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The old no-op check skipped updates whenever the converted latency matched the stored min_lat_nsec. That check ignored whether the current WBT state already matched the state requested by the write. For a queue disabled by default, attempting to enable WBT by writing the default value through sysfs could return success while the enable state was left unchanged. Treat a write as a no-op only when both the stored latency and the effective WBT enabled state already match the converted value. Signed-off-by: Guzebing --- Background: The issue can be reproduced on an NVMe namespace when BFQ is available: echo bfq > /sys/block/nvme0n1/queue/scheduler cat /sys/block/nvme0n1/queue/wbt_lat_usec echo 2000 > /sys/block/nvme0n1/queue/wbt_lat_usec cat /sys/block/nvme0n1/queue/wbt_lat_usec After BFQ selects the queue, WBT is disabled by default. On a non-rotational NVMe namespace the stored default latency remains 2000000 nsec, while the sysfs file reports 0 because the effective WBT state is disabled: queue/wbt_lat_usec = 0 debugfs enabled = 3 debugfs min_lat_nsec = 2000000 Writing the default value succeeds, but the old no-op check skips the state transition because min_lat_nsec already matches the converted value: echo 2000 > /sys/block/nvme0n1/queue/wbt_lat_usec # echo returns success, but: queue/wbt_lat_usec = 0 debugfs enabled = 3 debugfs min_lat_nsec = 2000000 As a control, writing a non-default value first does work: echo 5000 > /sys/block/nvme0n1/queue/wbt_lat_usec queue/wbt_lat_usec = 5000 debugfs enabled = 2 debugfs min_lat_nsec = 5000000 Writing the default value after that also works, because the stored latency changes from 5000000 nsec back to 2000000 nsec: echo 2000 > /sys/block/nvme0n1/queue/wbt_lat_usec queue/wbt_lat_usec = 2000 debugfs enabled = 2 debugfs min_lat_nsec = 2000000 With this patch, writing the default value after BFQ default-disables WBT also re-enables WBT as expected: queue/wbt_lat_usec = 2000 debugfs enabled = 2 debugfs min_lat_nsec = 2000000 block/blk-wbt.c | 21 ++++++++++++++++++++- 1 file changed, 20 insertions(+), 1 deletion(-) diff --git a/block/blk-wbt.c b/block/blk-wbt.c index dcc2438ca16dc..953d400fd0137 100644 --- a/block/blk-wbt.c +++ b/block/blk-wbt.c @@ -813,6 +813,21 @@ static void wbt_queue_depth_changed(struct rq_qos *rqos) wbt_update_limits(RQWB(rqos)); } +static bool wbt_set_lat_changed(struct request_queue *q, u64 val) +{ + struct rq_qos *rqos = wbt_rq_qos(q); + struct rq_wb *rwb; + + if (!rqos) + return true; + + rwb = RQWB(rqos); + if (rwb->min_lat_nsec != val) + return true; + + return rwb_enabled(rwb) != !!val; +} + static void wbt_exit(struct rq_qos *rqos) { struct rq_wb *rwb = RQWB(rqos); @@ -1005,8 +1020,12 @@ int wbt_set_lat(struct gendisk *disk, s64 val) else if (val >= 0) val *= 1000ULL; - if (wbt_get_min_lat(q) == val) + mutex_lock(&disk->rqos_state_mutex); + if (!wbt_set_lat_changed(q, val)) { + mutex_unlock(&disk->rqos_state_mutex); goto out; + } + mutex_unlock(&disk->rqos_state_mutex); blk_mq_quiesce_queue(q); -- 2.20.1