Re: [PATCH] vhost: improve dirty pages logging performance

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, May 07, 2018 at 11:49:49AM +0800, Tiwei Bie wrote:
> On Fri, May 04, 2018 at 05:48:05PM +0200, Maxime Coquelin wrote:
> > Hi Tiwei,
> > 
> > On 05/03/2018 01:56 PM, Tiwei Bie wrote:
> > > On Mon, Apr 30, 2018 at 05:59:54PM +0200, Maxime Coquelin wrote:
> [...]
> > > > +static __rte_always_inline void
> > > > +vhost_log_cache_sync(struct virtio_net *dev, struct vhost_virtqueue *vq)
> > > > +{
> > > > +	uint32_t *log_base;
> > > > +	int i;
> > > > +
> > > > +	if (likely(((dev->features & (1ULL << VHOST_F_LOG_ALL)) == 0) ||
> > > > +		   !dev->log_base))
> > > > +		return;
> > > > +
> > > > +	log_base = (uint32_t *)(uintptr_t)dev->log_base;
> > > > +
> > > > +	/* To make sure guest memory updates are committed before logging */
> > > > +	rte_smp_wmb();
> > > 
> > > It seems that __sync_fetch_and_or() can be considered a full
> > > barrier [1]. So do we really need this rte_smp_wmb()?
> > 
> > That's a good point, thanks for the pointer.
> > 
> > > Besides, based on the same doc [1], it seems that the __sync_
> > > version is deprecated in favor of the __atomic_ one.
> > 
> > I will change to __atomic_. For the memory model, do you agree I should
> > use __ATOMIC_SEQ_CST?
> 
> Maybe we can use __ATOMIC_RELAXED and keep rte_smp_wmb().
> 
> Best regards,
> Tiwei Bie

Yes I think if you do your own barriers, you can use __ATOMIC_RELAXED
in theory.

One issue is that as one lwn article noted
"there will be some seriously suboptimal code production before
gcc-7.1".  So if you are using these new APIs, you should also check
that a relatively new compiler is used.


> > 
> > > [1] https://gcc.gnu.org/onlinedocs/gcc/_005f_005fsync-Builtins.html
> > > 
> > > > +
> > > > +	for (i = 0; i < vq->log_cache_nb_elem; i++) {
> > > > +		struct log_cache_entry *elem = vq->log_cache + i;
> > > > +
> > > > +		__sync_fetch_and_or(log_base + elem->offset, elem->val);
> > > > +	}
> > > > +
> > > > +	vq->log_cache_nb_elem = 0;
> > > > +}
> > > > +
> > > [...]
> > > 
> > 
> > Thanks,
> > Maxime