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

[James Almer]

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.

> +            if (!c->shm_pool)
> +                return AVERROR(ENOMEM);
> +#endif
>              return 0;
>          }
>  
> @@ -680,8 +696,7 @@ static av_cold int xcbgrab_read_header(AVFormatContext *s)
>      }
>  
>  #if CONFIG_LIBXCB_SHM
> -    if ((c->has_shm = check_shm(c->conn)))
> -        c->segment = xcb_generate_id(c->conn);
> +    c->has_shm = check_shm(c->conn);
>  #endif
>  
>  #if CONFIG_LIBXCB_XFIXES
>