Re: wbt_lat_usec still set despite wbt disabled by BFQ

[Holger Hoffstätte <holger@xxxxxxxxxxxxxxxxxxxxxx>]

On 2022-09-14 03:35, Yu Kuai wrote:
[snip]
> Thanks for the test, it turns out this way doesn't select bfq as default
> as I expected...
>
> wbt can show min_lat_nsec despite that wbt can be disabled by
> wbt_disable_default(), I do miss that previously...
>
> Can you try the following patch again?
>
> diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> index a630d657c054..3e8adb95ff02 100644
> --- a/block/blk-sysfs.c
> +++ b/block/blk-sysfs.c
> @@ -467,10 +467,14 @@ static ssize_t queue_io_timeout_store(struct request_queue *q, const char *page,
>
>   static ssize_t queue_wb_lat_show(struct request_queue *q, char *page)
>   {
> +       u64 lat;
> +
>          if (!wbt_rq_qos(q))
>                  return -EINVAL;
>
> -       return sprintf(page, "%llu\n", div_u64(wbt_get_min_lat(q), 1000));
> +       lat = wbt_disabled(q) ? 0 : div_u64(wbt_get_min_lat(q), 1000);
> +
> +       return sprintf(page, "%llu\n", lat);
>   }
>
>   static ssize_t queue_wb_lat_store(struct request_queue *q, const char *page,
> @@ -493,6 +497,9 @@ static ssize_t queue_wb_lat_store(struct request_queue *q, const char *page,
>                          return ret;
>          }
>
> +       if (wbt_disabled(q))
> +               return -EINVAL;
> +
>          if (val == -1)
>                  val = wbt_default_latency_nsec(q);
>          else if (val >= 0)
> diff --git a/block/blk-wbt.c b/block/blk-wbt.c
> index 26ee6ca66a93..55d1015ef117 100644
> --- a/block/blk-wbt.c
> +++ b/block/blk-wbt.c
> @@ -423,6 +423,13 @@ static void wbt_update_limits(struct rq_wb *rwb)
>          rwb_wake_all(rwb);
>   }
>
> +bool wbt_disabled(struct request_queue *q)
> +{
> +       struct rq_qos *rqos = wbt_rq_qos(q);
> +
> +       return !rqos || RQWB(rqos)->enable_state == WBT_STATE_OFF_DEFAULT;
> +}
> +
>   u64 wbt_get_min_lat(struct request_queue *q)
>   {
>          struct rq_qos *rqos = wbt_rq_qos(q);
> diff --git a/block/blk-wbt.h b/block/blk-wbt.h
> index 1a49b6ac397c..4252b8077257 100644
> --- a/block/blk-wbt.h
> +++ b/block/blk-wbt.h
> @@ -94,6 +94,7 @@ void wbt_enable_default(struct request_queue *, bool);
>
>   u64 wbt_get_min_lat(struct request_queue *q);
>   void wbt_set_min_lat(struct request_queue *q, u64 val);
> +bool wbt_disabled(struct request_queue *);
>
>   void wbt_set_write_cache(struct request_queue *, bool);

This one works! :)

After boot:

$cat /sys/block/sdc/queue/scheduler
mq-deadline [bfq] none
$cat /sys/block/sdc/queue/wbt_lat_usec
0

Changing schedulers  back and forth - here on a device with different
default - also works:

$cat /sys/block/sda/queue/scheduler
[mq-deadline] bfq none
$cat /sys/block/sda/queue/wbt_lat_usec
2000
$echo bfq > /sys/block/sda/queue/scheduler
$cat /sys/block/sda/queue/wbt_lat_usec
0
$echo deadline > /sys/block/sda/queue/scheduler
$cat /sys/block/sda/queue/wbt_lat_usec
2000

Feel free to add my Reported-by and Tested-by.

cheers!
Holger