netdev - Re: Performance regression with virtio

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Message-ID: <20170730222552.4fxxnx3jxg4yv65g@multivac.euank.com>
Date:   Sun, 30 Jul 2017 15:25:52 -0700
From:   Euan Kemp <euan.kemp@...eos.com>
To:     mst@...hat.com, seth.forshee@...onical.com
Cc:     netdev@...r.kernel.org
Subject: Re: Performance regression with virtio_net

I've also observed this performance regression.

The minimal fix for me is removing the two
> if (unlikely(len > (unsigned long)ctx))
checks added in 680557c.

After digging a little more, the reason that check can fail appears to
be that add_recvbuf_mergeable sometimes includes a hole at the end,
which is included in len but not ctx.

I'd send a patch removing those conditions, but I'm not certain
whether "truesize" in receive_mergeable should also be changed back to
be the max of len/ctx, or should remain as-is.

- Euan