Re: [PATCH RFC v5 net-next 1/6] virtio_ring: fix virtqueue_enable_cb() when only 1 buffers were pending

On Tue, Feb 10, 2015 at 6:18 PM, Michael S. Tsirkin [off-list ref] 
wrote:

On Tue, Feb 10, 2015 at 11:33:52AM +1030, Rusty Russell wrote:

quoted

 Jason Wang [off-list ref] writes:
 > We currently does:
 >
 > bufs = (avail->idx - last_used_idx) * 3 / 4;
 >
 > This is ok now since we only try to enable the delayed callbacks 
when
 > the queue is about to be full. This may not work well when there 
is
 > only one pending buffer in the virtqueue (this may be the case 
after
 > tx interrupt was enabled). Since virtqueue_enable_cb() will return
 > false which may cause unnecessary triggering of napis. This patch
 > correct this by only calculate the four thirds when bufs is not 
one.
 
 I mildly prefer to avoid the branch, by changing the calculation 
like
 so:
 
         /* Set bufs >= 1, even if there's only one pending buffer */
         bufs = (bufs + 1) * 3 / 4;

Or bus * 3/4 + 1

quoted

 But it's not clear to me how much this happens.  I'm happy with the
 patch though, as currently virtqueue_enable_cb_delayed() is the same
 as virtqueue_enable_cb() if there's only been one buffer added.
 
 Cheers,
 Rusty.

But isn't this by design?
The documentation says:

 * This re-enables callbacks but hints to the other side to delay
 * interrupts until most of the available buffers have been processed;

Clearly, this implies that when there's one buffer it must behave
exactly the same.

So I'm not very happy - this changes the meaning of the API without
updating the documentation.

Think hard about this. And looks like your are right. And the patch may 
in fact cause more trouble e.g the only pending buffer were consumed 
before the final check between used idx and last_used_idx.

Will drop this.

Thanks