From: David Laight <David.Laight@ACULAB.COM>
To: 'David Howells' <dhowells@redhat.com>,
Linus Torvalds <torvalds@linux-foundation.org>
Cc: Pavel Begunkov <asml.silence@gmail.com>,
Matthew Wilcox <willy@infradead.org>,
Jens Axboe <axboe@kernel.dk>,
Alexander Viro <viro@zeniv.linux.org.uk>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
linux-block <linux-block@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH 01/29] iov_iter: Switch to using a table of operations
Date: Sun, 22 Nov 2020 13:58:10 +0000 [thread overview]
Message-ID: <4890290b302e480fb0d1cc66bd0d6ce9@AcuMS.aculab.com> (raw)
In-Reply-To: <254318.1606051984@warthog.procyon.org.uk>
From: David Howells
> Sent: 22 November 2020 13:33
>
> Linus Torvalds <torvalds@linux-foundation.org> wrote:
>
> > - I worry a bit about the indirect call overhead and spectre v2.
>
> I don't know enough about how spectre v2 works to say if this would be a
> problem for the ops-table approach, but wouldn't it also affect the chain of
> conditional branches that we currently use, since it's branch-prediction
> based?
The advantage of the 'chain of branches' is that it can be converted
into a 'tree of branches' because the values are all separate bits.
So as well as putting the (expected) common one first; you can do:
if (likely((a & (A | B))) {
if (a & A) {
code for A;
} else {
code for B;
} else ...
So get better control over the branch sequence.
(Hopefully the compiler doesn't change the logic.
I want a dumb compiler that (mostly) compiles what I write!)
Part of the difficulty is deciding the common case.
There'll always be a benchmark that exercises an uncommon case.
Adding an indirect call does let you do things like adding
ITER_IOVER_SINGLE and ITER_KVEC_SINGLE that are used in the
common case of a single buffer fragment.
That might be a measurable gain.
It is also possible to optimise the common case to a direct
call (or even inline code) and use an indirect call for
everything else.
David
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
next prev parent reply other threads:[~2020-11-22 13:58 UTC|newest]
Thread overview: 63+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-21 14:13 [PATCH 00/29] RFC: iov_iter: Switch to using an ops table David Howells
2020-11-21 14:13 ` [PATCH 01/29] iov_iter: Switch to using a table of operations David Howells
2020-11-21 14:31 ` Pavel Begunkov
2020-11-23 23:21 ` Pavel Begunkov
2020-11-21 18:21 ` Linus Torvalds
2020-12-11 1:30 ` Al Viro
2020-11-22 13:33 ` David Howells
2020-11-22 13:58 ` David Laight [this message]
2020-11-22 19:22 ` Linus Torvalds
2020-11-22 22:34 ` David Laight
2020-11-22 22:46 ` David Laight
2020-11-23 8:05 ` Christoph Hellwig
2020-11-23 10:31 ` David Howells
2020-11-23 23:42 ` Pavel Begunkov
2020-11-24 12:50 ` David Howells
2020-11-24 15:30 ` Jens Axboe
2020-11-27 17:14 ` David Howells
2020-11-23 11:14 ` David Howells
2020-12-03 6:45 ` [iov_iter] 9bd0e337c6: will-it-scale.per_process_ops -4.8% regression kernel test robot
2020-12-03 6:45 ` kernel test robot
2020-12-03 17:47 ` Linus Torvalds
2020-12-03 17:47 ` Linus Torvalds
2020-12-03 17:50 ` Jens Axboe
2020-12-03 17:50 ` Jens Axboe
2020-12-04 11:50 ` David Howells
2020-12-04 11:50 ` David Howells
2020-12-04 11:51 ` David Howells
2020-12-04 11:51 ` David Howells
2020-12-07 13:10 ` Oliver Sang
2020-12-07 13:10 ` Oliver Sang
2020-12-07 13:20 ` David Howells
2020-12-07 13:20 ` David Howells
2020-11-21 14:13 ` [PATCH 02/29] iov_iter: Split copy_page_to_iter() David Howells
2020-11-21 14:13 ` [PATCH 03/29] iov_iter: Split iov_iter_fault_in_readable David Howells
2020-11-21 14:13 ` [PATCH 04/29] iov_iter: Split the iterate_and_advance() macro David Howells
2020-11-21 14:14 ` [PATCH 05/29] iov_iter: Split copy_to_iter() David Howells
2020-11-21 14:14 ` [PATCH 06/29] iov_iter: Split copy_mc_to_iter() David Howells
2020-11-21 14:14 ` [PATCH 07/29] iov_iter: Split copy_from_iter() David Howells
2020-11-21 14:14 ` [PATCH 08/29] iov_iter: Split the iterate_all_kinds() macro David Howells
2020-11-21 14:14 ` [PATCH 09/29] iov_iter: Split copy_from_iter_full() David Howells
2020-11-21 14:14 ` [PATCH 10/29] iov_iter: Split copy_from_iter_nocache() David Howells
2020-11-21 14:14 ` [PATCH 11/29] iov_iter: Split copy_from_iter_flushcache() David Howells
2020-11-21 14:14 ` [PATCH 12/29] iov_iter: Split copy_from_iter_full_nocache() David Howells
2020-11-21 14:15 ` [PATCH 13/29] iov_iter: Split copy_page_from_iter() David Howells
2020-11-21 14:15 ` [PATCH 14/29] iov_iter: Split iov_iter_zero() David Howells
2020-11-21 14:15 ` [PATCH 15/29] iov_iter: Split copy_from_user_atomic() David Howells
2020-11-21 14:15 ` [PATCH 16/29] iov_iter: Split iov_iter_advance() David Howells
2020-11-21 14:15 ` [PATCH 17/29] iov_iter: Split iov_iter_revert() David Howells
2020-11-21 14:15 ` [PATCH 18/29] iov_iter: Split iov_iter_single_seg_count() David Howells
2020-11-21 14:15 ` [PATCH 19/29] iov_iter: Split iov_iter_alignment() David Howells
2020-11-21 14:15 ` [PATCH 20/29] iov_iter: Split iov_iter_gap_alignment() David Howells
2020-11-21 14:16 ` [PATCH 21/29] iov_iter: Split iov_iter_get_pages() David Howells
2020-11-21 14:16 ` [PATCH 22/29] iov_iter: Split iov_iter_get_pages_alloc() David Howells
2020-11-21 14:16 ` [PATCH 23/29] iov_iter: Split csum_and_copy_from_iter() David Howells
2020-11-21 14:16 ` [PATCH 24/29] iov_iter: Split csum_and_copy_from_iter_full() David Howells
2020-11-21 14:16 ` [PATCH 25/29] iov_iter: Split csum_and_copy_to_iter() David Howells
2020-11-21 14:16 ` [PATCH 26/29] iov_iter: Split iov_iter_npages() David Howells
2020-11-21 14:16 ` [PATCH 27/29] iov_iter: Split dup_iter() David Howells
2020-11-21 14:17 ` [PATCH 28/29] iov_iter: Split iov_iter_for_each_range() David Howells
2020-11-21 14:17 ` [PATCH 29/29] iov_iter: Remove iterate_all_kinds() and iterate_and_advance() David Howells
2020-11-21 14:34 ` [PATCH 00/29] RFC: iov_iter: Switch to using an ops table Pavel Begunkov
2020-11-21 18:23 ` Linus Torvalds
2020-12-11 3:24 ` Matthew Wilcox
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