Re: [PATCH] block: don't quiesce the queue when setting the default elevator

[Ming Lei]

On Wed, Sep 30, 2026 at 10:12 PM Qiliang Yuan <odys.yuan@xxxxxxxxx> wrote:
>
> elevator_set_default() runs from blk_register_queue() while the disk is
> still being added: the block device is not visible yet and uevents are
> suppressed, so no file system I/O can be issued. elevator_change()
> freezes the queue and cancels the dispatch work before switching, which
> is enough to drain dispatch activity from passthrough requests, as the
> comment in elevator_change() already states.
>
> elevator_switch() still quiesces the queue unconditionally, though, so
> every disk added with a default elevator waits for an RCU grace period.
> Commit 245a489e81e1 ("block: avoid to quiesce queue in
> elevator_init_mq") had removed this wait from the default elevator
> setup, and it came back when that setup was folded into
> elevator_change().
>
> The wait is paid in full by callers that add disks one at a time. ublk
> adds the disk of a single-queue device from START_DEV, which takes 7.7ms
> at p50 in a 16 vCPU KVM guest, most of it in blk_mq_quiesce_queue()
> called from elevator_set_default().
>
> Skip quiescing in elevator_switch() when setting the default elevator.
>
> ublk null target, 1000 single-queue devices created one by one:
>
>                      before      after
>   START_DEV p50      7.74ms     0.68ms
>   START_DEV p99     14.61ms     1.32ms
>   devices/s             114        971
>
> Fixes: 1e44bedbc921 ("block: unifying elevator change")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Qiliang Yuan <odys.yuan@xxxxxxxxx>
> ---
>  block/elevator.c | 7 +++++--
>  block/elevator.h | 2 ++
>  2 files changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/block/elevator.c b/block/elevator.c
> index 2161b6eea680c..da9ba70ac9321 100644
> --- a/block/elevator.c
> +++ b/block/elevator.c
> @@ -573,7 +573,8 @@ static int elevator_switch(struct request_queue *q, struct elv_change_ctx *ctx)
>                         return -EINVAL;
>         }
>
> -       blk_mq_quiesce_queue(q);
> +       if (!ctx->no_quiesce)
> +               blk_mq_quiesce_queue(q);
>
>         if (q->elevator) {
>                 ctx->old = q->elevator;
> @@ -594,7 +595,8 @@ static int elevator_switch(struct request_queue *q, struct elv_change_ctx *ctx)
>         blk_add_trace_msg(q, "elv switch: %s", ctx->name);
>
>  out_unfreeze:
> -       blk_mq_unquiesce_queue(q);
> +       if (!ctx->no_quiesce)
> +               blk_mq_unquiesce_queue(q);
>
>         if (ret) {
>                 pr_warn("elv: switch to \"%s\" failed, falling back to \"none\"\n",
> @@ -731,6 +733,7 @@ void elevator_set_default(struct request_queue *q)
>         struct elv_change_ctx ctx = {
>                 .name = "mq-deadline",
>                 .no_uevent = true,
> +               .no_quiesce = true,
>         };
>         int err;

Looks good given elevator_set_default() is only called on new added disk,

Reviewed-by: Ming Lei <tom.leiming@xxxxxxxxx>



Thanks,
Ming Lei