[PATCH v2 4/4] ext4: Make cache hits/misses per-cpu counts

Waiman Long
2016-04-08T16:16:51Z

This patch changes the es_stats_cache_hits and es_stats_cache_misses
statistics counts to per-cpu variables to reduce cacheline contention
issues whem multiple threads are trying to update those counts
simultaneously. It uses the new per-cpu stats APIs provided by the
percpu_stats.h header file.

With a 38-threads fio I/O test with 2 shared files (on DAX-mount
NVDIMM) running on a 4-socket Haswell-EX server with 4.6-rc1 kernel,
the aggregated bandwidths before and after the patch were:

Test W/O patch With patch % change
---- --------- ---------- --------
Read-only 10173MB/s 16141MB/s +58.7%
Read-write 2830MB/s 4315MB/s +52.5%

Signed-off-by: Waiman Long <Waiman.Long@hpe.com>
---
fs/ext4/extents_status.c | 22 +++++++++++++---------
fs/ext4/extents_status.h | 11 +++++++++--
2 files changed, 22 insertions(+), 11 deletions(-)

diff --git a/fs/ext4/extents_status.c b/fs/ext4/extents_status.c
index e38b987..bfa8d63 100644
--- a/fs/ext4/extents_status.c
+++ b/fs/ext4/extents_status.c
@@ -825,9 +825,9 @@ out:
es->es_pblk = es1->es_pblk;
if (!ext4_es_is_referenced(es1))
ext4_es_set_referenced(es1);
- stats->es_stats_cache_hits++;
+ percpu_stats_inc(&stats->es_stats, es_stats_cache_hits);
} else {
- stats->es_stats_cache_misses++;
+ percpu_stats_inc(&stats->es_stats, es_stats_cache_misses);
}

read_unlock(&EXT4_I(inode)->i_es_lock);
@@ -1113,9 +1113,9 @@ int ext4_seq_es_shrinker_info_show(struct seq_file *seq, void *v)
seq_printf(seq, "stats:\n %lld objects\n %lld reclaimable objects\n",
percpu_counter_sum_positive(&es_stats->es_stats_all_cnt),
percpu_counter_sum_positive(&es_stats->es_stats_shk_cnt));
- seq_printf(seq, " %lu/%lu cache hits/misses\n",
- es_stats->es_stats_cache_hits,
- es_stats->es_stats_cache_misses);
+ seq_printf(seq, " %llu/%llu cache hits/misses\n",
+ percpu_stats_sum(&es_stats->es_stats, es_stats_cache_hits),
+ percpu_stats_sum(&es_stats->es_stats, es_stats_cache_misses));
if (inode_cnt)
seq_printf(seq, " %d inodes on list\n", inode_cnt);

@@ -1142,8 +1142,6 @@ int ext4_es_register_shrinker(struct ext4_sb_info *sbi)
sbi->s_es_nr_inode = 0;
spin_lock_init(&sbi->s_es_lock);
sbi->s_es_stats.es_stats_shrunk = 0;
- sbi->s_es_stats.es_stats_cache_hits = 0;
- sbi->s_es_stats.es_stats_cache_misses = 0;
sbi->s_es_stats.es_stats_scan_time = 0;
sbi->s_es_stats.es_stats_max_scan_time = 0;
err = percpu_counter_init(&sbi->s_es_stats.es_stats_all_cnt, 0, GFP_KERNEL);
@@ -1153,15 +1151,20 @@ int ext4_es_register_shrinker(struct ext4_sb_info *sbi)
if (err)
goto err1;

+ err = percpu_stats_init(&sbi->s_es_stats.es_stats, es_stats_cnt, 0);
+ if (err)
+ goto err2;
+
sbi->s_es_shrinker.scan_objects = ext4_es_scan;
sbi->s_es_shrinker.count_objects = ext4_es_count;
sbi->s_es_shrinker.seeks = DEFAULT_SEEKS;
err = register_shrinker(&sbi->s_es_shrinker);
if (err)
- goto err2;
+ goto err3;

return 0;
-
+err3:
+ percpu_stats_destroy(&sbi->s_es_stats.es_stats);
err2:
percpu_counter_destroy(&sbi->s_es_stats.es_stats_shk_cnt);
err1:
@@ -1173,6 +1176,7 @@ void ext4_es_unregister_shrinker(struct ext4_sb_info *sbi)
{
percpu_counter_destroy(&sbi->s_es_stats.es_stats_all_cnt);
percpu_counter_destroy(&sbi->s_es_stats.es_stats_shk_cnt);
+ percpu_stats_destroy(&sbi->s_es_stats.es_stats);
unregister_shrinker(&sbi->s_es_shrinker);
}

diff --git a/fs/ext4/extents_status.h b/fs/ext4/extents_status.h
index f7aa24f..c163189 100644
--- a/fs/ext4/extents_status.h
+++ b/fs/ext4/extents_status.h
@@ -11,6 +11,8 @@
#ifndef _EXT4_EXTENTS_STATUS_H
#define _EXT4_EXTENTS_STATUS_H

+#include <linux/percpu_stats.h>
+
/*
* Turn on ES_DEBUG__ to get lots of info about extent status operations.
*/
@@ -67,10 +69,15 @@ struct ext4_es_tree {
struct extent_status *cache_es; /* recently accessed extent */
};

+enum ext4_es_stat_type {
+ es_stats_cache_hits,
+ es_stats_cache_misses,
+ es_stats_cnt,
+};
+
struct ext4_es_stats {
unsigned long es_stats_shrunk;
- unsigned long es_stats_cache_hits;
- unsigned long es_stats_cache_misses;
+ struct percpu_stats es_stats;
u64 es_stats_scan_time;
u64 es_stats_max_scan_time;
struct percpu_counter es_stats_all_cnt;
--
1.7.1