From: Christian Brauner <brauner@kernel.org>
To: David Howells <dhowells@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>, Al Viro <viro@zeniv.linux.org.uk>,
Christoph Hellwig <hch@infradead.org>,
Matthew Wilcox <willy@infradead.org>, Jan Kara <jack@suse.cz>,
Jeff Layton <jlayton@kernel.org>,
David Hildenbrand <david@redhat.com>,
Jason Gunthorpe <jgg@nvidia.com>,
Logan Gunthorpe <logang@deltatee.com>,
Hillf Danton <hdanton@sina.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
Christoph Hellwig <hch@lst.de>,
John Hubbard <jhubbard@nvidia.com>
Subject: Re: [PATCH v21 2/6] block: Fix bio_flagged() so that gcc can better optimise it
Date: Tue, 23 May 2023 14:37:35 +0200 [thread overview]
Message-ID: <20230523-kommst-gewechselt-e7b94e891a12@brauner> (raw)
In-Reply-To: <20230522205744.2825689-3-dhowells@redhat.com>
On Mon, May 22, 2023 at 09:57:40PM +0100, David Howells wrote:
> Fix bio_flagged() so that multiple instances of it, such as:
>
> if (bio_flagged(bio, BIO_PAGE_REFFED) ||
> bio_flagged(bio, BIO_PAGE_PINNED))
>
> can be combined by the gcc optimiser into a single test in assembly
> (arguably, this is a compiler optimisation issue[1]).
>
> The missed optimisation stems from bio_flagged() comparing the result of
> the bitwise-AND to zero. This results in an out-of-line bio_release_page()
> being compiled to something like:
>
> <+0>: mov 0x14(%rdi),%eax
> <+3>: test $0x1,%al
> <+5>: jne 0xffffffff816dac53 <bio_release_pages+11>
> <+7>: test $0x2,%al
> <+9>: je 0xffffffff816dac5c <bio_release_pages+20>
> <+11>: movzbl %sil,%esi
> <+15>: jmp 0xffffffff816daba1 <__bio_release_pages>
> <+20>: jmp 0xffffffff81d0b800 <__x86_return_thunk>
>
> However, the test is superfluous as the return type is bool. Removing it
> results in:
>
> <+0>: testb $0x3,0x14(%rdi)
> <+4>: je 0xffffffff816e4af4 <bio_release_pages+15>
> <+6>: movzbl %sil,%esi
> <+10>: jmp 0xffffffff816dab7c <__bio_release_pages>
> <+15>: jmp 0xffffffff81d0b7c0 <__x86_return_thunk>
>
> instead.
>
> Also, the MOVZBL instruction looks unnecessary[2] - I think it's just
> 're-booling' the mark_dirty parameter.
>
> Signed-off-by: David Howells <dhowells@redhat.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> Reviewed-by: John Hubbard <jhubbard@nvidia.com>
> cc: Jens Axboe <axboe@kernel.dk>
> cc: linux-block@vger.kernel.org
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108370 [1]
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108371 [2]
> Link: https://lore.kernel.org/r/167391056756.2311931.356007731815807265.stgit@warthog.procyon.org.uk/ # v6
> ---
Reviewed-by: Christian Brauner <brauner@kernel.org>
next prev parent reply other threads:[~2023-05-23 12:37 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-05-22 20:57 [PATCH v21 0/6] block: Use page pinning David Howells
2023-05-22 20:57 ` [PATCH v21 1/6] iomap: Don't get an reference on ZERO_PAGE for direct I/O block zeroing David Howells
2023-05-23 8:07 ` Jan Kara
2023-05-23 12:35 ` Christian Brauner
2023-05-22 20:57 ` [PATCH v21 2/6] block: Fix bio_flagged() so that gcc can better optimise it David Howells
2023-05-23 8:07 ` Jan Kara
2023-05-23 12:37 ` Christian Brauner [this message]
2023-05-22 20:57 ` [PATCH v21 3/6] block: Replace BIO_NO_PAGE_REF with BIO_PAGE_REFFED with inverted logic David Howells
2023-05-23 8:07 ` Jan Kara
2023-05-22 20:57 ` [PATCH v21 4/6] block: Add BIO_PAGE_PINNED and associated infrastructure David Howells
2023-05-23 8:08 ` Jan Kara
2023-05-22 20:57 ` [PATCH v21 5/6] block: Convert bio_iov_iter_get_pages to use iov_iter_extract_pages David Howells
2023-05-23 8:15 ` Jan Kara
2023-05-22 20:57 ` [PATCH v21 6/6] block: convert bio_map_user_iov " David Howells
2023-05-23 8:14 ` Jan Kara
2023-05-23 6:39 ` [PATCH v21 0/6] block: Use page pinning Christoph Hellwig
2023-05-23 20:16 ` Extending page pinning into fs/direct-io.c David Howells
2023-05-24 5:55 ` Christoph Hellwig
2023-05-24 7:06 ` David Hildenbrand
2023-05-24 8:47 ` David Howells
2023-05-25 9:51 ` Christoph Hellwig
2023-05-25 16:31 ` Linus Torvalds
2023-05-25 16:45 ` David Hildenbrand
2023-05-25 17:04 ` Linus Torvalds
2023-05-25 17:15 ` David Howells
2023-05-25 17:25 ` Linus Torvalds
2023-05-25 17:07 ` David Howells
2023-05-25 17:17 ` Linus Torvalds
2023-05-25 17:00 ` David Howells
2023-05-25 17:13 ` Linus Torvalds
2023-05-23 21:38 ` [PATCH v21 0/6] block: Use page pinning Jens Axboe
2023-05-24 5:52 ` Christoph Hellwig
2023-05-24 14:43 ` Jens Axboe
2023-05-24 7:35 ` David Howells
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