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

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Date:   Wed, 12 Jan 2022 16:43:32 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     David Laight <David.Laight@...LAB.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

On 1/12/22 02:41, David Laight wrote:
> 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.

Ah yes, might be better indeed

-- 
Pavel Begunkov