packet and allocates
>> a buffer for an AVFrame with ff_thread_get_ext_buffer()
>> (which also allocates a small structure that is shared
>> with other references to this frame) and sets its fields,
>> including side data. Then said thread calls ff_thread_finish_setup().
>> From that moment onward it is not allowed to change any
>> of the AVFrame fields at all any more, but it may change
>> fields which are an indirection away, like the content of
>> AVFrame.data or already existing side data.
>>
>> After thread A has called ff_thread_finish_setup(),
>> another thread (the user one) calls the codec's update_thread_context
>> callback which in turn calls ff_thread_ref_frame() which
>> calls av_frame_ref() which reads every field of A's
>> AVFrame; hence the above restriction on modifications
>> of the AVFrame (as any modification of the AVFrame by A after
>> ff_thread_finish_setup() would be a data race). Of course,
>> this av_frame_ref() also incurs allocations and therefore
>> needs to be checked. ff_thread_ref_frame() also references
>> the small structure used for communicating progress.
>>
>> This av_frame_ref() makes it awkward to propagate values that
>> only become known during decoding to later threads (in case of
>> frame reordering or other mechanisms of delayed output (like
>> show-existing-frames) it's not the decoding thread, but a later
>> thread that returns the AVFrame). E.g. for VP9 when exporting video
>> encoding parameters as side data the number of blocks only
>> becomes known during decoding, so one can't allocate the side data
>> before ff_thread_finish_setup(). It is currently being done afterwards
>> and this leads to a data race in the vp9-encparams test when using
>> frame-threading. Returning decode_error_flags is also complicated
>> by this.
>>
>> To perform this exchange a buffer shared between the references
>> is needed (notice that simply giving the later threads a pointer
>> to the original AVFrame does not work, because said AVFrame will
>> be reused lateron when thread A decodes the next packet given to it).
>> One could extend the buffer already used for progress for this
>> or use a new one (requiring yet another allocation), yet both
>> of these approaches have the drawback of being unnatural, ugly
>> and requiring quite a lot of ad-hoc code. E.g. in case of the VP9
>> side data mentioned above one could not simply use the helper
>> that allocates and adds the side data to an AVFrame in one go.
>>
>> The ProgressFrame API meanwhile offers a different solution to all
>> of this. It is based around the idea that the most natural
>> shared object for sharing information about an AVFrame between
>> decoding threads is the AVFrame itself. To actually implement this
>> the AVFrame needs to be reference counted. This is achieved by
>> putting a (ownership) pointer into a shared (and opaque) structure
>> that is managed by the RefStruct API and which also contains
>> the stuff necessary for progress reporting.
>> The users get a pointer to this AVFrame with the understanding
>> that the owner may set all the fields until it has indicated
>> that it has finished decoding this AVFrame; then the users are
>> allowed to read everything. Every decoder may of course employ
>> a different contract than the one outlined above.
>>
>> Given that there is no underlying av_frame_ref(), creating
>> references to a ProgressFrame can't fail. Only
>> ff_thread_progress_get_buffer() can fail, but given that
>> it will replace calls to ff_thread_get_ext_buffer() it is
>> at places where errors are already expected and properly
>> taken care of.
>>
>> The ProgressFrames are empty (i.e. the AVFrame pointer is NULL
>> and the AVFrames are not allocated during init at all)
>> while not being in use; ff_thread_progress_get_buffer() both
>> sets up the actual ProgressFrame and already calls
>> ff_thread_get_buffer(). So instead of checking for
>> ThreadFrame.f->data[0] or ThreadFrame.f->buf[0] being NULL
>> for "this reference frame is non-existing" one should check for
>> ProgressFrame.f.
>>
>> This also implies that one can only set AVFrame properties
>> after having allocated the buffer. This restriction is not deep:
>> if it becomes onerous for any codec, ff_thread_progress_get_buffer()
>> can be broken up. The user would then have to get a buffer
>> himself.
>>
>> In order to avoid unnecessary allocations, the shared structure
>> is pooled, so that both the structure as well as the AVFrame
>> itself are reused. This means that there won't be lots of
>> unnecessary allocations in case of non-frame-threaded decoding.
>> It might even turn out to have fewer than the current code
>> (the current code allocates AVFrames for every DPB slot, but
>> these are often excessively large and not completely used;
>> the new code allocates them on demand). Pooling relies on the
>> reset function of the RefStruct pool API, it would be impossible
>> to implement with the AVBufferPool API.
>>
>> Finally, ProgressFrames have no notion of owner; they are built
>> on top of the ThreadProgress API which also lacks such a concept.
>> Instead every ThreadProgress and every ProgressFrame contains
>> its own mutex and condition variable, making it completely independent
>> of pthread_frame.c. Just like the ThreadFrame API it is simply
>> presumed that only the actual owner/producer of a frame reports
>> progress on said frame.
>>
>> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
>> ---
>>  libavcodec/Makefile         |   1 +
>>  libavcodec/avcodec.c        |   1 +
>>  libavcodec/codec_internal.h |   4 ++
>>  libavcodec/decode.c         | 122 +++++++++++++++++++++++++++++++++
>>  libavcodec/internal.h       |   2 +
>>  libavcodec/progressframe.h  | 133 ++++++++++++++++++++++++++++++++++++
>>  libavcodec/pthread_frame.c  |   1 +
>>  libavcodec/tests/avcodec.c  |   3 +-
>>  8 files changed, 266 insertions(+), 1 deletion(-)
>>  create mode 100644 libavcodec/progressframe.h
> 
> breaks build without threads
> 
> ./configure --disable-pthreads --cc='ccache gcc' && make -j32
> ...
> libavcodec/threadprogress.c: In function ‘ff_thread_progress_report’:
> libavcodec/threadprogress.c:54:5: error: implicit declaration of function ‘pthread_mutex_lock’; did you mean ‘ff_mutex_lock’? [-Werror=implicit-function-declaration]
>      pthread_mutex_lock(&pro->progress_mutex);
>      ^~~~~~~~~~~~~~~~~~
>      ff_mutex_lock
> libavcodec/threadprogress.c:55:5: error: implicit declaration of function ‘pthread_cond_broadcast’; did you mean ‘ff_cond_broadcast’? [-Werror=implicit-function-declaration]
>      pthread_cond_broadcast(&pro->progress_cond);
>      ^~~~~~~~~~~~~~~~~~~~~~
>      ff_cond_broadcast
> libavcodec/threadprogress.c:56:5: error: implicit declaration of function ‘pthread_mutex_unlock’; did you mean ‘ff_mutex_unlock’? [-Werror=implicit-function-declaration]
>      pthread_mutex_unlock(&pro->progress_mutex);
>      ^~~~~~~~~~~~~~~~~~~~
>      ff_mutex_unlock
> libavcodec/threadprogress.c: In function ‘ff_thread_progress_await’:
> libavcodec/threadprogress.c:71:9: error: implicit declaration of function ‘pthread_cond_wait’; did you mean ‘ff_cond_wait’? [-Werror=implicit-function-declaration]
>          pthread_cond_wait(&pro->progress_cond, &pro->progress_mutex);
>          ^~~~~~~~~~~~~~~~~
>          ff_cond_wait
> cc1: some warnings being treated as errors
> ffbuild/common.mak:81: recipe for target 'libavcodec/threadprogress.o' failed
> make: *** [libavcodec/threadprogress.o] Error 1
> make: *** Waiting for unfinished jobs....
> 

Thanks for testing; fixed by v3 of patch #1.

- Andreas

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