Re: [PATCH v2] linux-aio: add IO_CMD_FDSYNC command support

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Mon, Mar 11, 2024 at 11:13:33AM +0530, Prasad Pandit wrote:
> From: Prasad Pandit <pjp@fedoraproject.org>
> 
> Libaio defines IO_CMD_FDSYNC command to sync all outstanding
> asynchronous I/O operations, by flushing out file data to the
> disk storage.
> 
> Enable linux-aio to submit such aio request. This helps to
> reduce latency induced via pthread_create calls by
> thread-pool (aio=threads).
> 
> Signed-off-by: Prasad Pandit <pjp@fedoraproject.org>
> ---
>  block/file-posix.c | 12 ++++++++++++
>  block/linux-aio.c  |  5 ++++-
>  2 files changed, 16 insertions(+), 1 deletion(-)
> 
> v2: if IO_CMD_FDSYNC is not supported by the kernel,
>     fallback on thread-pool flush.
>   -> https://lists.nongnu.org/archive/html/qemu-devel/2024-03/msg01986.html
> 
> diff --git a/block/file-posix.c b/block/file-posix.c
> index 35684f7e21..4f2195d01d 100644
> --- a/block/file-posix.c
> +++ b/block/file-posix.c
> @@ -2599,6 +2599,18 @@ static int coroutine_fn raw_co_flush_to_disk(BlockDriverState *bs)
>      if (raw_check_linux_io_uring(s)) {
>          return luring_co_submit(bs, s->fd, 0, NULL, QEMU_AIO_FLUSH);
>      }
> +#endif
> +#ifdef CONFIG_LINUX_AIO
> +    if (raw_check_linux_aio(s)) {
> +        ret = laio_co_submit(s->fd, 0, NULL, QEMU_AIO_FLUSH, 0);
> +        if (ret >= 0) {
> +            /*
> +             * if AIO_FLUSH is supported return
> +             * else fallback on thread-pool flush.
> +             */
> +            return ret;
> +        }

Falling back every time on an older host kernel might be a noticeable
performance regression. That can be avoided with a variable that keeps
track of whether -EINVAL was seen before and skips Linux AIO in that
case.

However, it appears that popular distributions starting from Debian 10,
Ubuntu 20.04, Fedora 27, CentOS 8, and OpenSUSE Leap 15.5 have the
necessary minimum Linux 4.18 kernel:
https://repology.org/project/linux/versions

Fallback should be very rare, so I don't think it needs to be optimized:

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

> +    }
>  #endif
>      return raw_thread_pool_submit(handle_aiocb_flush, &acb);
>  }
> diff --git a/block/linux-aio.c b/block/linux-aio.c
> index ec05d946f3..d940d029e3 100644
> --- a/block/linux-aio.c
> +++ b/block/linux-aio.c
> @@ -384,6 +384,9 @@ static int laio_do_submit(int fd, struct qemu_laiocb *laiocb, off_t offset,
>      case QEMU_AIO_READ:
>          io_prep_preadv(iocbs, fd, qiov->iov, qiov->niov, offset);
>          break;
> +    case QEMU_AIO_FLUSH:
> +        io_prep_fdsync(iocbs, fd);
> +        break;
>      /* Currently Linux kernel does not support other operations */
>      default:
>          fprintf(stderr, "%s: invalid AIO request type 0x%x.\n",
> @@ -412,7 +415,7 @@ int coroutine_fn laio_co_submit(int fd, uint64_t offset, QEMUIOVector *qiov,
>      AioContext *ctx = qemu_get_current_aio_context();
>      struct qemu_laiocb laiocb = {
>          .co         = qemu_coroutine_self(),
> -        .nbytes     = qiov->size,
> +        .nbytes     = qiov ? qiov->size : 0,
>          .ctx        = aio_get_linux_aio(ctx),
>          .ret        = -EINPROGRESS,
>          .is_read    = (type == QEMU_AIO_READ),
> -- 
> 2.44.0
> 

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