Dear ladies and gentlemen,
I created a patch to make the bypasses for readahead and background IO
that were added in late 2017 configurable via SysFS switches. Since
receiving that original patch in my distro's kernel I noticed
performance degradation and found a few people asking about similar
symptoms as I noticed online who weren't able to identify the problem.
I have more details and my new patch itself outlined at
https://gitlab.com/snippets/1820316, for convenience I have attached
that same patch to this email.
I just thought this may be interesting to you and wanted to let you
know. Do with it as you please.
With kind regards
Andreas
diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
index fdf75352e16a..5f9d9d59bdb5 100644
--- a/drivers/md/bcache/bcache.h
+++ b/drivers/md/bcache/bcache.h
@@ -372,6 +372,9 @@ struct cached_dev {
unsigned char writeback_percent;
unsigned int writeback_delay;
+ unsigned int bypass_readahead_io:1;
+ unsigned int bypass_background_io:1;
+
uint64_t writeback_rate_target;
int64_t writeback_rate_proportional;
int64_t writeback_rate_integral;
diff --git a/drivers/md/bcache/request.c b/drivers/md/bcache/request.c
index 15070412a32e..8028638b348e 100644
--- a/drivers/md/bcache/request.c
+++ b/drivers/md/bcache/request.c
@@ -394,9 +394,13 @@ static bool check_should_bypass(struct cached_dev *dc, struct bio *bio)
* Flag for bypass if the IO is for read-ahead or background,
* unless the read-ahead request is for metadata (eg, for gfs2).
*/
- if (bio->bi_opf & (REQ_RAHEAD|REQ_BACKGROUND) &&
- !(bio->bi_opf & REQ_PRIO))
- goto skip;
+ if (!(bio->bi_opf & REQ_PRIO))
+ {
+ if (dc->bypass_readahead_io && (bio->bi_opf & REQ_RAHEAD))
+ goto skip;
+ if (dc->bypass_background_io && (bio->bi_opf & REQ_BACKGROUND))
+ goto skip;
+ }
if (bio->bi_iter.bi_sector & (c->sb.block_size - 1) ||
bio_sectors(bio) & (c->sb.block_size - 1)) {
diff --git a/drivers/md/bcache/sysfs.c b/drivers/md/bcache/sysfs.c
index 557a8a3270a1..12cbee7f7d89 100644
--- a/drivers/md/bcache/sysfs.c
+++ b/drivers/md/bcache/sysfs.c
@@ -123,6 +123,8 @@ rw_attribute(errors);
rw_attribute(io_error_limit);
rw_attribute(io_error_halflife);
rw_attribute(verify);
+rw_attribute(bypass_readahead_io);
+rw_attribute(bypass_background_io);
rw_attribute(bypass_torture_test);
rw_attribute(key_merging_disabled);
rw_attribute(gc_always_rewrite);
@@ -171,6 +173,8 @@ SHOW(__bch_cached_dev)
sysfs_printf(data_csum, "%i", dc->disk.data_csum);
var_printf(verify, "%i");
+ var_printf(bypass_readahead_io, "%i");
+ var_printf(bypass_background_io, "%i");
var_printf(bypass_torture_test, "%i");
var_printf(writeback_metadata, "%i");
var_printf(writeback_running, "%i");
@@ -262,6 +266,8 @@ STORE(__cached_dev)
sysfs_strtoul(data_csum, dc->disk.data_csum);
d_strtoul(verify);
+ d_strtoul(bypass_readahead_io);
+ d_strtoul(bypass_background_io);
d_strtoul(bypass_torture_test);
d_strtoul(writeback_metadata);
d_strtoul(writeback_running);
@@ -448,6 +454,8 @@ static struct attribute *bch_cached_dev_files[] = {
&sysfs_state,
&sysfs_label,
&sysfs_readahead,
+ &sysfs_bypass_readahead_io,
+ &sysfs_bypass_background_io,
#ifdef CONFIG_BCACHE_DEBUG
&sysfs_verify,
&sysfs_bypass_torture_test,
