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

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Date:   Wed, 12 Jan 2022 02:41:42 +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 20:48
> On 1/11/22 17:25, David Laight wrote:
> > 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.)
> 
> I was more thinking about non-sctp path without NETIF_F_HW_CSUM

In which case you need the body of __skb_csum_hw_offload()
and end up doing the 'sctp' check once inside it.
The 'sctp' check is only done twice for sctp.

	David

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