block, bfq: print SHARED instead of pid for shared queues in logs

The function "bfq_log_bfqq" prints the pid of the process
associated with the queue passed as input.

Unfortunately, if the queue is shared, then more than one process
is associated with the queue. The pid that gets printed in this
case is the pid of one of the associated processes.
Which process gets printed depends on the exact sequence of merge
events the queue underwent. So printing such a pid is rather
useless and above all is often rather confusing because it
reports a random pid between those of the associated processes.

This commit addresses this issue by printing SHARED instead of a pid
if the queue is shared.

Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Francesco Pollicino <fra.fra.800@gmail.com>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 10 insertions, 0 deletions

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 49bde428f7f2..37cc8f127cf6 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -2590,6 +2590,16 @@ bfq_merge_bfqqs(struct bfq_data *bfqd, struct bfq_io_cq *bic,
 	 *   assignment causes no harm).
 	 */
 	new_bfqq->bic = NULL;
+	/*
+	 * If the queue is shared, the pid is the pid of one of the associated
+	 * processes. Which pid depends on the exact sequence of merge events
+	 * the queue underwent. So printing such a pid is useless and confusing
+	 * because it reports a random pid between those of the associated
+	 * processes.
+	 * We mark such a queue with a pid -1, and then print SHARED instead of
+	 * a pid in logging messages.
+	 */
+	new_bfqq->pid = -1;
 	bfqq->bic = NULL;
 	/* release process reference to bfqq */
 	bfq_put_queue(bfqq);