From: Bart Van Assche <bvanassche@acm.org>
To: Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
Bart Van Assche <bvanassche@acm.org>,
Ming Lei <ming.lei@redhat.com>, Keith Busch <kbusch@kernel.org>,
Steven Rostedt <rostedt@goodmis.org>,
Guenter Roeck <linux@roeck-us.net>
Subject: [PATCH v2 3/3] block: Micro-optimize get_max_segment_size()
Date: Tue, 25 Oct 2022 12:17:55 -0700 [thread overview]
Message-ID: <20221025191755.1711437-4-bvanassche@acm.org> (raw)
In-Reply-To: <20221025191755.1711437-1-bvanassche@acm.org>
This patch removes a conditional jump from get_max_segment_size(). The
x86-64 assembler code for this function without this patch is as follows:
206 return min_not_zero(mask - offset + 1,
0x0000000000000118 <+72>: not %rax
0x000000000000011b <+75>: and 0x8(%r10),%rax
0x000000000000011f <+79>: add $0x1,%rax
0x0000000000000123 <+83>: je 0x138 <bvec_split_segs+104>
0x0000000000000125 <+85>: cmp %rdx,%rax
0x0000000000000128 <+88>: mov %rdx,%r12
0x000000000000012b <+91>: cmovbe %rax,%r12
0x000000000000012f <+95>: test %rdx,%rdx
0x0000000000000132 <+98>: mov %eax,%edx
0x0000000000000134 <+100>: cmovne %r12d,%edx
With this patch applied:
206 return min(mask - offset, (unsigned long)lim->max_segment_size - 1) + 1;
0x000000000000003f <+63>: mov 0x28(%rdi),%ebp
0x0000000000000042 <+66>: not %rax
0x0000000000000045 <+69>: and 0x8(%rdi),%rax
0x0000000000000049 <+73>: sub $0x1,%rbp
0x000000000000004d <+77>: cmp %rbp,%rax
0x0000000000000050 <+80>: cmova %rbp,%rax
0x0000000000000054 <+84>: add $0x1,%eax
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Keith Busch <kbusch@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
block/blk-merge.c | 15 +++++++++++----
1 file changed, 11 insertions(+), 4 deletions(-)
diff --git a/block/blk-merge.c b/block/blk-merge.c
index 58fdc3f8905b..35a8f75cc45d 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -186,6 +186,14 @@ static inline unsigned get_max_io_size(struct bio *bio,
return max_sectors & ~(lbs - 1);
}
+/**
+ * get_max_segment_size() - maximum number of bytes to add as a single segment
+ * @lim: Request queue limits.
+ * @start_page: See below.
+ * @offset: Offset from @start_page where to add a segment.
+ *
+ * Returns the maximum number of bytes that can be added as a single segment.
+ */
static inline unsigned get_max_segment_size(const struct queue_limits *lim,
struct page *start_page, unsigned long offset)
{
@@ -194,11 +202,10 @@ static inline unsigned get_max_segment_size(const struct queue_limits *lim,
offset = mask & (page_to_phys(start_page) + offset);
/*
- * overflow may be triggered in case of zero page physical address
- * on 32bit arch, use queue's max segment size when that happens.
+ * Prevent an overflow if mask = ULONG_MAX and offset = 0 by adding 1
+ * after having calculated the minimum.
*/
- return min_not_zero(mask - offset + 1,
- (unsigned long)lim->max_segment_size);
+ return min(mask - offset, (unsigned long)lim->max_segment_size - 1) + 1;
}
/**
next prev parent reply other threads:[~2022-10-25 19:18 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-10-25 19:17 [PATCH 0/3] Block layer cleanup patches Bart Van Assche
2022-10-25 19:17 ` [PATCH v2 1/3] block: Remove request.write_hint Bart Van Assche
2022-10-25 19:17 ` [PATCH v2 2/3] block: Constify most queue limits pointers Bart Van Assche
2022-10-25 19:17 ` Bart Van Assche [this message]
2022-10-25 19:40 ` [PATCH 0/3] Block layer cleanup patches Keith Busch
2022-10-25 19:42 ` Jens Axboe
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