flow: fix hang with flow control and zoned block devices

When flow control is specified between 2 or more threads executing a workload
with zonemode=zbd and flow_sleep is not specified by the user (default to zero),
then a job A can lock a zone Z with the flow ratio exceeded, resulting in the
job to not issue the IO until other jobs IOs balance the flow ratio.
Since job A will wait with the Zone Z locked, other jobs will deadlock waiting
for this zone lock if their workload resulted in this zone Z being chosen
as an IO target.

Executing io_u_quiesce() when the flow ratio is exceeded solves this potential
deadlock as job A will wait for the flow ratio to balance without holding the
zone A lock. To preserve the behavior for regular block devices and to avoid
extra overhead, io_u_quiesce() only needs to be executed when the workload
specifies zonedmode=zbd.

Signed-off-by: Aravind Ramesh <aravind.ramesh@wdc.com>
Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/flow.c b/flow.c
index ea6b0ec9b3a39b712c5dec770722d4fceb6bd63d..c64bb3b27559053d27a79fa2c9a45f68de6e71c2 100644 (file)
--- a/flow.c
+++ b/flow.c
@@ -37,6 +37,8 @@ int flow_threshold_exceeded(struct thread_data *td)
                if (td->o.flow_sleep) {
                        io_u_quiesce(td);
                        usleep(td->o.flow_sleep);
+               } else if (td->o.zone_mode == ZONE_MODE_ZBD) {
+                       io_u_quiesce(td);
                }
 
                return 1;