netdev - RE: [RFC PATCH 9/9] iov_iter: Add benchmarking kunit tests for UBUF/IOVEC

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Date: Fri, 15 Sep 2023 12:10:07 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'David Howells' <dhowells@...hat.com>
CC: Al Viro <viro@...iv.linux.org.uk>, Linus Torvalds
	<torvalds@...ux-foundation.org>, Jens Axboe <axboe@...nel.dk>, "Christoph
 Hellwig" <hch@....de>, Christian Brauner <christian@...uner.io>, "Matthew
 Wilcox" <willy@...radead.org>, Brendan Higgins <brendanhiggins@...gle.com>,
	David Gow <davidgow@...gle.com>, "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-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
	"kunit-dev@...glegroups.com" <kunit-dev@...glegroups.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Andrew Morton
	<akpm@...ux-foundation.org>, Christian Brauner <brauner@...nel.org>, "David
 Hildenbrand" <david@...hat.com>, John Hubbard <jhubbard@...dia.com>
Subject: RE: [RFC PATCH 9/9] iov_iter: Add benchmarking kunit tests for
 UBUF/IOVEC

From: David Howells
> Sent: 15 September 2023 12:23
> 
> David Laight <David.Laight@...LAB.COM> wrote:
> 
> > > > Some measurements can be made using readv() and writev()
> > > > on /dev/zero and /dev/null.
> > >
> > > Forget /dev/null; that doesn't actually engage any iteration code.  The same
> > > for writing to /dev/zero.  Reading from /dev/zero does its own iteration thing
> > > rather than using iterate_and_advance(), presumably because it checks for
> > > signals and resched.
> >
> > Using /dev/null does exercise the 'copy iov from user' code.
> 
> Ummm....  Not really:

I was thinking of import_iovec() - or whatever its current
name is.

That really needs a single structure that contains the iov_iter
and the cache[] (which the caller pretty much always allocates
in the same place).
Fiddling with that is ok until you find what io_uring does.
Then it all gets entirely horrid.

	David

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