Re: [dpdk-dev] [PATCH v2 2/3] vhost: protect dirty logging against logging base change

[Victor Kaplansky <vkaplans@redhat.com>]

----- Original Message -----
> From: "Maxime Coquelin" <maxime.coquelin@redhat.com>
> To: "Victor Kaplansky" <vkaplans@redhat.com>
> Cc: dev@dpdk.org, yliu@fridaylinux.org, "tiwei bie" <tiwei.bie@intel.com>, "jianfeng tan" <jianfeng.tan@intel.com>,
> stable@dpdk.org, jfreiman@redhat.com
> Sent: Monday, November 27, 2017 10:27:22 AM
> Subject: Re: [PATCH v2 2/3] vhost: protect dirty logging against logging base change
> 
> Hi Victor,
> 
> On 11/27/2017 09:16 AM, Victor Kaplansky wrote:
> > Hi,
> > 
> > While I agree that taking full fledged lock by rte_rwlock_read_lock()
> > solves the race condition,
> > I'm afraid that it would be too expensive in case when logging is off,
> > since it introduces
> > acquiring and releasing lock into the main flow of ring updates.
> 
> Actually my v2 fixes the performance penalty when logging is off. The
> lock is now taken after the logging feature check.
> 
> But still, I agree logging on case will suffer from a performance
> penalty.

Yes, checking of logging feature is better than nothing, but VHOST_F_LOG_ALL
marks only whether logging is supported by the device and not if
logging is in the action. Thus, any guest will hit the performance
degradation even not during migration.


> 
> > It is OK for now, as it fixes the bug, but we need to perform more careful
> > performance measurements,
> > and see whether the performance degradation is not too prohibitive.
> > 
> > As alternative, we may consider using more light weighted busy looping.
> 
> I think it will end up almost being the same, as both threads will need
> to busy loop. PMD thread to be sure the protocol thread isn't being
> unmapping the region before doing the logging, and protocol thread to be
> sure the PMD thread is not doing logging before handling the set log
> base.
> 

I'm not fully aware how rte_rwlock_read_lock() is implemented, but
theoretically busy looping should be much cheaper in cases when
taking lock by one side is very rare.

> Maybe you have something else in mind?
> 
> > Also, lets fix by this series the __sync_fetch_and_or_8 ->
> > __sync_fetch_and_or,
> > as it may improve the performance slightly.
> 
> Sure, this can be done, but it would need to be benchmarked first.

Agree.
> 
> Regards,
> Maxime
>