[dpdk-stable] patch 'vhost: avoid concurrency when logging dirty pages' has been queued to stable release 18.02.2

[Maxime Coquelin]

Hi Luca,

Please don't backport this patch to -stable releases.
It has already been reverted in upstream master, and a new patch is to
be posted in the coming hours.

I already notified Yuanhan for the same for v17.11 LTS.

Thanks,
Maxime

On 04/30/2018 04:40 PM, luca.boccassi at gmail.com wrote:
> Hi,
> 
> FYI, your patch has been queued to stable release 18.02.2
> 
> Note it hasn't been pushed to http://dpdk.org/browse/dpdk-stable yet.
> It will be pushed if I get no objections before 05/02/18. So please
> shout if anyone has objections.
> 
> Thanks.
> 
> Luca Boccassi
> 
> ---
>  From a5d642c446cc3a3be41694715d99f3e9f9ea44df Mon Sep 17 00:00:00 2001
> From: Maxime Coquelin <maxime.coquelin at redhat.com>
> Date: Wed, 21 Mar 2018 16:44:13 +0100
> Subject: [PATCH] vhost: avoid concurrency when logging dirty pages
> 
> [ upstream commit 394313fff39d0f994325c47f7eab39daf5dc9e11 ]
> 
> 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")
> 
> Signed-off-by: Maxime Coquelin <maxime.coquelin at redhat.com>
> Reviewed-by: Jianfeng Tan <jianfeng.tan at intel.com>
> ---
>   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 bab0b456e..a4f23f714 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
>