Re: [PATCH 9/9] qemu-file: Account for rate_limit usage on qemu_fflush()

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, May 04, 2023 at 01:38:41PM +0200, Juan Quintela wrote:
> That is the moment we know we have transferred something.
> 
> Signed-off-by: Juan Quintela <quintela@redhat.com>
> ---
>  migration/qemu-file.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/migration/qemu-file.c b/migration/qemu-file.c
> index ddebfac847..309b4c56f4 100644
> --- a/migration/qemu-file.c
> +++ b/migration/qemu-file.c
> @@ -300,7 +300,9 @@ void qemu_fflush(QEMUFile *f)
>                                     &local_error) < 0) {
>              qemu_file_set_error_obj(f, -EIO, local_error);
>          } else {
> -            f->total_transferred += iov_size(f->iov, f->iovcnt);
> +            uint64_t size = iov_size(f->iov, f->iovcnt);
> +            qemu_file_acct_rate_limit(f, size);
> +            f->total_transferred += size;
>          }
>  
>          qemu_iovec_release_ram(f);
> @@ -527,7 +529,6 @@ void qemu_put_buffer_async(QEMUFile *f, const uint8_t *buf, size_t size,
>          return;
>      }
>  
> -    f->rate_limit_used += size;
>      add_to_iovec(f, buf, size, may_free);
>  }
>  
> @@ -545,7 +546,6 @@ void qemu_put_buffer(QEMUFile *f, const uint8_t *buf, size_t size)
>              l = size;
>          }
>          memcpy(f->buf + f->buf_index, buf, l);
> -        f->rate_limit_used += l;
>          add_buf_to_iovec(f, l);
>          if (qemu_file_get_error(f)) {
>              break;
> @@ -562,7 +562,6 @@ void qemu_put_byte(QEMUFile *f, int v)
>      }
>  
>      f->buf[f->buf_index] = v;
> -    f->rate_limit_used++;
>      add_buf_to_iovec(f, 1);
>  }

This has a slight semantic behavioural change.

By accounting for rate limit in the qemu_put functions, we ensure
that we stop growing the iovec when rate limiting activates.

If we only apply rate limit in the the flush function, that will
let the  f->iov continue to accumulate buffers, while we have
rate limited the actual transfer. This makes me uneasy - it feels
like a bad idea to continue to accumulate buffers if we're not
ready to send them

With regards,
Daniel
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