netdev - Re: [PATCH net 1/2] geneve, vxlan: Don't check skb

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Message-ID: <20181015130830.1c177301@redhat.com>
Date:   Mon, 15 Oct 2018 13:08:30 +0200
From:   Stefano Brivio <sbrivio@...hat.com>
To:     Nicolas Dichtel <nicolas.dichtel@...nd.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Xin Long <lucien.xin@...il.com>,
        Sabrina Dubroca <sd@...asysnail.net>, netdev@...r.kernel.org
Subject: Re: [PATCH net 1/2] geneve, vxlan: Don't check skb_dst() twice

On Mon, 15 Oct 2018 12:19:41 +0200
Nicolas Dichtel <nicolas.dichtel@...nd.com> wrote:

> Le 12/10/2018 à 23:53, Stefano Brivio a écrit :
> > Commit f15ca723c1eb ("net: don't call update_pmtu unconditionally") avoids
> > that we try updating PMTU for a non-existent destination, but didn't clean
> > up cases where the check was already explicit. Drop those redundant checks.  
> Yes, I leave them to avoid calculating the new mtu value when not needed. We are
> in the xmit path.

Before 2/2 of this series, though, we call skb_dst_update_pmtu() (and
in turn dst->ops->update_pmtu()) for *every* packet with a dst, which
I'd dare saying is by far the most common case. Besides, 2/2 needs
anyway to calculate the MTU to fix a bug.

So I think this is a vast improvement overall.

If we want to improve this further and avoid any indirect calls in the
most common path, we would need to cache the MTU in the dst -- it's
probably doable, but I would fix the specific issue addressed by 2/2
first.

-- 
Stefano