Commit 85686d0dc194 ("block, bfq: keep shared queues out of the waker
mechanism") leaves shared bfq_queues out of the waker-detection
mechanism. It attains this goal by not updating the pointer
last_completed_rq_bfqq, if the last request completed belongs to a
shared bfq_queue (so that the pointer will not point to the shared
bfq_queue).
Yet this has a side effect: the pointer last_completed_rq_bfqq keeps
pointing, deceptively, to a bfq_queue that actually is not the last
one to have had a request completed. As a consequence, such a
bfq_queue may deceptively be considered as a waker of some bfq_queue,
even of some shared bfq_queue.
To address this issue, reset last_completed_rq_bfqq if the last
request completed belongs to a shared queue.
Fixes: 85686d0dc194 ("block, bfq: keep shared queues out of the waker mechanism")
Signed-off-by: Paolo Valente <paolo.valente@xxxxxxxxxx>
---
block/bfq-iosched.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index a273b2bcea2a..fedb0a8fd388 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -6165,11 +6165,13 @@ static void bfq_completed_request(struct bfq_queue *bfqq, struct bfq_data *bfqd)
* of other queues. But a false waker will unjustly steal
* bandwidth to its supposedly woken queue. So considering
* also shared queues in the waking mechanism may cause more
- * control troubles than throughput benefits. Then do not set
- * last_completed_rq_bfqq to bfqq if bfqq is a shared queue.
+ * control troubles than throughput benefits. Then reset
+ * last_completed_rq_bfqq if bfqq is a shared queue.
*/
if (!bfq_bfqq_coop(bfqq))
bfqd->last_completed_rq_bfqq = bfqq;
+ else
+ bfqd->last_completed_rq_bfqq = NULL;
/*
* If we are waiting to discover whether the request pattern
--
2.20.1
