netdev - Re: [PATCH 2/3] virtio-net: correct capacity math on ring full

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Message-ID: <878vbn2ify.fsf@rustcorp.com.au>
Date:	Thu, 04 Oct 2012 09:54:01 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	"Michael S. Tsirkin" <mst@...hat.com>,
	Thomas Lendacky <tahm@...ux.vnet.ibm.com>
Cc:	Sasha Levin <levinsasha928@...il.com>,
	virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, avi@...hat.com, kvm@...r.kernel.org,
	netdev@...r.kernel.org
Subject: Re: [PATCH 2/3] virtio-net: correct capacity math on ring full

"Michael S. Tsirkin" <mst@...hat.com> writes:
> Capacity math on ring full is wrong: we are
> looking at num_sg but that might be optimistic
> because of indirect buffer use.
>
> The implementation also penalizes fast path
> with extra memory accesses for the benefit of
> ring full condition handling which is slow path.
>
> It's easy to query ring capacity so let's do just that.

This path will reduce the actual queue use to worst-case assumptions.
With bufferbloat maybe that's a good thing, but it's true.

If we do this, the code is now wrong:

	/* This can happen with OOM and indirect buffers. */
	if (unlikely(capacity < 0)) {

Because this should now *never* happen.

But I do like the cleanup; returning capacity from add_buf() was always
hacky.  I've got an idea, we'll see what it looks like...

Cheers,
Rusty.
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