linux-kernel - [PATCH 3/3] bcache: count cache_available

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Date:   Sat, 27 Jul 2019 18:19:59 +0800
From:   Yaowei Bai <baiyaowei@...s.chinamobile.com>
To:     colyli@...e.de, kent.overstreet@...il.com
Cc:     linux-bcache@...r.kernel.org, linux-kernel@...r.kernel.org,
        baiyaowei@...s.chinamobile.com
Subject: [PATCH 3/3] bcache: count cache_available_percent accurately

The interface cache_available_percent is used to indicate how
many buckets in percent are available to be used to cache data
at a specific moment. It should include the unused and clean
buckets which we get from bch_btree_gc_finish function:

	if (!GC_MARK(b) || GC_MARK(b) == GC_MARK_RECLAIMABLE)
		 c->avail_nbuckets++;

However currently in the allocation code we didn't distinguish
these available buckets with the metadata and dirty buckets, we
just decrease the c->avail_nbuckets everytime we allocate a bucket,
and correct it after a gc completes. With this, in a read-only
scenario, you can observe that cache_available_percent bounces,
it first go down to a number, like 95, and then bounces back to
100. It goes on and on, making users confused.

This patch fixes this problem by decreasing c->avail_nbuckets
only when allocate metadata and dirty buckets. With this patch,
cache_available_percent will always be accurate and avoid the
confusion.

Signed-off-by: Yaowei Bai <baiyaowei@...s.chinamobile.com>
---
 drivers/md/bcache/alloc.c   | 10 +++++-----
 drivers/md/bcache/request.c |  9 ++++++++-
 2 files changed, 13 insertions(+), 6 deletions(-)

diff --git a/drivers/md/bcache/alloc.c b/drivers/md/bcache/alloc.c
index 609df38..dc7f6c2 100644
--- a/drivers/md/bcache/alloc.c
+++ b/drivers/md/bcache/alloc.c
@@ -443,17 +443,17 @@ long bch_bucket_alloc(struct cache *ca, unsigned int reserve, bool wait)
 		SET_GC_MARK(b, GC_MARK_METADATA);
 		SET_GC_MOVE(b, 0);
 		b->prio = BTREE_PRIO;
+
+		if (ca->set->avail_nbuckets > 0) {
+			ca->set->avail_nbuckets--;
+			bch_update_bucket_in_use(ca->set, &ca->set->gc_stats);
+		}
 	} else {
 		SET_GC_MARK(b, GC_MARK_RECLAIMABLE);
 		SET_GC_MOVE(b, 0);
 		b->prio = INITIAL_PRIO;
 	}

-	if (ca->set->avail_nbuckets > 0) {
-		ca->set->avail_nbuckets--;
-		bch_update_bucket_in_use(ca->set, &ca->set->gc_stats);
-	}
-
 	return r;
 }

diff --git a/drivers/md/bcache/request.c b/drivers/md/bcache/request.c
index 41adcd1..b69bd8d 100644
--- a/drivers/md/bcache/request.c
+++ b/drivers/md/bcache/request.c
@@ -244,9 +244,16 @@ static void bch_data_insert_start(struct closure *cl)
 		if (op->writeback) {
 			SET_KEY_DIRTY(k, true);

-			for (i = 0; i < KEY_PTRS(k); i++)
+			for (i = 0; i < KEY_PTRS(k); i++) {
 				SET_GC_MARK(PTR_BUCKET(op->c, k, i),
 					    GC_MARK_DIRTY);
+
+				if (op->c->avail_nbuckets > 0) {
+					op->c->avail_nbuckets--;
+					bch_update_bucket_in_use(op->c,
+								 &op->c->gc_stats);
+				}
+			}
 		}

 		SET_KEY_CSUM(k, op->csum);
-- 
1.8.3.1