Re: [dpdk-dev] [PATCH] vhost: avoid concurrency when logging dirty pages

["Tan, Jianfeng" <jianfeng.tan@intel.com>]

> -----Original Message-----
> From: Maxime Coquelin [mailto:maxime.coquelin@redhat.com]
> Sent: Wednesday, March 21, 2018 11:44 PM
> To: dev@dpdk.org; Tan, Jianfeng; Bie, Tiwei; jfreimann@redhat.com
> Cc: stable@dpdk.org; Maxime Coquelin
> Subject: [PATCH] vhost: avoid concurrency when logging dirty pages
> 
> This patch aims at fixing a migration performance regression
> faced since atomic operation is used to log pages as dirty when
> doing live migration.
> 
> Instead of setting a single bit by doing an atomic read-modify-write
> operation to log a page as dirty, this patch write 0xFF to the
> corresponding byte, and so logs 8 page as dirty.
> 
> The advantage is that it avoids concurrent atomic operations by
> multiple PMD threads, the drawback is that some clean pages are
> marked as dirty and so are transferred twice.
> 
> Fixes: 897f13a1f726 ("vhost: make page logging atomic")
> Cc: stable@dpdk.org
> 
> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>

As you mentioned, the concern is if it affects the rate of convergence. Hope we could have some tests on that.

Reviewed-by: Jianfeng Tan <jianfeng.tan@intel.com>

Thanks!

> ---
>  lib/librte_vhost/vhost.h | 12 ++++--------
>  1 file changed, 4 insertions(+), 8 deletions(-)
> 
> diff --git a/lib/librte_vhost/vhost.h b/lib/librte_vhost/vhost.h
> index d947bc9e3..aa2606f8a 100644
> --- a/lib/librte_vhost/vhost.h
> +++ b/lib/librte_vhost/vhost.h
> @@ -247,18 +247,14 @@ struct virtio_net {
>  #define VHOST_LOG_PAGE	4096
> 
>  /*
> - * Atomically set a bit in memory.
> + * Mark all pages belonging to the same dirty log bitmap byte
> + * as dirty. The goal is to avoid concurrency between different
> + * threads doing atomic read-modify-writes on the same byte.
>   */
> -static __rte_always_inline void
> -vhost_set_bit(unsigned int nr, volatile uint8_t *addr)
> -{
> -	__sync_fetch_and_or_8(addr, (1U << nr));
> -}
> -
>  static __rte_always_inline void
>  vhost_log_page(uint8_t *log_base, uint64_t page)
>  {
> -	vhost_set_bit(page % 8, &log_base[page / 8]);
> +	log_base[page / 8] = 0xff;
>  }
> 
>  static __rte_always_inline void
> --
> 2.14.3