Re: [PATCH v3 3/3] migration/multifd: Warn user when zerocopy not working

[Peter Xu]

On Mon, Jul 04, 2022 at 05:23:15PM -0300, Leonardo Bras wrote:
> Some errors, like the lack of Scatter-Gather support by the network
> interface(NETIF_F_SG) may cause sendmsg(...,MSG_ZEROCOPY) to fail on using
> zero-copy, which causes it to fall back to the default copying mechanism.
> 
> After each full dirty-bitmap scan there should be a zero-copy flush
> happening, which checks for errors each of the previous calls to
> sendmsg(...,MSG_ZEROCOPY). If all of them failed to use zero-copy, then
> increment dirty_sync_missed_zero_copy migration stat to let the user know
> about it.
> 
> Signed-off-by: Leonardo Bras <leobras@redhat.com>
> ---
>  migration/ram.h     | 2 ++
>  migration/multifd.c | 2 ++
>  migration/ram.c     | 5 +++++
>  3 files changed, 9 insertions(+)
> 
> diff --git a/migration/ram.h b/migration/ram.h
> index ded0a3a086..d3c7eb96f5 100644
> --- a/migration/ram.h
> +++ b/migration/ram.h
> @@ -87,4 +87,6 @@ void ram_write_tracking_prepare(void);
>  int ram_write_tracking_start(void);
>  void ram_write_tracking_stop(void);
>  
> +void dirty_sync_missed_zero_copy(void);
> +
>  #endif
> diff --git a/migration/multifd.c b/migration/multifd.c
> index 684c014c86..3909b34967 100644
> --- a/migration/multifd.c
> +++ b/migration/multifd.c
> @@ -624,6 +624,8 @@ int multifd_send_sync_main(QEMUFile *f)
>              if (ret < 0) {
>                  error_report_err(err);
>                  return -1;
> +            } else if (ret == 1) {
> +                dirty_sync_missed_zero_copy();
>              }
>          }
>      }

I know that Juan is working on some patch to only do
multifd_send_sync_main() for each dirty sync, but that's not landed, right?

Can we name it without "dirty-sync" at all (so it'll work before/after
Juan's patch will be applied)?  Something like "zero-copy-send-fallbacks"?

The other thing is the subject may need to be touched up as right now with
the field we don't warn the user anymore on zero-copy-send fallbacks.

Thanks,

> diff --git a/migration/ram.c b/migration/ram.c
> index 01f9cc1d72..db948c4787 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -407,6 +407,11 @@ static void ram_transferred_add(uint64_t bytes)
>      ram_counters.transferred += bytes;
>  }
>  
> +void dirty_sync_missed_zero_copy(void)
> +{
> +    ram_counters.dirty_sync_missed_zero_copy++;
> +}
> +
>  /* used by the search for pages to send */
>  struct PageSearchStatus {
>      /* Current block being searched */
> -- 
> 2.36.1
> 

-- 
Peter Xu