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From: David Laight <David.Laight@ACULAB.COM>
To: 'Yadu Kishore' <kyk.segfault@gmail.com>
Cc: David Miller <davem@davemloft.net>,
	Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
	Network Development <netdev@vger.kernel.org>
Subject: RE: [PATCH v2] net: Make skb_segment not to compute checksum if network controller supports checksumming
Date: Tue, 3 Mar 2020 09:56:27 +0000	[thread overview]
Message-ID: <de1012794ec54314b6fe790c01dee60b@AcuMS.aculab.com> (raw)
In-Reply-To: <CABGOaVQMq-AxwQOJ5DdDY6yLBOXqBg6G7qC_MdOYj_z4y-QQiw@mail.gmail.com>

From: Yadu Kishore
> Sent: 03 March 2020 09:15
...
> The perf data I presented was collected on an arm64 platform (hikey960) where
> the do_csum implementation that is called is not in assembly but in C
> (lib/checksum.c)

It is a long time since I've written any arm assembler, but an
asm checksum loop ought to be faster than a C one because using
'add with carry' ought to be a gain.
(Unlike mips style instruction sets without a carry flag.)

However what it more interesting is that do_csum() is being
called at all.
It implies that a large data block is being checksummed 'in situ'
whereas the expectation is that 'linearising' the skb requires
all the data be copied - so the checksum would be done during the
copy.

Additionally unless the copy loop is 'load + store' and
'load + store + adc' can be executed in the same number of
clocks (without excessive loop unrolling) then doing the
checksum in the copy loop isn't 'free'.

For x86 (including old intel cpu where adc is 2 clocks)
the 'checksum in copy' isn't free.

Clearly, if you have to do a copy and a software checksum
it is very likely that doing them together is faster.
(Although a fast 'rep movs' copy and an ad[co]x (or AVX2?)
checksum may be faster on very recent Intel cpu for large
enough buffers.)

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

  reply	other threads:[~2020-03-03  9:56 UTC|newest]

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-04  5:55 TCP checksum not offloaded during GSO Yadu Kishore
2020-02-05 21:43 ` Willem de Bruijn
2020-02-21  5:13   ` [PATCH] net: Make skb_segment not to compute checksum if network controller supports checksumming Yadu Kishore
2020-02-23  2:41     ` Willem de Bruijn
2020-02-28  5:24       ` Yadu Kishore
2020-02-28 14:30         ` Willem de Bruijn
2020-02-28 20:01           ` David Miller
2020-03-02  6:51             ` [PATCH v2] " Yadu Kishore
2020-03-02  8:42               ` Yadu Kishore
2020-03-02 15:19                 ` David Laight
2020-03-03  9:15                   ` Yadu Kishore
2020-03-03  9:56                     ` David Laight [this message]
2020-03-05  6:32                       ` Yadu Kishore
2020-03-05 16:06                         ` Willem de Bruijn
2020-03-05 17:00                           ` David Laight
2020-03-05 17:19                             ` Eric Dumazet
2020-03-06 17:12                               ` David Laight
2020-03-13  6:36                                 ` Yadu Kishore
2020-03-13 13:57                                   ` Willem de Bruijn
2020-03-13 14:04                                     ` David Laight
2020-03-13 18:05                                   ` David Miller
2020-03-17  8:38                                     ` [PATCH v3] " Yadu Kishore
2020-03-22  3:06                                       ` David Miller
2020-03-22 14:40                                       ` Willem de Bruijn
2020-03-23 20:00                                       ` David Miller
2020-03-22 19:53                                 ` [PATCH v2] " Tom Herbert
2020-03-23  9:15                                   ` David Laight
2020-02-06 12:13 ` TCP checksum not offloaded during GSO David Laight

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