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

[Yuanhan Liu <yliu@fridaylinux.org>]

On Mon, Apr 23, 2018 at 11:03:22AM +0200, Maxime Coquelin wrote:
> 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.

Hi Maxime,

Thank you for letting me know. I have now dropped it.

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