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Message-ID: <20111129152106.GN5635@quack.suse.cz>
Date: Tue, 29 Nov 2011 16:21:06 +0100
From: Jan Kara <jack@...e.cz>
To: Wu Fengguang <fengguang.wu@...el.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Andi Kleen <andi@...stfloor.org>, Ingo Molnar <mingo@...e.hu>,
Jens Axboe <axboe@...nel.dk>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Rik van Riel <riel@...hat.com>,
Linux Memory Management List <linux-mm@...ck.org>,
linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 6/9] readahead: add /debug/readahead/stats
On Tue 29-11-11 21:09:06, Wu Fengguang wrote:
> 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
>
> The added overheads are two readahead_stats() calls per readahead.
> Which is trivial costs unless there are concurrent random reads on
> super fast SSDs, which may lead to cache bouncing when updating the
> global ra_stats[][]. Considering that normal users won't need this
> except when debugging performance problems, it's disabled by default.
> So it looks reasonable to keep this debug code simple rather than trying
> to improve its scalability.
>
> 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@...e.hu>
> CC: Jens Axboe <axboe@...nel.dk>
> CC: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Acked-by: Rik van Riel <riel@...hat.com>
> Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
This looks all inherently racy (which doesn't matter much as you suggest)
so I just wanted to suggest that if you used per-cpu counters you'd get
race-free and faster code at the cost of larger data structures and using
percpu_counter_add() instead of ++ (which doesn't seem like a big
complication to me).
Honza
> ---
> mm/Kconfig | 15 +++
> mm/readahead.c | 194 +++++++++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 209 insertions(+)
>
> --- linux-next.orig/mm/readahead.c 2011-11-29 20:48:05.000000000 +0800
> +++ linux-next/mm/readahead.c 2011-11-29 20:58:53.000000000 +0800
> @@ -18,6 +18,17 @@
> #include <linux/pagevec.h>
> #include <linux/pagemap.h>
>
> +static const char * const ra_pattern_names[] = {
> + [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",
> +};
> +
> /*
> * Initialise a struct file's readahead state. Assumes that the caller has
> * memset *ra to zero.
> @@ -32,6 +43,181 @@ 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/seq_file.h>
> +#include <linux/debugfs.h>
> +
> +static u32 readahead_stats_enable __read_mostly;
> +
> +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,
> +};
> +
> +static unsigned long ra_stats[RA_PATTERN_MAX][RA_ACCOUNT_MAX];
> +
> +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;
> +
> +recount:
> + ra_stats[pattern][RA_ACCOUNT_COUNT]++;
> + ra_stats[pattern][RA_ACCOUNT_SIZE] += size;
> + ra_stats[pattern][RA_ACCOUNT_ASYNC_SIZE] += async_size;
> + ra_stats[pattern][RA_ACCOUNT_ACTUAL] += actual;
> +
> + if (start + size >= eof)
> + ra_stats[pattern][RA_ACCOUNT_EOF]++;
> + if (actual < size)
> + ra_stats[pattern][RA_ACCOUNT_CACHE_HIT]++;
> +
> + if (actual) {
> + ra_stats[pattern][RA_ACCOUNT_IOCOUNT]++;
> +
> + if (start <= offset && offset < start + size)
> + ra_stats[pattern][RA_ACCOUNT_SYNC]++;
> +
> + if (for_mmap)
> + ra_stats[pattern][RA_ACCOUNT_MMAP]++;
> + if (for_metadata)
> + ra_stats[pattern][RA_ACCOUNT_METADATA]++;
> + }
> +
> + if (pattern != RA_PATTERN_ALL) {
> + pattern = RA_PATTERN_ALL;
> + goto recount;
> + }
> +}
> +
> +static int readahead_stats_show(struct seq_file *s, void *_)
> +{
> + unsigned long 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");
> +
> + 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 %10lu %10lu %10lu %10lu %10lu "
> + "%10lu %10lu %10lu %10lu %10lu\n",
> + ra_pattern_names[i],
> + 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)
> +{
> + memset(ra_stats, 0, sizeof(ra_stats));
> + 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;
> +
> + 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;
> +
> + 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,
> + bool for_mmap,
> + bool for_metadata,
> + enum readahead_pattern pattern,
> + pgoff_t start,
> + unsigned long size,
> + 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
> +}
> +
> +
> /*
> * 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 +414,9 @@ 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;
> @@ -267,6 +456,11 @@ 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;
> --- linux-next.orig/mm/Kconfig 2011-11-29 20:48:05.000000000 +0800
> +++ linux-next/mm/Kconfig 2011-11-29 20:48:05.000000000 +0800
> @@ -373,3 +373,18 @@ 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 y
> + 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
>
>
> --
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--
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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