linux-kernel - RE: [PATCH v5 00/11] iov_iter: Convert the iterator macros into inline funcs

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Date:   Sat, 23 Sep 2023 10:31:38 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Willem de Bruijn' <willemdebruijn.kernel@...il.com>,
        David Howells <dhowells@...hat.com>
CC:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Christoph Hellwig <hch@....de>,
        Christian Brauner <christian@...uner.io>,
        Matthew Wilcox <willy@...radead.org>,
        Jeff Layton <jlayton@...nel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v5 00/11] iov_iter: Convert the iterator macros into
 inline funcs

From: Willem de Bruijn
> Sent: 23 September 2023 07:59
> 
> On Fri, Sep 22, 2023 at 2:01 PM David Howells <dhowells@...hat.com> wrote:
> >
> > David Laight <David.Laight@...LAB.COM> wrote:
> >
> > > >  (8) Move the copy-and-csum code to net/ where it can be in proximity with
> > > >      the code that uses it.  This eliminates the code if CONFIG_NET=n and
> > > >      allows for the slim possibility of it being inlined.
> > > >
> > > >  (9) Fold memcpy_and_csum() in to its two users.
> > > >
> > > > (10) Move csum_and_copy_from_iter_full() out of line and merge in
> > > >      csum_and_copy_from_iter() since the former is the only caller of the
> > > >      latter.
> > >
> > > I thought that the real idea behind these was to do the checksum
> > > at the same time as the copy to avoid loading the data into the L1
> > > data-cache twice - especially for long buffers.
> > > I wonder how often there are multiple iov[] that actually make
> > > it better than just check summing the linear buffer?
> >
> > It also reduces the overhead for finding the data to checksum in the case the
> > packet gets split since we're doing the checksumming as we copy - but with a
> > linear buffer, that's negligible.
> >
> > > I had a feeling that check summing of udp data was done during
> > > copy_to/from_user, but the code can't be the copy-and-csum here
> > > for that because it is missing support form odd-length buffers.
> >
> > Is there a bug there?

No, I misread the code - i shouldn't scan patches when I'd
got a viral head code...

...
> > You may be right.  That's more a question for the networking folks than for
> > me.  It's entirely possible that the checksumming code is just not used on
> > modern systems these days.
> >
> > Maybe Willem can comment since he's the UDP maintainer?
> 
> Perhaps these days it is more relevant to embedded systems than high
> end servers.

The checksum and copy are done together.
I probably missed it because the function isn't passed the
old checksum (which it can pretty much process for free).
Instead the caller is adding it afterwards - which involves
and extra explicit csum_add().

The x86-x84 ip checksum loops are all horrid though.
The unrolling in them is so 1990's.
With the out-of-order pipeline the memory accesses tend
to take care of themselves.
Not to mention that a whole raft of (now oldish) cpu take two
clocks to execute 'adc'.

	David

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