Patch "bcache: correct cache_dirty_target in __update_writeback_rate()" has been added to the 4.9-stable tree

Patch "bcache: correct cache_dirty_target in __update_writeback_rate()" has been added to the 4.9-stable tree

[gregkh]

This is a note to let you know that I've just added the patch titled

    bcache: correct cache_dirty_target in __update_writeback_rate()

to the 4.9-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     bcache-correct-cache_dirty_target-in-__update_writeback_rate.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From a8394090a9129b40f9d90dcb7f4a49d60c727ca6 Mon Sep 17 00:00:00 2001
From: Tang Junhui <tang.junhui@zte.com.cn>
Date: Wed, 6 Sep 2017 14:25:56 +0800
Subject: bcache: correct cache_dirty_target in __update_writeback_rate()

From: Tang Junhui <tang.junhui@zte.com.cn>

commit a8394090a9129b40f9d90dcb7f4a49d60c727ca6 upstream.

__update_write_rate() uses a Proportion-Differentiation Controller
algorithm to control writeback rate. A dirty target number is used in
this PD controller to control writeback rate. A larger target number
will make the writeback rate smaller, on the versus, a smaller target
number will make the writeback rate larger.

bcache uses the following steps to calculate the target number,
1) cache_sectors = all-buckets-of-cache-set * buckets-size
2) cache_dirty_target = cache_sectors * cached-device-writeback_percent
3) target = cache_dirty_target *
(sectors-of-cached-device/sectors-of-all-cached-devices-of-this-cache-set)

The calculation at step 1) for cache_sectors is incorrect, which does
not consider dirty blocks occupied by flash only volume.

A flash only volume can be took as a bcache device without cached
device. All data sectors allocated for it are persistent on cache device
and marked dirty, they are not touched by bcache writeback and garbage
collection code. So data blocks of flash only volume should be ignore
when calculating cache_sectors of cache set.

Current code does not subtract dirty sectors of flash only volume, which
results a larger target number from the above 3 steps. And in sequence
the cache device's writeback rate is smaller then a correct value,
writeback speed is slower on all cached devices.

This patch fixes the incorrect slower writeback rate by subtracting
dirty sectors of flash only volumes in __update_writeback_rate().

(Commit log composed by Coly Li to pass checkpatch.pl checking)

Signed-off-by: Tang Junhui <tang.junhui@zte.com.cn>
Reviewed-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 drivers/md/bcache/writeback.c |    3 ++-
 drivers/md/bcache/writeback.h |   19 +++++++++++++++++++
 2 files changed, 21 insertions(+), 1 deletion(-)

--- a/drivers/md/bcache/writeback.c
+++ b/drivers/md/bcache/writeback.c
@@ -20,7 +20,8 @@
 static void __update_writeback_rate(struct cached_dev *dc)
 {
 	struct cache_set *c = dc->disk.c;
-	uint64_t cache_sectors = c->nbuckets * c->sb.bucket_size;
+	uint64_t cache_sectors = c->nbuckets * c->sb.bucket_size -
+				bcache_flash_devs_sectors_dirty(c);
 	uint64_t cache_dirty_target =
 		div_u64(cache_sectors * dc->writeback_percent, 100);
 
--- a/drivers/md/bcache/writeback.h
+++ b/drivers/md/bcache/writeback.h
@@ -14,6 +14,25 @@ static inline uint64_t bcache_dev_sector
 	return ret;
 }
 
+static inline uint64_t  bcache_flash_devs_sectors_dirty(struct cache_set *c)
+{
+	uint64_t i, ret = 0;
+
+	mutex_lock(&bch_register_lock);
+
+	for (i = 0; i < c->nr_uuids; i++) {
+		struct bcache_device *d = c->devices[i];
+
+		if (!d || !UUID_FLASH_ONLY(&c->uuids[i]))
+			continue;
+	   ret += bcache_dev_sectors_dirty(d);
+	}
+
+	mutex_unlock(&bch_register_lock);
+
+	return ret;
+}
+
 static inline unsigned offset_to_stripe(struct bcache_device *d,
 					uint64_t offset)
 {


Patches currently in stable-queue which might be from tang.junhui@zte.com.cn are

queue-4.9/bcache-do-not-subtract-sectors_to_gc-for-bypassed-io.patch
queue-4.9/bcache-correct-cache_dirty_target-in-__update_writeback_rate.patch
queue-4.9/bcache-fix-for-gc-and-write-back-race.patch
queue-4.9/bcache-initialize-dirty-stripes-in-flash_dev_run.patch