[FFmpeg-devel] [PATCH 2/2] avdevice/xcbgrab: use a buffer pool for shared memory segments

Marton Balint cus at passwd.hu
Sat Dec 14 22:19:57 EET 2019


On Sat, 14 Dec 2019, James Almer wrote:

> On 12/7/2019 6:57 PM, Marton Balint wrote:
>> Also by wrapping the SHM buffer in an AVBufferRef we eliminate yet another
>> possible memcpy improving performance.
>> 
>> Signed-off-by: Marton Balint <cus at passwd.hu>
>> ---
>>  libavdevice/xcbgrab.c | 79 ++++++++++++++++++++++++++++++---------------------
>>  1 file changed, 47 insertions(+), 32 deletions(-)
>> 
>> diff --git a/libavdevice/xcbgrab.c b/libavdevice/xcbgrab.c
>> index 158f381c04..82ea09ca96 100644
>> --- a/libavdevice/xcbgrab.c
>> +++ b/libavdevice/xcbgrab.c
>> @@ -49,13 +49,11 @@
>>  typedef struct XCBGrabContext {
>>      const AVClass *class;
>> 
>> -    uint8_t *buffer;
>> -
>>      xcb_connection_t *conn;
>>      xcb_screen_t *screen;
>>      xcb_window_t window;
>>  #if CONFIG_LIBXCB_SHM
>> -    xcb_shm_seg_t segment;
>> +    AVBufferPool *shm_pool;
>>  #endif
>>      int64_t time_frame;
>>      AVRational time_base;
>> @@ -231,31 +229,35 @@ static int check_shm(xcb_connection_t *conn)
>>      return 0;
>>  }
>> 
>> -static int allocate_shm(AVFormatContext *s)
>> +static void free_shm_buffer(void *opaque, uint8_t *data)
>>  {
>> -    XCBGrabContext *c = s->priv_data;
>> -    int size = c->frame_size + AV_INPUT_BUFFER_PADDING_SIZE;
>> +    shmdt(data);
>> +}
>> +
>> +static AVBufferRef *allocate_shm_buffer(void *opaque, int size)
>> +{
>> +    xcb_connection_t *conn = opaque;
>> +    xcb_shm_seg_t segment;
>> +    AVBufferRef *ref;
>>      uint8_t *data;
>>      int id;
>> 
>> -    if (c->buffer)
>> -        return 0;
>>      id = shmget(IPC_PRIVATE, size, IPC_CREAT | 0777);
>> -    if (id == -1) {
>> -        char errbuf[1024];
>> -        int err = AVERROR(errno);
>> -        av_strerror(err, errbuf, sizeof(errbuf));
>> -        av_log(s, AV_LOG_ERROR, "Cannot get %d bytes of shared memory: %s.\n",
>> -               size, errbuf);
>> -        return err;
>> -    }
>> -    xcb_shm_attach(c->conn, c->segment, id, 0);
>> +    if (id == -1)
>> +        return NULL;
>> +
>> +    segment = xcb_generate_id(conn);
>> +    xcb_shm_attach(conn, segment, id, 0);
>>      data = shmat(id, NULL, 0);
>>      shmctl(id, IPC_RMID, 0);
>>      if ((intptr_t)data == -1 || !data)
>> -        return AVERROR(errno);
>> -    c->buffer = data;
>> -    return 0;
>> +        return NULL;
>> +
>> +    ref = av_buffer_create(data, size, free_shm_buffer, (void *)(ptrdiff_t)segment, 0);
>> +    if (!ref)
>> +        shmdt(data);
>> +
>> +    return ref;
>>  }
>>
>>  static int xcbgrab_frame_shm(AVFormatContext *s, AVPacket *pkt)
>> @@ -265,15 +267,19 @@ static int xcbgrab_frame_shm(AVFormatContext *s, AVPacket *pkt)
>>      xcb_shm_get_image_reply_t *img;
>>      xcb_drawable_t drawable = c->screen->root;
>>      xcb_generic_error_t *e = NULL;
>> -    int ret;
>> +    AVBufferRef *buf;
>> +    xcb_shm_seg_t segment;
>> 
>> -    ret = allocate_shm(s);
>> -    if (ret < 0)
>> -        return ret;
>> +    buf = av_buffer_pool_get(c->shm_pool);
>> +    if (!buf) {
>> +        av_log(s, AV_LOG_ERROR, "Could not get shared memory buffer.\n");
>> +        return AVERROR(ENOMEM);
>> +    }
>> +    segment = (xcb_shm_seg_t)av_buffer_pool_buffer_get_opaque(buf);
>>
>>      iq = xcb_shm_get_image(c->conn, drawable,
>>                             c->x, c->y, c->width, c->height, ~0,
>> -                           XCB_IMAGE_FORMAT_Z_PIXMAP, c->segment, 0);
>> +                           XCB_IMAGE_FORMAT_Z_PIXMAP, segment, 0);
>>      img = xcb_shm_get_image_reply(c->conn, iq, &e);
>>
>>      xcb_flush(c->conn);
>> @@ -287,12 +293,16 @@ static int xcbgrab_frame_shm(AVFormatContext *s, AVPacket *pkt)
>>                 e->sequence, e->resource_id, e->minor_code, e->major_code);
>>
>>          free(e);
>> +        av_buffer_unref(&buf);
>>          return AVERROR(EACCES);
>>      }
>>
>>      free(img);
>> 
>> -    pkt->data = c->buffer;
>> +    av_init_packet(pkt);
>> +
>> +    pkt->buf = buf;
>> +    pkt->data = buf->data;
>>      pkt->size = c->frame_size;
>>
>>      return 0;
>> @@ -425,8 +435,10 @@ static int xcbgrab_read_packet(AVFormatContext *s, AVPacket *pkt)
>>          xcbgrab_update_region(s);
>>
>>  #if CONFIG_LIBXCB_SHM
>> -    if (c->has_shm && xcbgrab_frame_shm(s, pkt) < 0)
>> +    if (c->has_shm && xcbgrab_frame_shm(s, pkt) < 0) {
>> +        av_log(s, AV_LOG_WARNING, "Continuing without shared memory.\n");
>>          c->has_shm = 0;
>> +    }
>>  #endif
>>      if (!c->has_shm)
>>          ret = xcbgrab_frame(s, pkt);
>> @@ -447,9 +459,7 @@ static av_cold int xcbgrab_read_close(AVFormatContext *s)
>>      XCBGrabContext *ctx = s->priv_data;
>>
>>  #if CONFIG_LIBXCB_SHM
>> -    if (ctx->buffer) {
>> -        shmdt(ctx->buffer);
>> -    }
>> +    av_buffer_pool_uninit(&ctx->shm_pool);
>>  #endif
>>
>>      xcb_disconnect(ctx->conn);
>> @@ -515,6 +525,12 @@ static int pixfmt_from_pixmap_format(AVFormatContext *s, int depth,
>>          if (*pix_fmt) {
>>              c->bpp        = fmt->bits_per_pixel;
>>              c->frame_size = c->width * c->height * fmt->bits_per_pixel / 8;
>> +#if CONFIG_LIBXCB_SHM
>> +            c->shm_pool = av_buffer_pool_init2(c->frame_size + AV_INPUT_BUFFER_PADDING_SIZE,
>> +                                               c->conn, allocate_shm_buffer, NULL);
>
> I didn't look too much into it, but shouldn't you add a free() callback
> here and move the xcb_disconnect(conn) call from xcbgrab_read_close() to
> it? If the AVBufferRefs created here outlive the demuxer/outdev, then it
> may result in issues if the xcb_connection_t is expected to be valid
> until the last one of them is freed.

In the destructor of the buffers I only do an shmdt() on the actual shared 
memory segments. My assumption is that when I do xcb_disconnect in 
xcb_read_close() the X server cleans up shared memory objects and segments 
on the X server side automatically, but since shared memory segments are 
refcounted (by the kernel itself), I am good to use them in my process 
until I actually do an shmdt myself.

The opaque parameter of the buffers become stale after xcb_disconnect, but 
that does not matter, because I only used them during capture. So I think 
it is safe to use the buffers even after the X connection is closed 
becuase the buffers are ordinary shared memory segments, nothing X server 
specific.

Regards,
Marton


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