Re: [Qemu-devel] [PATCH 09/15] linux-aio: fix submit aio as a batch

[Paolo Bonzini]

Il 30/07/2014 13:39, Ming Lei ha scritto:
> In the enqueue path, we can't complete request, otherwise
> "Co-routine re-entered recursively" may be caused, so this
> patch fixes the issue with below ideas:
> 
>       - for -EAGAIN or partial completion, retry the submission by
>       an introduced event handler
>       - for part of completion, also update the io queue
>       - for other failure, return the failure if in enqueue path,
>       otherwise, abort all queued I/O
> 
> Signed-off-by: Ming Lei <address@hidden>
> ---
>  block/linux-aio.c |   90 
> ++++++++++++++++++++++++++++++++++++++++-------------
>  1 file changed, 68 insertions(+), 22 deletions(-)
> 
> diff --git a/block/linux-aio.c b/block/linux-aio.c
> index 7ac7e8c..5eb9c92 100644
> --- a/block/linux-aio.c
> +++ b/block/linux-aio.c
> @@ -51,6 +51,7 @@ struct qemu_laio_state {
>  
>      /* io queue for submit at batch */
>      LaioQueue io_q;
> +    EventNotifier retry;      /* handle -EAGAIN and partial completion */
>  };
>  
>  static inline ssize_t io_event_ret(struct io_event *ev)
> @@ -154,45 +155,80 @@ static void ioq_init(LaioQueue *io_q)
>      io_q->plugged = 0;
>  }
>  
> -static int ioq_submit(struct qemu_laio_state *s)
> +static void abort_queue(struct qemu_laio_state *s)
> +{
> +    int i;
> +    for (i = 0; i < s->io_q.idx; i++) {
> +        struct qemu_laiocb *laiocb = container_of(s->io_q.iocbs[i],
> +                                                  struct qemu_laiocb,
> +                                                  iocb);
> +        laiocb->ret = -EIO;
> +        qemu_laio_process_completion(s, laiocb);
> +    }
> +}
> +
> +static int ioq_submit(struct qemu_laio_state *s, bool enqueue)
>  {
>      int ret, i = 0;
>      int len = s->io_q.idx;
> +    int j = 0;
>  
> -    do {
> -        ret = io_submit(s->ctx, len, s->io_q.iocbs);
> -    } while (i++ < 3 && ret == -EAGAIN);
> +    if (!len) {
> +        return 0;
> +    }
>  
> -    /* empty io queue */
> -    s->io_q.idx = 0;
> +    ret = io_submit(s->ctx, len, s->io_q.iocbs);
> +    if (ret == -EAGAIN) {
> +        event_notifier_set(&s->retry);

Retrying immediately (and just doing a couple of system calls to waste
time) is not an improvement.  The right place to retry is in
qemu_laio_completion_cb, after io_getevents has been called and
presumably the queue depth has decreased.

If !s->io_q.plugged but io_submit fails you can call ioq_enqueue and it
will just work.  Then you can only go to out_free_aiocb if the queue is
full (independent of the "plug" state).

Paolo

> +        return 0;
> +    } else if (ret < 0) {
> +        if (enqueue) {
> +            return ret;
> +        }
>  
> -    if (ret < 0) {
> -        i = 0;
> -    } else {
> -        i = ret;
> +        /* in non-queue path, all IOs have to be completed */
> +        abort_queue(s);
> +        ret = len;
> +    } else if (ret == 0) {
> +        goto out;
>      }
>  
> -    for (; i < len; i++) {
> -        struct qemu_laiocb *laiocb =
> -            container_of(s->io_q.iocbs[i], struct qemu_laiocb, iocb);
> -
> -        laiocb->ret = (ret < 0) ? ret : -EIO;
> -        qemu_laio_process_completion(s, laiocb);
> +    for (i = ret; i < len; i++) {
> +        s->io_q.iocbs[j++] = s->io_q.iocbs[i];
>      }
> +
> + out:
> +    /* update io queue */
> +    s->io_q.idx -= ret;
> +
>      return ret;
>  }
>  
> -static void ioq_enqueue(struct qemu_laio_state *s, struct iocb *iocb)
> +static void ioq_submit_retry(EventNotifier *e)
> +{
> +    struct qemu_laio_state *s = container_of(e, struct qemu_laio_state, 
> retry);
> +
> +    event_notifier_test_and_clear(e);
> +    ioq_submit(s, false);
> +}
> +
> +static int ioq_enqueue(struct qemu_laio_state *s, struct iocb *iocb)
>  {
>      unsigned int idx = s->io_q.idx;
>  
> +    if (unlikely(idx == s->io_q.size)) {
> +        return -1;
> +    }
> +
>      s->io_q.iocbs[idx++] = iocb;
>      s->io_q.idx = idx;
>  
> -    /* submit immediately if queue is full */
> -    if (idx == s->io_q.size) {
> -        ioq_submit(s);
> +    /* submit immediately if queue depth is above 2/3 */
> +    if (idx > s->io_q.size * 2 / 3) {
> +        return ioq_submit(s, true);
>      }
> +
> +    return 0;
>  }
>  
>  void laio_io_plug(BlockDriverState *bs, void *aio_ctx)
> @@ -214,7 +250,7 @@ int laio_io_unplug(BlockDriverState *bs, void *aio_ctx, 
> bool unplug)
>      }
>  
>      if (s->io_q.idx > 0) {
> -        ret = ioq_submit(s);
> +        ret = ioq_submit(s, false);
>      }
>  
>      return ret;
> @@ -258,7 +294,9 @@ BlockDriverAIOCB *laio_submit(BlockDriverState *bs, void 
> *aio_ctx, int fd,
>              goto out_free_aiocb;
>          }
>      } else {
> -        ioq_enqueue(s, iocbs);
> +        if (ioq_enqueue(s, iocbs) < 0) {
> +            goto out_free_aiocb;
> +        }
>      }
>      return &laiocb->common;
>  
> @@ -272,6 +310,7 @@ void laio_detach_aio_context(void *s_, AioContext 
> *old_context)
>      struct qemu_laio_state *s = s_;
>  
>      aio_set_event_notifier(old_context, &s->e, NULL);
> +    aio_set_event_notifier(old_context, &s->retry, NULL);
>  }
>  
>  void laio_attach_aio_context(void *s_, AioContext *new_context)
> @@ -279,6 +318,7 @@ void laio_attach_aio_context(void *s_, AioContext 
> *new_context)
>      struct qemu_laio_state *s = s_;
>  
>      aio_set_event_notifier(new_context, &s->e, qemu_laio_completion_cb);
> +    aio_set_event_notifier(new_context, &s->retry, ioq_submit_retry);
>  }
>  
>  void *laio_init(void)
> @@ -295,9 +335,14 @@ void *laio_init(void)
>      }
>  
>      ioq_init(&s->io_q);
> +    if (event_notifier_init(&s->retry, false) < 0) {
> +        goto out_notifer_init;
> +    }
>  
>      return s;
>  
> +out_notifer_init:
> +    io_destroy(s->ctx);
>  out_close_efd:
>      event_notifier_cleanup(&s->e);
>  out_free_state:
> @@ -310,6 +355,7 @@ void laio_cleanup(void *s_)
>      struct qemu_laio_state *s = s_;
>  
>      event_notifier_cleanup(&s->e);
> +    event_notifier_cleanup(&s->retry);
>  
>      if (io_destroy(s->ctx) != 0) {
>          fprintf(stderr, "%s: destroy AIO context %p failed\n",
>