bcache: make cutoff_writeback and cutoff_writeback_sync tunable

Currently the cutoff writeback and cutoff writeback sync thresholds are
defined by CUTOFF_WRITEBACK (40) and CUTOFF_WRITEBACK_SYNC (70) as
static values. Most of time these they work fine, but when people want
to do research on bcache writeback mode performance tuning, there is no
chance to modify the soft and hard cutoff writeback values.

This patch introduces two module parameters bch_cutoff_writeback_sync
and bch_cutoff_writeback which permit people to tune the values when
loading bcache.ko. If they are not specified by module loading, current
values CUTOFF_WRITEBACK_SYNC and CUTOFF_WRITEBACK will be used as
default and nothing changes.

When people want to tune this two values,
- cutoff_writeback can be set in range [1, 70]
- cutoff_writeback_sync can be set in range [1, 90]
- cutoff_writeback always <= cutoff_writeback_sync

The default values are strongly recommended to most of users for most of
workloads. Anyway, if people wants to take their own risk to do research
on new writeback cutoff tuning for their own workload, now they can make
it.

Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 40 insertions, 0 deletions

diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index 61d3b63fa617..4dee119c3664 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -25,6 +25,9 @@
 #include <linux/reboot.h>
 #include <linux/sysfs.h>
 
+unsigned int bch_cutoff_writeback;
+unsigned int bch_cutoff_writeback_sync;
+
 static const char bcache_magic[] = {
 	0xc6, 0x85, 0x73, 0xf6, 0x4e, 0x1a, 0x45, 0xca,
 	0x82, 0x65, 0xf5, 0x7f, 0x48, 0xba, 0x6d, 0x81
@@ -2420,6 +2423,32 @@ static void bcache_exit(void)
 	mutex_destroy(&bch_register_lock);
 }
 
+/* Check and fixup module parameters */
+static void check_module_parameters(void)
+{
+	if (bch_cutoff_writeback_sync == 0)
+		bch_cutoff_writeback_sync = CUTOFF_WRITEBACK_SYNC;
+	else if (bch_cutoff_writeback_sync > CUTOFF_WRITEBACK_SYNC_MAX) {
+		pr_warn("set bch_cutoff_writeback_sync (%u) to max value %u",
+			bch_cutoff_writeback_sync, CUTOFF_WRITEBACK_SYNC_MAX);
+		bch_cutoff_writeback_sync = CUTOFF_WRITEBACK_SYNC_MAX;
+	}
+
+	if (bch_cutoff_writeback == 0)
+		bch_cutoff_writeback = CUTOFF_WRITEBACK;
+	else if (bch_cutoff_writeback > CUTOFF_WRITEBACK_MAX) {
+		pr_warn("set bch_cutoff_writeback (%u) to max value %u",
+			bch_cutoff_writeback, CUTOFF_WRITEBACK_MAX);
+		bch_cutoff_writeback = CUTOFF_WRITEBACK_MAX;
+	}
+
+	if (bch_cutoff_writeback > bch_cutoff_writeback_sync) {
+		pr_warn("set bch_cutoff_writeback (%u) to %u",
+			bch_cutoff_writeback, bch_cutoff_writeback_sync);
+		bch_cutoff_writeback = bch_cutoff_writeback_sync;
+	}
+}
+
 static int __init bcache_init(void)
 {
 	static const struct attribute *files[] = {
@@ -2428,6 +2457,8 @@ static int __init bcache_init(void)
 		NULL
 	};
 
+	check_module_parameters();
+
 	mutex_init(&bch_register_lock);
 	init_waitqueue_head(&unregister_wait);
 	register_reboot_notifier(&reboot);
@@ -2464,9 +2495,18 @@ err:
 	return -ENOMEM;
 }
 
+/*
+ * Module hooks
+ */
 module_exit(bcache_exit);
 module_init(bcache_init);
 
+module_param(bch_cutoff_writeback, uint, 0);
+MODULE_PARM_DESC(bch_cutoff_writeback, "threshold to cutoff writeback");
+
+module_param(bch_cutoff_writeback_sync, uint, 0);
+MODULE_PARM_DESC(bch_cutoff_writeback_sync, "hard threshold to cutoff writeback");
+
 MODULE_DESCRIPTION("Bcache: a Linux block layer cache");
 MODULE_AUTHOR("Kent Overstreet <kent.overstreet@gmail.com>");
 MODULE_LICENSE("GPL");