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

[Maxime Coquelin <maxime.coquelin@redhat.com>]

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.

> 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.

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.

Regards,
Maxime