> 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).
OK, here is the incremental patch to use per-cpu counters :)
---
mm/readahead.c | 61 +++++++++++++++++++++++++++++++++--------------
1 file changed, 44 insertions(+), 17 deletions(-)
--- linux-next.orig/mm/readahead.c 2011-12-14 09:50:37.000000000 +0800
+++ linux-next/mm/readahead.c 2011-12-14 14:16:15.000000000 +0800
@@ -68,7 +68,7 @@ enum ra_account {
RA_ACCOUNT_MAX,
};
-static unsigned long ra_stats[RA_PATTERN_MAX][RA_ACCOUNT_MAX];
+static DEFINE_PER_CPU(unsigned long[RA_PATTERN_ALL][RA_ACCOUNT_MAX], ra_stat);
static void readahead_stats(struct address_space *mapping,
pgoff_t offset,
@@ -83,38 +83,62 @@ static void readahead_stats(struct addre
{
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;
+ preempt_disable();
+
+ __this_cpu_inc(ra_stat[pattern][RA_ACCOUNT_COUNT]);
+ __this_cpu_add(ra_stat[pattern][RA_ACCOUNT_SIZE], size);
+ __this_cpu_add(ra_stat[pattern][RA_ACCOUNT_ASYNC_SIZE], async_size);
+ __this_cpu_add(ra_stat[pattern][RA_ACCOUNT_ACTUAL], actual);
if (start + size >= eof)
- ra_stats[pattern][RA_ACCOUNT_EOF]++;
+ __this_cpu_inc(ra_stat[pattern][RA_ACCOUNT_EOF]);
if (actual < size)
- ra_stats[pattern][RA_ACCOUNT_CACHE_HIT]++;
+ __this_cpu_inc(ra_stat[pattern][RA_ACCOUNT_CACHE_HIT]);
if (actual) {
- ra_stats[pattern][RA_ACCOUNT_IOCOUNT]++;
+ __this_cpu_inc(ra_stat[pattern][RA_ACCOUNT_IOCOUNT]);
if (start <= offset && offset < start + size)
- ra_stats[pattern][RA_ACCOUNT_SYNC]++;
+ __this_cpu_inc(ra_stat[pattern][RA_ACCOUNT_SYNC]);
if (for_mmap)
- ra_stats[pattern][RA_ACCOUNT_MMAP]++;
+ __this_cpu_inc(ra_stat[pattern][RA_ACCOUNT_MMAP]);
if (for_metadata)
- ra_stats[pattern][RA_ACCOUNT_METADATA]++;
+ __this_cpu_inc(ra_stat[pattern][RA_ACCOUNT_METADATA]);
}
- if (pattern != RA_PATTERN_ALL) {
- pattern = RA_PATTERN_ALL;
- goto recount;
- }
+ preempt_enable();
+}
+
+static void ra_stats_clear(void)
+{
+ int cpu;
+ int i, j;
+
+ for_each_online_cpu(cpu)
+ for (i = 0; i < RA_PATTERN_ALL; i++)
+ for (j = 0; j < RA_ACCOUNT_MAX; j++)
+ per_cpu(ra_stat[i][j], cpu) = 0;
+}
+
+static void ra_stats_sum(unsigned long ra_stats[RA_PATTERN_MAX][RA_ACCOUNT_MAX])
+{
+ int cpu;
+ int i, j;
+
+ for_each_online_cpu(cpu)
+ for (i = 0; i < RA_PATTERN_ALL; i++)
+ for (j = 0; j < RA_ACCOUNT_MAX; j++) {
+ unsigned long n = per_cpu(ra_stat[i][j], cpu);
+ ra_stats[i][j] += n;
+ ra_stats[RA_PATTERN_ALL][j] += n;
+ }
}
static int readahead_stats_show(struct seq_file *s, void *_)
{
unsigned long i;
+ unsigned long ra_stats[RA_PATTERN_MAX][RA_ACCOUNT_MAX];
seq_printf(s,
"%-10s %10s %10s %10s %10s %10s %10s %10s %10s %10s %10s\n",
@@ -122,6 +146,9 @@ static int readahead_stats_show(struct s
"io", "sync_io", "mmap_io", "meta_io",
"size", "async_size", "io_size");
+ memset(ra_stats, 0, sizeof(ra_stats));
+ ra_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];
@@ -159,7 +186,7 @@ static int readahead_stats_open(struct i
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));
+ ra_stats_clear();
return size;
}
--
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