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

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Message-ID: <72e93605b28742c2a496ce4890ecaa80@AcuMS.aculab.com>
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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