Re: [PATCH] nvme: core: freeze multipath queue early in nvme_update_ns_info()

[Niklas Cassel <cassel@kernel.org>]

On Thu, Aug 22, 2024 at 10:14:13PM +0200, Martin Wilck wrote:
> For multipath devices, nvme_update_ns_info() needs to freeze both
> the queue of the path and the queue of the multipath device. For
> both operations, it waits for one RCU grace period to pass, ~25ms
> on my test system. By calling blk_freeze_queue_start() for the
> multipath queue early, we avoid waiting twice; tests using ftrace
> have shown that the second blk_mq_freeze_queue_wait() call finishes
> in just a few microseconds. The path queue is unfrozen before
> calling blk_mq_freeze_queue_wait() on the multipath queue, so that
> possibly outstanding IO in the multipath queue can be flushed.
> 
> I tested this using the "controller rescan under I/O load" test
> I submitted recently [1].
> 
> [1] https://lore.kernel.org/linux-nvme/20240822193814.106111-3-mwilck@suse.com/T/#u
> 
> Signed-off-by: Martin Wilck <mwilck@suse.com>
> ---
>  drivers/nvme/host/core.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
> index 33fa01c599ad..e2454398c660 100644
> --- a/drivers/nvme/host/core.c
> +++ b/drivers/nvme/host/core.c
> @@ -2217,6 +2217,9 @@ static int nvme_update_ns_info(struct nvme_ns *ns, struct nvme_ns_info *info)
>  	bool unsupported = false;
>  	int ret;
>  
> +	if (nvme_ns_head_multipath(ns->head))
> +		blk_freeze_queue_start(ns->head->disk->queue);
> +

From someone reading this code, it looks quite similar to
nvme_mpath_start_freeze().

Perhaps create a new helper, with proper kdoc, and possibly also add kdoc to
nvme_mpath_start_freeze(), so that a user can easily tell (from the kdoc)
when to use which function.


Kind regards,
Niklas