[PATCH 0/9] readahead stats/tracing, backwards prefetching and more (v4)

[PATCH 0/9] readahead stats/tracing, backwards prefetching and more (v4)

[Wu Fengguang]

Andrew,

Will you include it into the -mm tree?

This introduces the per-cpu readahead stats, tracing, backwards prefetching,
fixes context readahead for SSD random reads and does some other minor changes.

Changes since v3:
- default to CONFIG_READAHEAD_STATS=n
- drop "block: limit default readahead size for small devices"
  (and expect some distro udev rules to do the job)
- use percpu_counter for the readahead stats

Changes since v2:
- use per-cpu counters for readahead stats
- make context readahead more conservative
- simplify readahead tracing format and use __print_symbolic()
- backwards prefetching and snap to EOF fixes and cleanups

Changes since v1:
- use bit fields: pattern, for_mmap, for_metadata, lseek
- comment the various readahead patterns
- drop boot options "readahead=" and "readahead_stats="
- add for_metadata
- add snapping to EOF

 [PATCH 1/9] readahead: make context readahead more conservative
 [PATCH 2/9] readahead: record readahead patterns
 [PATCH 3/9] readahead: tag mmap page fault call sites
 [PATCH 4/9] readahead: tag metadata call sites
 [PATCH 5/9] readahead: add vfs/readahead tracing event
 [PATCH 6/9] readahead: add /debug/readahead/stats
 [PATCH 7/9] readahead: basic support for backwards prefetching
 [PATCH 8/9] readahead: dont do start-of-file readahead after lseek()
 [PATCH 9/9] readahead: snap readahead request to EOF

 fs/Makefile                |    1 
 fs/ext3/dir.c              |    1 
 fs/ext4/dir.c              |    1 
 fs/read_write.c            |    3 
 fs/trace.c                 |    2 
 include/linux/fs.h         |   41 ++++
 include/linux/mm.h         |    4 
 include/trace/events/vfs.h |   78 ++++++++
 mm/Kconfig                 |   15 +
 mm/filemap.c               |    9 -
 mm/readahead.c             |  310 +++++++++++++++++++++++++++++++++--
 11 files changed, 450 insertions(+), 15 deletions(-)

Thanks,
Fengguang

[PATCH 1/9] readahead: make context readahead more conservative

[Wu Fengguang]

[-- Attachment #1: readahead-context-tt --]
[-- Type: text/plain, Size: 1998 bytes --]

Try to prevent negatively impact moderately dense random reads on SSD.

Transaction-Per-Second numbers provided by Taobao:

		QPS	case
		-------------------------------------------------------
		7536	disable context readahead totally
w/ patch:	7129	slower size rampup and start RA on the 3rd read
		6717	slower size rampup
w/o patch:	5581	unmodified context readahead

Before, readahead will be started whenever reading page N+1 when it
happen to read N recently. After patch, we'll only start readahead
when *three* random reads happen to access pages N, N+1, N+2. The
probability of this happening is extremely low for pure random reads,
unless they are very dense, which actually deserves some readahead.

Also start with a smaller readahead window. The impact to interleaved
sequential reads should be small, because for a long run stream, the
the small readahead window rampup phase is negletable.

The context readahead actually benefits clustered random reads on HDD
whose seek cost is pretty high.  However as SSD is increasingly used for
random read workloads it's better for the context readahead to
concentrate on interleaved sequential reads.

Tested-by: Tao Ma <tm@tao.ma>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 mm/readahead.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

--- linux-next.orig/mm/readahead.c	2012-01-25 15:57:47.000000000 +0800
+++ linux-next/mm/readahead.c	2012-01-25 15:57:49.000000000 +0800
@@ -369,10 +369,10 @@ static int try_context_readahead(struct 
 	size = count_history_pages(mapping, ra, offset, max);
 
 	/*
-	 * no history pages:
+	 * not enough history pages:
 	 * it could be a random read
 	 */
-	if (!size)
+	if (size <= req_size)
 		return 0;
 
 	/*
@@ -383,8 +383,8 @@ static int try_context_readahead(struct 
 		size *= 2;
 
 	ra->start = offset;
-	ra->size = get_init_ra_size(size + req_size, max);
-	ra->async_size = ra->size;
+	ra->size = min(size + req_size, max);
+	ra->async_size = 1;
 
 	return 1;
 }

[PATCH 2/9] readahead: record readahead patterns

[Wu Fengguang]

[-- Attachment #1: readahead-tracepoints.patch --]
[-- Type: text/plain, Size: 6845 bytes --]

Record the readahead pattern in ra->pattern and extend ra_submit()
parameters, to be used by the next readahead tracing/stats patches.

7 patterns are defined:

      	pattern			readahead for
-----------------------------------------------------------
	RA_PATTERN_INITIAL	start-of-file read
	RA_PATTERN_SUBSEQUENT	trivial sequential read
	RA_PATTERN_CONTEXT	interleaved sequential read
	RA_PATTERN_OVERSIZE	oversize read
	RA_PATTERN_MMAP_AROUND	mmap fault
	RA_PATTERN_FADVISE	posix_fadvise()
	RA_PATTERN_RANDOM	random read

Note that random reads will be recorded in file_ra_state now.
This won't deteriorate cache bouncing because the ra->prev_pos update
in do_generic_file_read() already pollutes the data cache, and
filemap_fault() will stop calling into us after MMAP_LOTSAMISS.

CC: Ingo Molnar <mingo@elte.hu>
CC: Jens Axboe <axboe@kernel.dk>
CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Jan Kara <jack@suse.cz>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 include/linux/fs.h |   36 +++++++++++++++++++++++++++++++++++-
 include/linux/mm.h |    4 +++-
 mm/filemap.c       |    3 ++-
 mm/readahead.c     |   29 ++++++++++++++++++++++-------
 4 files changed, 62 insertions(+), 10 deletions(-)

--- linux-next.orig/include/linux/fs.h	2012-01-25 15:57:47.000000000 +0800
+++ linux-next/include/linux/fs.h	2012-01-25 15:57:50.000000000 +0800
@@ -952,11 +952,45 @@ struct file_ra_state {
 					   there are only # of pages ahead */
 
 	unsigned int ra_pages;		/* Maximum readahead window */
-	unsigned int mmap_miss;		/* Cache miss stat for mmap accesses */
+	u16 mmap_miss;			/* Cache miss stat for mmap accesses */
+	u8 pattern;			/* one of RA_PATTERN_* */
+
 	loff_t prev_pos;		/* Cache last read() position */
 };
 
 /*
+ * Which policy makes decision to do the current read-ahead IO?
+ *
+ * RA_PATTERN_INITIAL		readahead window is initially opened,
+ *				normally when reading from start of file
+ * RA_PATTERN_SUBSEQUENT	readahead window is pushed forward
+ * RA_PATTERN_CONTEXT		no readahead window available, querying the
+ *				page cache to decide readahead start/size.
+ *				This typically happens on interleaved reads (eg.
+ *				reading pages 0, 1000, 1, 1001, 2, 1002, ...)
+ *				where one file_ra_state struct is not enough
+ *				for recording 2+ interleaved sequential read
+ *				streams.
+ * RA_PATTERN_MMAP_AROUND	read-around on mmap page faults
+ *				(w/o any sequential/random hints)
+ * RA_PATTERN_FADVISE		triggered by POSIX_FADV_WILLNEED or FMODE_RANDOM
+ * RA_PATTERN_OVERSIZE		a random read larger than max readahead size,
+ *				do max readahead to break down the read size
+ * RA_PATTERN_RANDOM		a small random read
+ */
+enum readahead_pattern {
+	RA_PATTERN_INITIAL,
+	RA_PATTERN_SUBSEQUENT,
+	RA_PATTERN_CONTEXT,
+	RA_PATTERN_MMAP_AROUND,
+	RA_PATTERN_FADVISE,
+	RA_PATTERN_OVERSIZE,
+	RA_PATTERN_RANDOM,
+	RA_PATTERN_ALL,		/* for summary stats */
+	RA_PATTERN_MAX
+};
+
+/*
  * Check if @index falls in the readahead windows.
  */
 static inline int ra_has_index(struct file_ra_state *ra, pgoff_t index)
--- linux-next.orig/mm/readahead.c	2012-01-25 15:57:49.000000000 +0800
+++ linux-next/mm/readahead.c	2012-01-25 15:57:50.000000000 +0800
@@ -249,7 +249,10 @@ unsigned long max_sane_readahead(unsigne
  * Submit IO for the read-ahead request in file_ra_state.
  */
 unsigned long ra_submit(struct file_ra_state *ra,
-		       struct address_space *mapping, struct file *filp)
+			struct address_space *mapping,
+			struct file *filp,
+			pgoff_t offset,
+			unsigned long req_size)
 {
 	int actual;
 
@@ -382,6 +385,7 @@ static int try_context_readahead(struct 
 	if (size >= offset)
 		size *= 2;
 
+	ra->pattern = RA_PATTERN_CONTEXT;
 	ra->start = offset;
 	ra->size = min(size + req_size, max);
 	ra->async_size = 1;
@@ -403,8 +407,10 @@ ondemand_readahead(struct address_space 
 	/*
 	 * start of file
 	 */
-	if (!offset)
+	if (!offset) {
+		ra->pattern = RA_PATTERN_INITIAL;
 		goto initial_readahead;
+	}
 
 	/*
 	 * It's the expected callback offset, assume sequential access.
@@ -412,6 +418,7 @@ ondemand_readahead(struct address_space 
 	 */
 	if ((offset == (ra->start + ra->size - ra->async_size) ||
 	     offset == (ra->start + ra->size))) {
+		ra->pattern = RA_PATTERN_SUBSEQUENT;
 		ra->start += ra->size;
 		ra->size = get_next_ra_size(ra, max);
 		ra->async_size = ra->size;
@@ -434,6 +441,7 @@ ondemand_readahead(struct address_space 
 		if (!start || start - offset > max)
 			return 0;
 
+		ra->pattern = RA_PATTERN_CONTEXT;
 		ra->start = start;
 		ra->size = start - offset;	/* old async_size */
 		ra->size += req_size;
@@ -445,14 +453,18 @@ ondemand_readahead(struct address_space 
 	/*
 	 * oversize read
 	 */
-	if (req_size > max)
+	if (req_size > max) {
+		ra->pattern = RA_PATTERN_OVERSIZE;
 		goto initial_readahead;
+	}
 
 	/*
 	 * sequential cache miss
 	 */
-	if (offset - (ra->prev_pos >> PAGE_CACHE_SHIFT) <= 1UL)
+	if (offset - (ra->prev_pos >> PAGE_CACHE_SHIFT) <= 1UL) {
+		ra->pattern = RA_PATTERN_INITIAL;
 		goto initial_readahead;
+	}
 
 	/*
 	 * Query the page cache and look for the traces(cached history pages)
@@ -463,9 +475,12 @@ ondemand_readahead(struct address_space 
 
 	/*
 	 * standalone, small random read
-	 * Read as is, and do not pollute the readahead state.
 	 */
-	return __do_page_cache_readahead(mapping, filp, offset, req_size, 0);
+	ra->pattern = RA_PATTERN_RANDOM;
+	ra->start = offset;
+	ra->size = req_size;
+	ra->async_size = 0;
+	goto readit;
 
 initial_readahead:
 	ra->start = offset;
@@ -483,7 +498,7 @@ readit:
 		ra->size += ra->async_size;
 	}
 
-	return ra_submit(ra, mapping, filp);
+	return ra_submit(ra, mapping, filp, offset, req_size);
 }
 
 /**
--- linux-next.orig/include/linux/mm.h	2012-01-25 15:57:47.000000000 +0800
+++ linux-next/include/linux/mm.h	2012-01-25 15:57:50.000000000 +0800
@@ -1448,7 +1448,9 @@ void page_cache_async_readahead(struct a
 unsigned long max_sane_readahead(unsigned long nr);
 unsigned long ra_submit(struct file_ra_state *ra,
 			struct address_space *mapping,
-			struct file *filp);
+			struct file *filp,
+			pgoff_t offset,
+			unsigned long req_size);
 
 /* Generic expand stack which grows the stack according to GROWS{UP,DOWN} */
 extern int expand_stack(struct vm_area_struct *vma, unsigned long address);
--- linux-next.orig/mm/filemap.c	2012-01-25 15:57:47.000000000 +0800
+++ linux-next/mm/filemap.c	2012-01-25 15:57:50.000000000 +0800
@@ -1597,11 +1597,12 @@ static void do_sync_mmap_readahead(struc
 	/*
 	 * mmap read-around
 	 */
+	ra->pattern = RA_PATTERN_MMAP_AROUND;
 	ra_pages = max_sane_readahead(ra->ra_pages);
 	ra->start = max_t(long, 0, offset - ra_pages / 2);
 	ra->size = ra_pages;
 	ra->async_size = ra_pages / 4;
-	ra_submit(ra, mapping, file);
+	ra_submit(ra, mapping, file, offset, 1);
 }
 
 /*

[PATCH 3/9] readahead: tag mmap page fault call sites

[Wu Fengguang]

[-- Attachment #1: readahead-for-mmap --]
[-- Type: text/plain, Size: 2068 bytes --]

Introduce a bit field ra->for_mmap for tagging mmap reads.
The tag will be cleared immediate after submitting the IO.

Acked-by: Jan Kara <jack@suse.cz>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 include/linux/fs.h |    1 +
 mm/filemap.c       |    6 +++++-
 mm/readahead.c     |    1 +
 3 files changed, 7 insertions(+), 1 deletion(-)

--- linux-next.orig/include/linux/fs.h	2012-01-25 15:57:50.000000000 +0800
+++ linux-next/include/linux/fs.h	2012-01-25 15:57:51.000000000 +0800
@@ -954,6 +954,7 @@ struct file_ra_state {
 	unsigned int ra_pages;		/* Maximum readahead window */
 	u16 mmap_miss;			/* Cache miss stat for mmap accesses */
 	u8 pattern;			/* one of RA_PATTERN_* */
+	unsigned int for_mmap:1;	/* readahead for mmap accesses */
 
 	loff_t prev_pos;		/* Cache last read() position */
 };
--- linux-next.orig/mm/filemap.c	2012-01-25 15:57:50.000000000 +0800
+++ linux-next/mm/filemap.c	2012-01-25 15:57:51.000000000 +0800
@@ -1578,6 +1578,7 @@ static void do_sync_mmap_readahead(struc
 		return;
 
 	if (VM_SequentialReadHint(vma)) {
+		ra->for_mmap = 1;
 		page_cache_sync_readahead(mapping, ra, file, offset,
 					  ra->ra_pages);
 		return;
@@ -1597,6 +1598,7 @@ static void do_sync_mmap_readahead(struc
 	/*
 	 * mmap read-around
 	 */
+	ra->for_mmap = 1;
 	ra->pattern = RA_PATTERN_MMAP_AROUND;
 	ra_pages = max_sane_readahead(ra->ra_pages);
 	ra->start = max_t(long, 0, offset - ra_pages / 2);
@@ -1622,9 +1624,11 @@ static void do_async_mmap_readahead(stru
 		return;
 	if (ra->mmap_miss > 0)
 		ra->mmap_miss--;
-	if (PageReadahead(page))
+	if (PageReadahead(page)) {
+		ra->for_mmap = 1;
 		page_cache_async_readahead(mapping, ra, file,
 					   page, offset, ra->ra_pages);
+	}
 }
 
 /**
--- linux-next.orig/mm/readahead.c	2012-01-25 15:57:50.000000000 +0800
+++ linux-next/mm/readahead.c	2012-01-25 15:57:51.000000000 +0800
@@ -259,6 +259,7 @@ unsigned long ra_submit(struct file_ra_s
 	actual = __do_page_cache_readahead(mapping, filp,
 					ra->start, ra->size, ra->async_size);
 
+	ra->for_mmap = 0;
 	return actual;
 }
 

[PATCH 4/9] readahead: tag metadata call sites

[Wu Fengguang]

[-- Attachment #1: readahead-for-metadata --]
[-- Type: text/plain, Size: 1978 bytes --]

We may be doing more metadata readahead in future.

Acked-by: Jan Kara <jack@suse.cz>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 fs/ext3/dir.c      |    1 +
 fs/ext4/dir.c      |    1 +
 include/linux/fs.h |    1 +
 mm/readahead.c     |    1 +
 4 files changed, 4 insertions(+)

--- linux-next.orig/fs/ext3/dir.c	2012-01-25 15:57:46.000000000 +0800
+++ linux-next/fs/ext3/dir.c	2012-01-25 15:57:52.000000000 +0800
@@ -136,6 +136,7 @@ static int ext3_readdir(struct file * fi
 			pgoff_t index = map_bh.b_blocknr >>
 					(PAGE_CACHE_SHIFT - inode->i_blkbits);
 			if (!ra_has_index(&filp->f_ra, index))
+				filp->f_ra.for_metadata = 1;
 				page_cache_sync_readahead(
 					sb->s_bdev->bd_inode->i_mapping,
 					&filp->f_ra, filp,
--- linux-next.orig/fs/ext4/dir.c	2012-01-25 15:57:46.000000000 +0800
+++ linux-next/fs/ext4/dir.c	2012-01-25 15:57:52.000000000 +0800
@@ -153,6 +153,7 @@ static int ext4_readdir(struct file *fil
 			pgoff_t index = map.m_pblk >>
 					(PAGE_CACHE_SHIFT - inode->i_blkbits);
 			if (!ra_has_index(&filp->f_ra, index))
+				filp->f_ra.for_metadata = 1;
 				page_cache_sync_readahead(
 					sb->s_bdev->bd_inode->i_mapping,
 					&filp->f_ra, filp,
--- linux-next.orig/include/linux/fs.h	2012-01-25 15:57:51.000000000 +0800
+++ linux-next/include/linux/fs.h	2012-01-25 15:57:52.000000000 +0800
@@ -955,6 +955,7 @@ struct file_ra_state {
 	u16 mmap_miss;			/* Cache miss stat for mmap accesses */
 	u8 pattern;			/* one of RA_PATTERN_* */
 	unsigned int for_mmap:1;	/* readahead for mmap accesses */
+	unsigned int for_metadata:1;	/* readahead for meta data */
 
 	loff_t prev_pos;		/* Cache last read() position */
 };
--- linux-next.orig/mm/readahead.c	2012-01-25 15:57:51.000000000 +0800
+++ linux-next/mm/readahead.c	2012-01-25 15:57:52.000000000 +0800
@@ -260,6 +260,7 @@ unsigned long ra_submit(struct file_ra_s
 					ra->start, ra->size, ra->async_size);
 
 	ra->for_mmap = 0;
+	ra->for_metadata = 0;
 	return actual;
 }
 

[PATCH 5/9] readahead: add vfs/readahead tracing event

[Wu Fengguang]

[-- Attachment #1: readahead-tracer.patch --]
[-- Type: text/plain, Size: 5682 bytes --]

This is very useful for verifying whether the readahead algorithms are
working to the expectation.

Example output:

# echo 1 > /debug/tracing/events/vfs/readahead/enable
# cp test-file /dev/null
# cat /debug/tracing/trace  # trimmed output
pattern=initial bdi=0:16 ino=100177 req=0+2 ra=0+4-2 async=0 actual=4
pattern=subsequent bdi=0:16 ino=100177 req=2+2 ra=4+8-8 async=1 actual=8
pattern=subsequent bdi=0:16 ino=100177 req=4+2 ra=12+16-16 async=1 actual=16
pattern=subsequent bdi=0:16 ino=100177 req=12+2 ra=28+32-32 async=1 actual=32
pattern=subsequent bdi=0:16 ino=100177 req=28+2 ra=60+60-60 async=1 actual=24
pattern=subsequent bdi=0:16 ino=100177 req=60+2 ra=120+60-60 async=1 actual=0

CC: Ingo Molnar <mingo@elte.hu>
CC: Jens Axboe <axboe@kernel.dk>
CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Jan Kara <jack@suse.cz>
Acked-by: Rik van Riel <riel@redhat.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 fs/Makefile                |    1 
 fs/trace.c                 |    2 
 include/trace/events/vfs.h |   77 +++++++++++++++++++++++++++++++++++
 mm/readahead.c             |   24 ++++++++++
 4 files changed, 104 insertions(+)

--- /dev/null	1970-01-01 00:00:00.000000000 +0000
+++ linux-next/include/trace/events/vfs.h	2012-01-25 15:57:52.000000000 +0800
@@ -0,0 +1,77 @@
+#undef TRACE_SYSTEM
+#define TRACE_SYSTEM vfs
+
+#if !defined(_TRACE_VFS_H) || defined(TRACE_HEADER_MULTI_READ)
+#define _TRACE_VFS_H
+
+#include <linux/fs.h>
+#include <linux/blkdev.h>
+#include <linux/backing-dev.h>
+#include <linux/tracepoint.h>
+
+#define READAHEAD_PATTERNS						   \
+			{ RA_PATTERN_INITIAL,		"initial"	}, \
+			{ RA_PATTERN_SUBSEQUENT,	"subsequent"	}, \
+			{ RA_PATTERN_CONTEXT,		"context"	}, \
+			{ RA_PATTERN_MMAP_AROUND,	"around"	}, \
+			{ RA_PATTERN_FADVISE,		"fadvise"	}, \
+			{ RA_PATTERN_OVERSIZE,		"oversize"	}, \
+			{ RA_PATTERN_RANDOM,		"random"	}, \
+			{ RA_PATTERN_ALL,		"all"		}
+
+TRACE_EVENT(readahead,
+	TP_PROTO(struct address_space *mapping,
+		 pgoff_t offset,
+		 unsigned long req_size,
+		 enum readahead_pattern pattern,
+		 pgoff_t start,
+		 unsigned long size,
+		 unsigned long async_size,
+		 unsigned int actual),
+
+	TP_ARGS(mapping, offset, req_size, pattern, start, size, async_size,
+		actual),
+
+	TP_STRUCT__entry(
+		__array(char,		bdi, 32)
+		__field(ino_t,		ino)
+		__field(pgoff_t,	offset)
+		__field(unsigned long,	req_size)
+		__field(unsigned int,	pattern)
+		__field(pgoff_t,	start)
+		__field(unsigned int,	size)
+		__field(unsigned int,	async_size)
+		__field(unsigned int,	actual)
+	),
+
+	TP_fast_assign(
+		strncpy(__entry->bdi,
+			dev_name(mapping->backing_dev_info->dev), 32);
+		__entry->ino		= mapping->host->i_ino;
+		__entry->offset		= offset;
+		__entry->req_size	= req_size;
+		__entry->pattern	= pattern;
+		__entry->start		= start;
+		__entry->size		= size;
+		__entry->async_size	= async_size;
+		__entry->actual		= actual;
+	),
+
+	TP_printk("pattern=%s bdi=%s ino=%lu "
+		  "req=%lu+%lu ra=%lu+%d-%d async=%d actual=%d",
+			__print_symbolic(__entry->pattern, READAHEAD_PATTERNS),
+			__entry->bdi,
+			__entry->ino,
+			__entry->offset,
+			__entry->req_size,
+			__entry->start,
+			__entry->size,
+			__entry->async_size,
+			__entry->start > __entry->offset,
+			__entry->actual)
+);
+
+#endif /* _TRACE_VFS_H */
+
+/* This part must be outside protection */
+#include <trace/define_trace.h>
--- linux-next.orig/mm/readahead.c	2012-01-25 15:57:52.000000000 +0800
+++ linux-next/mm/readahead.c	2012-01-25 15:57:52.000000000 +0800
@@ -17,6 +17,7 @@
 #include <linux/task_io_accounting_ops.h>
 #include <linux/pagevec.h>
 #include <linux/pagemap.h>
+#include <trace/events/vfs.h>
 
 /*
  * Initialise a struct file's readahead state.  Assumes that the caller has
@@ -32,6 +33,21 @@ EXPORT_SYMBOL_GPL(file_ra_state_init);
 
 #define list_to_page(head) (list_entry((head)->prev, struct page, lru))
 
+static inline void readahead_event(struct address_space *mapping,
+				   pgoff_t offset,
+				   unsigned long req_size,
+				   bool for_mmap,
+				   bool for_metadata,
+				   enum readahead_pattern pattern,
+				   pgoff_t start,
+				   unsigned long size,
+				   unsigned long async_size,
+				   int actual)
+{
+	trace_readahead(mapping, offset, req_size,
+			pattern, start, size, async_size, actual);
+}
+
 /*
  * see if a page needs releasing upon read_cache_pages() failure
  * - the caller of read_cache_pages() may have set PG_private or PG_fscache
@@ -228,6 +244,9 @@ int force_page_cache_readahead(struct ad
 			ret = err;
 			break;
 		}
+		readahead_event(mapping, offset, nr_to_read, 0, 0,
+				RA_PATTERN_FADVISE, offset, this_chunk, 0,
+				err);
 		ret += err;
 		offset += this_chunk;
 		nr_to_read -= this_chunk;
@@ -259,6 +278,11 @@ unsigned long ra_submit(struct file_ra_s
 	actual = __do_page_cache_readahead(mapping, filp,
 					ra->start, ra->size, ra->async_size);
 
+	readahead_event(mapping, offset, req_size,
+			ra->for_mmap, ra->for_metadata,
+			ra->pattern, ra->start, ra->size, ra->async_size,
+			actual);
+
 	ra->for_mmap = 0;
 	ra->for_metadata = 0;
 	return actual;
--- /dev/null	1970-01-01 00:00:00.000000000 +0000
+++ linux-next/fs/trace.c	2012-01-25 15:57:52.000000000 +0800
@@ -0,0 +1,2 @@
+#define CREATE_TRACE_POINTS
+#include <trace/events/vfs.h>
--- linux-next.orig/fs/Makefile	2012-01-25 15:57:46.000000000 +0800
+++ linux-next/fs/Makefile	2012-01-25 15:57:52.000000000 +0800
@@ -50,6 +50,7 @@ obj-$(CONFIG_NFS_COMMON)	+= nfs_common/
 obj-$(CONFIG_GENERIC_ACL)	+= generic_acl.o
 
 obj-$(CONFIG_FHANDLE)		+= fhandle.o
+obj-$(CONFIG_TRACEPOINTS)	+= trace.o
 
 obj-y				+= quota/
 

[PATCH 6/9] readahead: add /debug/readahead/stats

[Wu Fengguang]

[-- Attachment #1: readahead-stats.patch --]
[-- Type: text/plain, Size: 8832 bytes --]

The accounting code will be compiled in by default (CONFIG_READAHEAD_STATS=y),
and will remain inactive by default.

It can be runtime enabled/disabled through the debugfs interface

	echo 1 > /debug/readahead/stats_enable
	echo 0 > /debug/readahead/stats_enable

Example output:
(taken from a fresh booted NFS-ROOT console box with rsize=524288)

$ cat /debug/readahead/stats
pattern     readahead    eof_hit  cache_hit         io    sync_io    mmap_io    meta_io       size async_size    io_size
initial           702        511          0        692        692          0          0          2          0          2
subsequent          7          0          1          7          1          1          0         23         22         23
context           160        161          0          2          0          1          0          0          0         16
around            184        184        177        184        184        184          0         58          0         53
backwards           2          0          2          2          2          0          0          4          0          3
fadvise          2593         47          8       2588       2588          0          0          1          0          1
oversize            0          0          0          0          0          0          0          0          0          0
random             45         20          0         44         44          0          0          1          0          1
all              3697        923        188       3519       3511        186          0          4          0          4

The two most important columns are
- io		number of readahead IO
- io_size	average readahead IO size

CC: Ingo Molnar <mingo@elte.hu>
CC: Jens Axboe <axboe@kernel.dk>
CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 mm/Kconfig     |   15 +++
 mm/readahead.c |  202 +++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 217 insertions(+)

--- linux-next.orig/mm/readahead.c	2012-01-25 15:57:52.000000000 +0800
+++ linux-next/mm/readahead.c	2012-01-25 15:57:53.000000000 +0800
@@ -33,6 +33,202 @@ EXPORT_SYMBOL_GPL(file_ra_state_init);
 
 #define list_to_page(head) (list_entry((head)->prev, struct page, lru))
 
+#ifdef CONFIG_READAHEAD_STATS
+#include <linux/ftrace_event.h>
+#include <linux/seq_file.h>
+#include <linux/debugfs.h>
+
+static u32 readahead_stats_enable __read_mostly;
+
+static const struct trace_print_flags ra_pattern_names[] = {
+	READAHEAD_PATTERNS
+};
+
+enum ra_account {
+	/* number of readaheads */
+	RA_ACCOUNT_COUNT,	/* readahead request */
+	RA_ACCOUNT_EOF,		/* readahead request covers EOF */
+	RA_ACCOUNT_CACHE_HIT,	/* readahead request covers some cached pages */
+	RA_ACCOUNT_IOCOUNT,	/* readahead IO */
+	RA_ACCOUNT_SYNC,	/* readahead IO that is synchronous */
+	RA_ACCOUNT_MMAP,	/* readahead IO by mmap page faults */
+	RA_ACCOUNT_METADATA,	/* readahead IO on metadata */
+	/* number of readahead pages */
+	RA_ACCOUNT_SIZE,	/* readahead size */
+	RA_ACCOUNT_ASYNC_SIZE,	/* readahead async size */
+	RA_ACCOUNT_ACTUAL,	/* readahead actual IO size */
+	/* end mark */
+	RA_ACCOUNT_MAX,
+};
+
+#define RA_STAT_BATCH	(INT_MAX / 2)
+static struct percpu_counter ra_stat[RA_PATTERN_ALL][RA_ACCOUNT_MAX];
+
+static inline void add_ra_stat(int i, int j, s64 amount)
+{
+	__percpu_counter_add(&ra_stat[i][j], amount, RA_STAT_BATCH);
+}
+
+static inline void inc_ra_stat(int i, int j)
+{
+	add_ra_stat(i, j, 1);
+}
+
+static void readahead_stats(struct address_space *mapping,
+			    pgoff_t offset,
+			    unsigned long req_size,
+			    bool for_mmap,
+			    bool for_metadata,
+			    enum readahead_pattern pattern,
+			    pgoff_t start,
+			    unsigned long size,
+			    unsigned long async_size,
+			    int actual)
+{
+	pgoff_t eof = ((i_size_read(mapping->host)-1) >> PAGE_CACHE_SHIFT) + 1;
+
+	inc_ra_stat(pattern, RA_ACCOUNT_COUNT);
+	add_ra_stat(pattern, RA_ACCOUNT_SIZE, size);
+	add_ra_stat(pattern, RA_ACCOUNT_ASYNC_SIZE, async_size);
+	add_ra_stat(pattern, RA_ACCOUNT_ACTUAL, actual);
+
+	if (start + size >= eof)
+		inc_ra_stat(pattern, RA_ACCOUNT_EOF);
+	if (actual < size)
+		inc_ra_stat(pattern, RA_ACCOUNT_CACHE_HIT);
+
+	if (actual) {
+		inc_ra_stat(pattern, RA_ACCOUNT_IOCOUNT);
+
+		if (start <= offset && offset < start + size)
+			inc_ra_stat(pattern, RA_ACCOUNT_SYNC);
+
+		if (for_mmap)
+			inc_ra_stat(pattern, RA_ACCOUNT_MMAP);
+		if (for_metadata)
+			inc_ra_stat(pattern, RA_ACCOUNT_METADATA);
+	}
+}
+
+static void readahead_stats_reset(void)
+{
+	int i, j;
+
+	for (i = 0; i < RA_PATTERN_ALL; i++)
+		for (j = 0; j < RA_ACCOUNT_MAX; j++)
+			percpu_counter_set(&ra_stat[i][j], 0);
+}
+
+static void
+readahead_stats_sum(long long ra_stats[RA_PATTERN_MAX][RA_ACCOUNT_MAX])
+{
+	int i, j;
+
+	for (i = 0; i < RA_PATTERN_ALL; i++)
+		for (j = 0; j < RA_ACCOUNT_MAX; j++) {
+			s64 n = percpu_counter_sum(&ra_stat[i][j]);
+			ra_stats[i][j] += n;
+			ra_stats[RA_PATTERN_ALL][j] += n;
+		}
+}
+
+static int readahead_stats_show(struct seq_file *s, void *_)
+{
+	long long ra_stats[RA_PATTERN_MAX][RA_ACCOUNT_MAX];
+	int i;
+
+	seq_printf(s,
+		   "%-10s %10s %10s %10s %10s %10s %10s %10s %10s %10s %10s\n",
+		   "pattern", "readahead", "eof_hit", "cache_hit",
+		   "io", "sync_io", "mmap_io", "meta_io",
+		   "size", "async_size", "io_size");
+
+	memset(ra_stats, 0, sizeof(ra_stats));
+	readahead_stats_sum(ra_stats);
+
+	for (i = 0; i < RA_PATTERN_MAX; i++) {
+		unsigned long count = ra_stats[i][RA_ACCOUNT_COUNT];
+		unsigned long iocount = ra_stats[i][RA_ACCOUNT_IOCOUNT];
+		/*
+		 * avoid division-by-zero
+		 */
+		if (count == 0)
+			count = 1;
+		if (iocount == 0)
+			iocount = 1;
+
+		seq_printf(s, "%-10s %10lld %10lld %10lld %10lld %10lld "
+			   "%10lld %10lld %10lld %10lld %10lld\n",
+				ra_pattern_names[i].name,
+				ra_stats[i][RA_ACCOUNT_COUNT],
+				ra_stats[i][RA_ACCOUNT_EOF],
+				ra_stats[i][RA_ACCOUNT_CACHE_HIT],
+				ra_stats[i][RA_ACCOUNT_IOCOUNT],
+				ra_stats[i][RA_ACCOUNT_SYNC],
+				ra_stats[i][RA_ACCOUNT_MMAP],
+				ra_stats[i][RA_ACCOUNT_METADATA],
+				ra_stats[i][RA_ACCOUNT_SIZE] / count,
+				ra_stats[i][RA_ACCOUNT_ASYNC_SIZE] / count,
+				ra_stats[i][RA_ACCOUNT_ACTUAL] / iocount);
+	}
+
+	return 0;
+}
+
+static int readahead_stats_open(struct inode *inode, struct file *file)
+{
+	return single_open(file, readahead_stats_show, NULL);
+}
+
+static ssize_t readahead_stats_write(struct file *file, const char __user *buf,
+				     size_t size, loff_t *offset)
+{
+	readahead_stats_reset();
+	return size;
+}
+
+static const struct file_operations readahead_stats_fops = {
+	.owner		= THIS_MODULE,
+	.open		= readahead_stats_open,
+	.write		= readahead_stats_write,
+	.read		= seq_read,
+	.llseek		= seq_lseek,
+	.release	= single_release,
+};
+
+static int __init readahead_create_debugfs(void)
+{
+	struct dentry *root;
+	struct dentry *entry;
+	int i, j;
+
+	root = debugfs_create_dir("readahead", NULL);
+	if (!root)
+		goto out;
+
+	entry = debugfs_create_file("stats", 0644, root,
+				    NULL, &readahead_stats_fops);
+	if (!entry)
+		goto out;
+
+	entry = debugfs_create_bool("stats_enable", 0644, root,
+				    &readahead_stats_enable);
+	if (!entry)
+		goto out;
+
+	for (i = 0; i < RA_PATTERN_ALL; i++)
+		for (j = 0; j < RA_ACCOUNT_MAX; j++)
+			percpu_counter_init(&ra_stat[i][j], 0);
+
+	return 0;
+out:
+	printk(KERN_ERR "readahead: failed to create debugfs entries\n");
+	return -ENOMEM;
+}
+
+late_initcall(readahead_create_debugfs);
+#endif
+
 static inline void readahead_event(struct address_space *mapping,
 				   pgoff_t offset,
 				   unsigned long req_size,
@@ -44,6 +240,12 @@ static inline void readahead_event(struc
 				   unsigned long async_size,
 				   int actual)
 {
+#ifdef CONFIG_READAHEAD_STATS
+	if (readahead_stats_enable)
+		readahead_stats(mapping, offset, req_size,
+				for_mmap, for_metadata,
+				pattern, start, size, async_size, actual);
+#endif
 	trace_readahead(mapping, offset, req_size,
 			pattern, start, size, async_size, actual);
 }
--- linux-next.orig/mm/Kconfig	2012-01-25 15:57:46.000000000 +0800
+++ linux-next/mm/Kconfig	2012-01-25 15:57:53.000000000 +0800
@@ -379,3 +379,18 @@ config CLEANCACHE
 	  in a negligible performance hit.
 
 	  If unsure, say Y to enable cleancache
+
+config READAHEAD_STATS
+	bool "Collect page cache readahead stats"
+	depends on DEBUG_FS
+	default n
+	help
+	  This provides the readahead events accounting facilities.
+
+	  To do readahead accounting for a workload:
+
+	  echo 1 > /sys/kernel/debug/readahead/stats_enable
+	  echo 0 > /sys/kernel/debug/readahead/stats  # reset counters
+	  # run the workload
+	  cat /sys/kernel/debug/readahead/stats       # check counters
+	  echo 0 > /sys/kernel/debug/readahead/stats_enable

[PATCH 7/9] readahead: basic support for backwards prefetching

[Wu Fengguang]

[-- Attachment #1: readahead-backwards.patch --]
[-- Type: text/plain, Size: 5588 bytes --]

Add the backwards prefetching feature. It's pretty simple if we don't
support async prefetching and interleaved reads.

tail and tac are observed to have the reverse read pattern:

tail-3501  [006]   111.881191: readahead: readahead-random(bdi=0:16, ino=1548450, req=750+1, ra=750+1-0, async=0) = 1
tail-3501  [006]   111.881506: readahead: readahead-backwards(bdi=0:16, ino=1548450, req=748+2, ra=746+5-0, async=0) = 4
tail-3501  [006]   111.882021: readahead: readahead-backwards(bdi=0:16, ino=1548450, req=744+2, ra=726+25-0, async=0) = 20
tail-3501  [006]   111.883713: readahead: readahead-backwards(bdi=0:16, ino=1548450, req=724+2, ra=626+125-0, async=0) = 100

 tac-3528  [001]   118.671924: readahead: readahead-random(bdi=0:16, ino=1548445, req=750+1, ra=750+1-0, async=0) = 1
 tac-3528  [001]   118.672371: readahead: readahead-backwards(bdi=0:16, ino=1548445, req=748+2, ra=746+5-0, async=0) = 4
 tac-3528  [001]   118.673039: readahead: readahead-backwards(bdi=0:16, ino=1548445, req=744+2, ra=726+25-0, async=0) = 20

Here is the behavior with an 8-page read sequence from 10000 down to 0.
(The readahead size is a bit large since it's an NFS mount.)

readahead-random(dev=0:16, ino=3948605, req=10000+8, ra=10000+8-0, async=0) = 8
readahead-backwards(dev=0:16, ino=3948605, req=9992+8, ra=9968+32-0, async=0) = 32
readahead-backwards(dev=0:16, ino=3948605, req=9960+8, ra=9840+128-0, async=0) = 128
readahead-backwards(dev=0:16, ino=3948605, req=9832+8, ra=9584+256-0, async=0) = 256
readahead-backwards(dev=0:16, ino=3948605, req=9576+8, ra=9072+512-0, async=0) = 512
readahead-backwards(dev=0:16, ino=3948605, req=9064+8, ra=8048+1024-0, async=0) = 1024
readahead-backwards(dev=0:16, ino=3948605, req=8040+8, ra=6128+1920-0, async=0) = 1920
readahead-backwards(dev=0:16, ino=3948605, req=6120+8, ra=4208+1920-0, async=0) = 1920
readahead-backwards(dev=0:16, ino=3948605, req=4200+8, ra=2288+1920-0, async=0) = 1920
readahead-backwards(dev=0:16, ino=3948605, req=2280+8, ra=368+1920-0, async=0) = 1920
readahead-backwards(dev=0:16, ino=3948605, req=360+8, ra=0+368-0, async=0) = 368

And a simple 1-page read sequence from 10000 down to 0.

readahead-random(dev=0:16, ino=3948605, req=10000+1, ra=10000+1-0, async=0) = 1
readahead-backwards(dev=0:16, ino=3948605, req=9999+1, ra=9996+4-0, async=0) = 4
readahead-backwards(dev=0:16, ino=3948605, req=9995+1, ra=9980+16-0, async=0) = 16
readahead-backwards(dev=0:16, ino=3948605, req=9979+1, ra=9916+64-0, async=0) = 64
readahead-backwards(dev=0:16, ino=3948605, req=9915+1, ra=9660+256-0, async=0) = 256
readahead-backwards(dev=0:16, ino=3948605, req=9659+1, ra=9148+512-0, async=0) = 512
readahead-backwards(dev=0:16, ino=3948605, req=9147+1, ra=8124+1024-0, async=0) = 1024
readahead-backwards(dev=0:16, ino=3948605, req=8123+1, ra=6204+1920-0, async=0) = 1920
readahead-backwards(dev=0:16, ino=3948605, req=6203+1, ra=4284+1920-0, async=0) = 1920
readahead-backwards(dev=0:16, ino=3948605, req=4283+1, ra=2364+1920-0, async=0) = 1920
readahead-backwards(dev=0:16, ino=3948605, req=2363+1, ra=444+1920-0, async=0) = 1920
readahead-backwards(dev=0:16, ino=3948605, req=443+1, ra=0+444-0, async=0) = 444

CC: Andi Kleen <andi@firstfloor.org>
CC: Li Shaohua <shaohua.li@intel.com>
Acked-by: Jan Kara <jack@suse.cz>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 include/linux/fs.h         |    2 ++
 include/trace/events/vfs.h |    1 +
 mm/readahead.c             |   20 ++++++++++++++++++++
 3 files changed, 23 insertions(+)

--- linux-next.orig/include/linux/fs.h	2012-01-25 15:57:52.000000000 +0800
+++ linux-next/include/linux/fs.h	2012-01-25 15:57:57.000000000 +0800
@@ -975,6 +975,7 @@ struct file_ra_state {
  *				streams.
  * RA_PATTERN_MMAP_AROUND	read-around on mmap page faults
  *				(w/o any sequential/random hints)
+ * RA_PATTERN_BACKWARDS		reverse reading detected
  * RA_PATTERN_FADVISE		triggered by POSIX_FADV_WILLNEED or FMODE_RANDOM
  * RA_PATTERN_OVERSIZE		a random read larger than max readahead size,
  *				do max readahead to break down the read size
@@ -985,6 +986,7 @@ enum readahead_pattern {
 	RA_PATTERN_SUBSEQUENT,
 	RA_PATTERN_CONTEXT,
 	RA_PATTERN_MMAP_AROUND,
+	RA_PATTERN_BACKWARDS,
 	RA_PATTERN_FADVISE,
 	RA_PATTERN_OVERSIZE,
 	RA_PATTERN_RANDOM,
--- linux-next.orig/mm/readahead.c	2012-01-25 15:57:53.000000000 +0800
+++ linux-next/mm/readahead.c	2012-01-25 15:57:57.000000000 +0800
@@ -695,6 +695,26 @@ ondemand_readahead(struct address_space 
 	}
 
 	/*
+	 * backwards reading
+	 */
+	if (offset < ra->start && offset + req_size >= ra->start) {
+		ra->pattern = RA_PATTERN_BACKWARDS;
+		ra->size = get_next_ra_size(ra, max);
+		if (ra->size > ra->start) {
+			/*
+			 * ra->start may be concurrently set to some huge
+			 * value, the min() at least avoids submitting huge IO
+			 * in this race condition
+			 */
+			ra->size = min(ra->start, max);
+			ra->start = 0;
+		} else
+			ra->start -= ra->size;
+		ra->async_size = 0;
+		goto readit;
+	}
+
+	/*
 	 * Query the page cache and look for the traces(cached history pages)
 	 * that a sequential stream would leave behind.
 	 */
--- linux-next.orig/include/trace/events/vfs.h	2012-01-25 15:57:52.000000000 +0800
+++ linux-next/include/trace/events/vfs.h	2012-01-25 15:57:57.000000000 +0800
@@ -14,6 +14,7 @@
 			{ RA_PATTERN_SUBSEQUENT,	"subsequent"	}, \
 			{ RA_PATTERN_CONTEXT,		"context"	}, \
 			{ RA_PATTERN_MMAP_AROUND,	"around"	}, \
+			{ RA_PATTERN_BACKWARDS,		"backwards"	}, \
 			{ RA_PATTERN_FADVISE,		"fadvise"	}, \
 			{ RA_PATTERN_OVERSIZE,		"oversize"	}, \
 			{ RA_PATTERN_RANDOM,		"random"	}, \

[PATCH 8/9] readahead: dont do start-of-file readahead after lseek()

[Wu Fengguang]

[-- Attachment #1: readahead-lseek.patch --]
[-- Type: text/plain, Size: 2250 bytes --]

Some applications (eg. blkid, id3tool etc.) seek around the file
to get information. For example, blkid does

	     seek to	0
	     read	1024
	     seek to	1536
	     read	16384

The start-of-file readahead heuristic is wrong for them, whose
access pattern can be identified by lseek() calls.

So test-and-set a READAHEAD_LSEEK flag on lseek() and don't
do start-of-file readahead on seeing it. Proposed by Linus.

Acked-by: Rik van Riel <riel@redhat.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 fs/read_write.c    |    3 +++
 include/linux/fs.h |    1 +
 mm/readahead.c     |    4 ++++
 3 files changed, 8 insertions(+)

--- linux-next.orig/mm/readahead.c	2012-01-25 15:57:57.000000000 +0800
+++ linux-next/mm/readahead.c	2012-01-25 15:57:58.000000000 +0800
@@ -485,6 +485,7 @@ unsigned long ra_submit(struct file_ra_s
 			ra->pattern, ra->start, ra->size, ra->async_size,
 			actual);
 
+	ra->lseek = 0;
 	ra->for_mmap = 0;
 	ra->for_metadata = 0;
 	return actual;
@@ -636,6 +637,8 @@ ondemand_readahead(struct address_space 
 	 * start of file
 	 */
 	if (!offset) {
+		if (ra->lseek && req_size < max)
+			goto random_read;
 		ra->pattern = RA_PATTERN_INITIAL;
 		goto initial_readahead;
 	}
@@ -721,6 +724,7 @@ ondemand_readahead(struct address_space 
 	if (try_context_readahead(mapping, ra, offset, req_size, max))
 		goto readit;
 
+random_read:
 	/*
 	 * standalone, small random read
 	 */
--- linux-next.orig/fs/read_write.c	2012-01-25 15:57:46.000000000 +0800
+++ linux-next/fs/read_write.c	2012-01-25 15:57:58.000000000 +0800
@@ -47,6 +47,9 @@ static loff_t lseek_execute(struct file 
 		file->f_pos = offset;
 		file->f_version = 0;
 	}
+
+	file->f_ra.lseek = 1;
+
 	return offset;
 }
 
--- linux-next.orig/include/linux/fs.h	2012-01-25 15:57:57.000000000 +0800
+++ linux-next/include/linux/fs.h	2012-01-25 15:57:58.000000000 +0800
@@ -956,6 +956,7 @@ struct file_ra_state {
 	u8 pattern;			/* one of RA_PATTERN_* */
 	unsigned int for_mmap:1;	/* readahead for mmap accesses */
 	unsigned int for_metadata:1;	/* readahead for meta data */
+	unsigned int lseek:1;		/* this read has a leading lseek */
 
 	loff_t prev_pos;		/* Cache last read() position */
 };

[PATCH 9/9] readahead: snap readahead request to EOF

[Wu Fengguang]

[-- Attachment #1: readahead-eof --]
[-- Type: text/plain, Size: 1623 bytes --]

If the file size is 20kb and readahead request is [0, 16kb),
it's better to expand the readahead request to [0, 20kb), which will
likely save one followup I/O for the ending [16kb, 20kb).

If the readahead request already covers EOF, trimm it down to EOF.
Also don't set the PG_readahead mark to avoid an unnecessary future
invocation of the readahead code.

This special handling looks worthwhile because small to medium sized
files are pretty common.

Acked-by: Jan Kara <jack@suse.cz>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 mm/readahead.c |   21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

--- linux-next.orig/mm/readahead.c	2012-01-25 15:57:58.000000000 +0800
+++ linux-next/mm/readahead.c	2012-01-25 15:57:59.000000000 +0800
@@ -466,6 +466,25 @@ unsigned long max_sane_readahead(unsigne
 		+ node_page_state(numa_node_id(), NR_FREE_PAGES)) / 2);
 }
 
+static void snap_to_eof(struct file_ra_state *ra, struct address_space *mapping)
+{
+	pgoff_t eof = ((i_size_read(mapping->host)-1) >> PAGE_CACHE_SHIFT) + 1;
+	pgoff_t start = ra->start;
+	unsigned int size = ra->size;
+
+	/*
+	 * skip backwards and random reads
+	 */
+	if (ra->pattern > RA_PATTERN_MMAP_AROUND)
+		return;
+
+	size += min(size / 2, ra->ra_pages / 4);
+	if (start + size > eof) {
+		ra->size = eof - start;
+		ra->async_size = 0;
+	}
+}
+
 /*
  * Submit IO for the read-ahead request in file_ra_state.
  */
@@ -477,6 +496,8 @@ unsigned long ra_submit(struct file_ra_s
 {
 	int actual;
 
+	snap_to_eof(ra, mapping);
+
 	actual = __do_page_cache_readahead(mapping, filp,
 					ra->start, ra->size, ra->async_size);
 

Re: [PATCH 6/9] readahead: add /debug/readahead/stats

[Christoph Lameter]

On Fri, 27 Jan 2012, Wu Fengguang wrote:

> +
> +#define RA_STAT_BATCH	(INT_MAX / 2)
> +static struct percpu_counter ra_stat[RA_PATTERN_ALL][RA_ACCOUNT_MAX];

Why use percpu counter here? The stats structures are not dynamically
allocated so you can just use a DECLARE_PER_CPU statement. That way you do
not have the overhead of percpu counter calls. Instead simple instructions
are generated to deal with the counter.

There are also no calls to any of the fast access functions for percpu
counter so percpu_counter has to always having to loop over all
counters anyways to get the results. The batching of the percpu_counters
is therefore not used.

Its simpler to just do a loop that sums over all counters when displaying
the results.

> +static inline void add_ra_stat(int i, int j, s64 amount)
> +{
> +	__percpu_counter_add(&ra_stat[i][j], amount, RA_STAT_BATCH);

	__this_cpu_add(ra_stat[i][j], amount);

> +}

> +
> +static void readahead_stats_reset(void)
> +{
> +	int i, j;
> +
> +	for (i = 0; i < RA_PATTERN_ALL; i++)
> +		for (j = 0; j < RA_ACCOUNT_MAX; j++)
> +			percpu_counter_set(&ra_stat[i][j], 0);

for_each_online(cpu)
	memset(per_cpu_ptr(&ra_stat, cpu), 0, sizeof(ra_stat));

> +}
> +
> +static void
> +readahead_stats_sum(long long ra_stats[RA_PATTERN_MAX][RA_ACCOUNT_MAX])
> +{
> +	int i, j;
> +
> +	for (i = 0; i < RA_PATTERN_ALL; i++)
> +		for (j = 0; j < RA_ACCOUNT_MAX; j++) {
> +			s64 n = percpu_counter_sum(&ra_stat[i][j]);
> +			ra_stats[i][j] += n;
> +			ra_stats[RA_PATTERN_ALL][j] += n;
> +		}
> +}

Define a function stats instead?

static long get_stat_sum(long __per_cpu *x)
{
	int cpu;
	long sum;

	for_each_online(cpu)
		sum += *per_cpu_ptr(x, cpu);

	return sum;
}

> +
> +static int readahead_stats_show(struct seq_file *s, void *_)
> +{

> +	readahead_stats_sum(ra_stats);
> +
> +	for (i = 0; i < RA_PATTERN_MAX; i++) {
> +		unsigned long count = ra_stats[i][RA_ACCOUNT_COUNT];

			= get_stats(&ra_stats[i][RA_ACCOUNT]);

...

?

Re: [PATCH 6/9] readahead: add /debug/readahead/stats

[Andrew Morton]

On Fri, 27 Jan 2012 10:21:36 -0600 (CST)
Christoph Lameter <cl@linux.com> wrote:

> > +
> > +static void readahead_stats_reset(void)
> > +{
> > +	int i, j;
> > +
> > +	for (i = 0; i < RA_PATTERN_ALL; i++)
> > +		for (j = 0; j < RA_ACCOUNT_MAX; j++)
> > +			percpu_counter_set(&ra_stat[i][j], 0);
> 
> for_each_online(cpu)
> 	memset(per_cpu_ptr(&ra_stat, cpu), 0, sizeof(ra_stat));

for_each_possible_cpu().  And that's one reason to not open-code the
operation.  Another is so we don't have tiresome open-coded loops all
over the place.

But before doing either of those things we should choose boring old
atomic_inc().  Has it been shown that the cost of doing so is
unacceptable?  Bearing this in mind:

> The accounting code will be compiled in by default
> (CONFIG_READAHEAD_STATS=y), and will remain inactive by default.

I agree with those choices.  They effectively mean that the stats will
be a developer-only/debugger-only thing.  So even if the atomic_inc()
costs are measurable during these develop/debug sessions, is anyone
likely to care?

Re: [PATCH 6/9] readahead: add /debug/readahead/stats

[Wu Fengguang]

On Fri, Jan 27, 2012 at 12:15:51PM -0800, Andrew Morton wrote:

> > The accounting code will be compiled in by default
> > (CONFIG_READAHEAD_STATS=y), and will remain inactive by default.
> 
> I agree with those choices.  They effectively mean that the stats will
> be a developer-only/debugger-only thing.  So even if the atomic_inc()
> costs are measurable during these develop/debug sessions, is anyone
> likely to care?

Sorry I have changed the default to CONFIG_READAHEAD_STATS=n to avoid
bloating the kernel (and forgot to edit the changelog accordingly).

I'm not sure how many people are going to check the readahead stats.

Thanks,
Fengguang

Re: [PATCH 6/9] readahead: add /debug/readahead/stats

[Dave Chinner]

On Fri, Jan 27, 2012 at 12:15:51PM -0800, Andrew Morton wrote:
> On Fri, 27 Jan 2012 10:21:36 -0600 (CST)
> Christoph Lameter <cl@linux.com> wrote:
> 
> > > +
> > > +static void readahead_stats_reset(void)
> > > +{
> > > +	int i, j;
> > > +
> > > +	for (i = 0; i < RA_PATTERN_ALL; i++)
> > > +		for (j = 0; j < RA_ACCOUNT_MAX; j++)
> > > +			percpu_counter_set(&ra_stat[i][j], 0);
> > 
> > for_each_online(cpu)
> > 	memset(per_cpu_ptr(&ra_stat, cpu), 0, sizeof(ra_stat));
> 
> for_each_possible_cpu().  And that's one reason to not open-code the
> operation.  Another is so we don't have tiresome open-coded loops all
> over the place.

Amen, brother!

> But before doing either of those things we should choose boring old
> atomic_inc().  Has it been shown that the cost of doing so is
> unacceptable?  Bearing this in mind:

atomics for stats in the IO path have long been known not to scale
well enough - especially now we have PCIe SSDs that can do hundreds
of thousands of reads per second if you have enough CPU concurrency
to drive them that hard. Under that sort of workload, atomics won't
scale.

> 
> > The accounting code will be compiled in by default
> > (CONFIG_READAHEAD_STATS=y), and will remain inactive by default.
> 
> I agree with those choices.  They effectively mean that the stats will
> be a developer-only/debugger-only thing.  So even if the atomic_inc()
> costs are measurable during these develop/debug sessions, is anyone
> likely to care?

I do.  If I need the debugging stats, the overhead must not perturb
the behaviour I'm trying to understand/debug for them to be
useful....

Cheers,

Dave.

-- 
Dave Chinner
david@fromorbit.com

[PATCH 6/9 update changelog] readahead: add /debug/readahead/stats

[Wu Fengguang]

[-- Attachment #1: readahead-stats.patch --]
[-- Type: text/plain, Size: 8864 bytes --]

This accounting code is effectively a no-op by default (CONFIG_READAHEAD_STATS=n).

It's expected to be runtime reset and enabled before using:

	echo 0 > /debug/readahead/stats		# reset counters
	echo 1 > /debug/readahead/stats_enable
	# run test workload
	echo 0 > /debug/readahead/stats_enable

Example output:
(taken from a fresh booted NFS-ROOT console box with rsize=524288)

$ cat /debug/readahead/stats
pattern     readahead    eof_hit  cache_hit         io    sync_io    mmap_io    meta_io       size async_size    io_size
initial           702        511          0        692        692          0          0          2          0          2
subsequent          7          0          1          7          1          1          0         23         22         23
context           160        161          0          2          0          1          0          0          0         16
around            184        184        177        184        184        184          0         58          0         53
backwards           2          0          2          2          2          0          0          4          0          3
fadvise          2593         47          8       2588       2588          0          0          1          0          1
oversize            0          0          0          0          0          0          0          0          0          0
random             45         20          0         44         44          0          0          1          0          1
all              3697        923        188       3519       3511        186          0          4          0          4

The two most important columns are
- io		number of readahead IO
- io_size	average readahead IO size

CC: Ingo Molnar <mingo@elte.hu>
CC: Jens Axboe <axboe@kernel.dk>
CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 mm/Kconfig     |   15 +++
 mm/readahead.c |  202 +++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 217 insertions(+)

--- linux-next.orig/mm/readahead.c	2012-01-25 15:57:52.000000000 +0800
+++ linux-next/mm/readahead.c	2012-01-25 15:57:53.000000000 +0800
@@ -33,6 +33,202 @@ EXPORT_SYMBOL_GPL(file_ra_state_init);
 
 #define list_to_page(head) (list_entry((head)->prev, struct page, lru))
 
+#ifdef CONFIG_READAHEAD_STATS
+#include <linux/ftrace_event.h>
+#include <linux/seq_file.h>
+#include <linux/debugfs.h>
+
+static u32 readahead_stats_enable __read_mostly;
+
+static const struct trace_print_flags ra_pattern_names[] = {
+	READAHEAD_PATTERNS
+};
+
+enum ra_account {
+	/* number of readaheads */
+	RA_ACCOUNT_COUNT,	/* readahead request */
+	RA_ACCOUNT_EOF,		/* readahead request covers EOF */
+	RA_ACCOUNT_CACHE_HIT,	/* readahead request covers some cached pages */
+	RA_ACCOUNT_IOCOUNT,	/* readahead IO */
+	RA_ACCOUNT_SYNC,	/* readahead IO that is synchronous */
+	RA_ACCOUNT_MMAP,	/* readahead IO by mmap page faults */
+	RA_ACCOUNT_METADATA,	/* readahead IO on metadata */
+	/* number of readahead pages */
+	RA_ACCOUNT_SIZE,	/* readahead size */
+	RA_ACCOUNT_ASYNC_SIZE,	/* readahead async size */
+	RA_ACCOUNT_ACTUAL,	/* readahead actual IO size */
+	/* end mark */
+	RA_ACCOUNT_MAX,
+};
+
+#define RA_STAT_BATCH	(INT_MAX / 2)
+static struct percpu_counter ra_stat[RA_PATTERN_ALL][RA_ACCOUNT_MAX];
+
+static inline void add_ra_stat(int i, int j, s64 amount)
+{
+	__percpu_counter_add(&ra_stat[i][j], amount, RA_STAT_BATCH);
+}
+
+static inline void inc_ra_stat(int i, int j)
+{
+	add_ra_stat(i, j, 1);
+}
+
+static void readahead_stats(struct address_space *mapping,
+			    pgoff_t offset,
+			    unsigned long req_size,
+			    bool for_mmap,
+			    bool for_metadata,
+			    enum readahead_pattern pattern,
+			    pgoff_t start,
+			    unsigned long size,
+			    unsigned long async_size,
+			    int actual)
+{
+	pgoff_t eof = ((i_size_read(mapping->host)-1) >> PAGE_CACHE_SHIFT) + 1;
+
+	inc_ra_stat(pattern, RA_ACCOUNT_COUNT);
+	add_ra_stat(pattern, RA_ACCOUNT_SIZE, size);
+	add_ra_stat(pattern, RA_ACCOUNT_ASYNC_SIZE, async_size);
+	add_ra_stat(pattern, RA_ACCOUNT_ACTUAL, actual);
+
+	if (start + size >= eof)
+		inc_ra_stat(pattern, RA_ACCOUNT_EOF);
+	if (actual < size)
+		inc_ra_stat(pattern, RA_ACCOUNT_CACHE_HIT);
+
+	if (actual) {
+		inc_ra_stat(pattern, RA_ACCOUNT_IOCOUNT);
+
+		if (start <= offset && offset < start + size)
+			inc_ra_stat(pattern, RA_ACCOUNT_SYNC);
+
+		if (for_mmap)
+			inc_ra_stat(pattern, RA_ACCOUNT_MMAP);
+		if (for_metadata)
+			inc_ra_stat(pattern, RA_ACCOUNT_METADATA);
+	}
+}
+
+static void readahead_stats_reset(void)
+{
+	int i, j;
+
+	for (i = 0; i < RA_PATTERN_ALL; i++)
+		for (j = 0; j < RA_ACCOUNT_MAX; j++)
+			percpu_counter_set(&ra_stat[i][j], 0);
+}
+
+static void
+readahead_stats_sum(long long ra_stats[RA_PATTERN_MAX][RA_ACCOUNT_MAX])
+{
+	int i, j;
+
+	for (i = 0; i < RA_PATTERN_ALL; i++)
+		for (j = 0; j < RA_ACCOUNT_MAX; j++) {
+			s64 n = percpu_counter_sum(&ra_stat[i][j]);
+			ra_stats[i][j] += n;
+			ra_stats[RA_PATTERN_ALL][j] += n;
+		}
+}
+
+static int readahead_stats_show(struct seq_file *s, void *_)
+{
+	long long ra_stats[RA_PATTERN_MAX][RA_ACCOUNT_MAX];
+	int i;
+
+	seq_printf(s,
+		   "%-10s %10s %10s %10s %10s %10s %10s %10s %10s %10s %10s\n",
+		   "pattern", "readahead", "eof_hit", "cache_hit",
+		   "io", "sync_io", "mmap_io", "meta_io",
+		   "size", "async_size", "io_size");
+
+	memset(ra_stats, 0, sizeof(ra_stats));
+	readahead_stats_sum(ra_stats);
+
+	for (i = 0; i < RA_PATTERN_MAX; i++) {
+		unsigned long count = ra_stats[i][RA_ACCOUNT_COUNT];
+		unsigned long iocount = ra_stats[i][RA_ACCOUNT_IOCOUNT];
+		/*
+		 * avoid division-by-zero
+		 */
+		if (count == 0)
+			count = 1;
+		if (iocount == 0)
+			iocount = 1;
+
+		seq_printf(s, "%-10s %10lld %10lld %10lld %10lld %10lld "
+			   "%10lld %10lld %10lld %10lld %10lld\n",
+				ra_pattern_names[i].name,
+				ra_stats[i][RA_ACCOUNT_COUNT],
+				ra_stats[i][RA_ACCOUNT_EOF],
+				ra_stats[i][RA_ACCOUNT_CACHE_HIT],
+				ra_stats[i][RA_ACCOUNT_IOCOUNT],
+				ra_stats[i][RA_ACCOUNT_SYNC],
+				ra_stats[i][RA_ACCOUNT_MMAP],
+				ra_stats[i][RA_ACCOUNT_METADATA],
+				ra_stats[i][RA_ACCOUNT_SIZE] / count,
+				ra_stats[i][RA_ACCOUNT_ASYNC_SIZE] / count,
+				ra_stats[i][RA_ACCOUNT_ACTUAL] / iocount);
+	}
+
+	return 0;
+}
+
+static int readahead_stats_open(struct inode *inode, struct file *file)
+{
+	return single_open(file, readahead_stats_show, NULL);
+}
+
+static ssize_t readahead_stats_write(struct file *file, const char __user *buf,
+				     size_t size, loff_t *offset)
+{
+	readahead_stats_reset();
+	return size;
+}
+
+static const struct file_operations readahead_stats_fops = {
+	.owner		= THIS_MODULE,
+	.open		= readahead_stats_open,
+	.write		= readahead_stats_write,
+	.read		= seq_read,
+	.llseek		= seq_lseek,
+	.release	= single_release,
+};
+
+static int __init readahead_create_debugfs(void)
+{
+	struct dentry *root;
+	struct dentry *entry;
+	int i, j;
+
+	root = debugfs_create_dir("readahead", NULL);
+	if (!root)
+		goto out;
+
+	entry = debugfs_create_file("stats", 0644, root,
+				    NULL, &readahead_stats_fops);
+	if (!entry)
+		goto out;
+
+	entry = debugfs_create_bool("stats_enable", 0644, root,
+				    &readahead_stats_enable);
+	if (!entry)
+		goto out;
+
+	for (i = 0; i < RA_PATTERN_ALL; i++)
+		for (j = 0; j < RA_ACCOUNT_MAX; j++)
+			percpu_counter_init(&ra_stat[i][j], 0);
+
+	return 0;
+out:
+	printk(KERN_ERR "readahead: failed to create debugfs entries\n");
+	return -ENOMEM;
+}
+
+late_initcall(readahead_create_debugfs);
+#endif
+
 static inline void readahead_event(struct address_space *mapping,
 				   pgoff_t offset,
 				   unsigned long req_size,
@@ -44,6 +240,12 @@ static inline void readahead_event(struc
 				   unsigned long async_size,
 				   int actual)
 {
+#ifdef CONFIG_READAHEAD_STATS
+	if (readahead_stats_enable)
+		readahead_stats(mapping, offset, req_size,
+				for_mmap, for_metadata,
+				pattern, start, size, async_size, actual);
+#endif
 	trace_readahead(mapping, offset, req_size,
 			pattern, start, size, async_size, actual);
 }
--- linux-next.orig/mm/Kconfig	2012-01-25 15:57:46.000000000 +0800
+++ linux-next/mm/Kconfig	2012-01-25 15:57:53.000000000 +0800
@@ -379,3 +379,18 @@ config CLEANCACHE
 	  in a negligible performance hit.
 
 	  If unsure, say Y to enable cleancache
+
+config READAHEAD_STATS
+	bool "Collect page cache readahead stats"
+	depends on DEBUG_FS
+	default n
+	help
+	  This provides the readahead events accounting facilities.
+
+	  To do readahead accounting for a workload:
+
+	  echo 0 > /sys/kernel/debug/readahead/stats  # reset counters
+	  echo 1 > /sys/kernel/debug/readahead/stats_enable
+	  # run the workload
+	  cat /sys/kernel/debug/readahead/stats       # check counters
+	  echo 0 > /sys/kernel/debug/readahead/stats_enable