[dpdk-stable] patch 'vhost: avoid concurrency when logging dirty pages' has been queued to LTS release 17.11.2

Maxime Coquelin maxime.coquelin at redhat.com
Mon Apr 23 11:03:22 CEST 2018


Hi Yuanhan,

On 04/22/2018 05:09 PM, Yuanhan Liu wrote:
> Hi,
> 
> FYI, your patch has been queued to LTS release 17.11.2
> 
> Note it hasn't been pushed to http://dpdk.org/browse/dpdk-stable yet.
> It will be pushed if I get no objections before 04/29/18. So please
> shout if anyone has objections.

Please don't pick this patch, it cause a regression with multiqueue,
I posted a revert of the patch on list last week.

I'm working on an alternative solution, but while it is ready, better to
keep performance penalty from initial patch.


Thanks,
Maxime
> Thanks.
> 
> 	--yliu
> 
> ---
>  From acdf634b3533a5ccb1528ee546ac9dcfb57b8a36 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 c8f2a8176..5c488fa57 100644
> --- a/lib/librte_vhost/vhost.h
> +++ b/lib/librte_vhost/vhost.h
> @@ -261,18 +261,14 @@ 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
> 


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