linux-kernel - RE: [PATCH 13/14] net: inline part of skb_csum_hwoffload

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Date:   Tue, 11 Jan 2022 17:25:35 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Pavel Begunkov' <asml.silence@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "David S . Miller" <davem@...emloft.net>,
        "Jakub Kicinski" <kuba@...nel.org>
CC:     Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 13/14] net: inline part of skb_csum_hwoffload_help

From: Pavel Begunkov
> Sent: 11 January 2022 16:59
> 
> On 1/11/22 09:24, David Laight wrote:
> > From: Pavel Begunkov
> >> Sent: 11 January 2022 01:22
> >>
> >> Inline a HW csum'ed part of skb_csum_hwoffload_help().
> >>
> >> Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
> >> ---
> >>   include/linux/netdevice.h | 16 ++++++++++++++--
> >>   net/core/dev.c            | 13 +++----------
> >>   2 files changed, 17 insertions(+), 12 deletions(-)
> >>
> >> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> >> index 3213c7227b59..fbe6c764ce57 100644
> >> --- a/include/linux/netdevice.h
> >> +++ b/include/linux/netdevice.h
> >> @@ -4596,8 +4596,20 @@ void netdev_rss_key_fill(void *buffer, size_t len);
> >>
> >>   int skb_checksum_help(struct sk_buff *skb);
> >>   int skb_crc32c_csum_help(struct sk_buff *skb);
> >> -int skb_csum_hwoffload_help(struct sk_buff *skb,
> >> -			    const netdev_features_t features);
> >> +int __skb_csum_hwoffload_help(struct sk_buff *skb,
> >> +			      const netdev_features_t features);
> >> +
> >> +static inline int skb_csum_hwoffload_help(struct sk_buff *skb,
> >> +					  const netdev_features_t features)
> >> +{
> >> +	if (unlikely(skb_csum_is_sctp(skb)))
> >> +		return !!(features & NETIF_F_SCTP_CRC) ? 0 :
> >
> > If that !! doing anything? - doesn't look like it.
> 
> It doesn't, but left the original style

It just makes you think it is needed...

> >> +			skb_crc32c_csum_help(skb);
> >> +
> >> +	if (features & NETIF_F_HW_CSUM)
> >> +		return 0;
> >> +	return __skb_csum_hwoffload_help(skb, features);
> >> +}
> >
> > Maybe you should remove some bloat by moving the sctp code
> > into the called function.
> > This probably needs something like?
> >
> > {
> > 	if (features & NETIF_F_HW_CSUM && !skb_csum_is_sctp(skb))
> > 		return 0;
> > 	return __skb_csum_hw_offload(skb, features);
> > }
> 
> I don't like inlining that sctp chunk myself. It seems your way would
> need another skb_csum_is_sctp() in __skb_csum_hw_offload(), if so I
> don't think it's worth it. Would've been great to put the
> NETIF_F_HW_CSUM check first and hide sctp, but don't think it's
> correct. Would be great to hear some ideas.

Given the definition:

static inline bool skb_csum_is_sctp(struct sk_buff *skb)
{
	return skb->csum_not_inet;
}

I wouldn't worry about doing it twice.

Also skb_crc32_csum_help() is only called one.
Make it static (so inlined) and pass 'features' into it.

In reality sctp is such a slow crappy protocol that a few extra
function calls will make diddly-squit difference.
(And yes, we do actually use the sctp stack.)

	David

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