Re: [PATCH v2 2/2] nvme: support io stats on the mpath device

[Hannes Reinecke <hare@suse.de>]

On 10/3/22 11:43, Sagi Grimberg wrote:
> Our mpath stack device is just a shim that selects a bottom namespace
> and submits the bio to it without any fancy splitting. This also means
> that we don't clone the bio or have any context to the bio beyond
> submission. However it really sucks that we don't see the mpath device
> io stats.
> 
> Given that the mpath device can't do that without adding some context
> to it, we let the bottom device do it on its behalf (somewhat similar
> to the approach taken in nvme_trace_bio_complete).
> 
> When the IO starts, we account the request for multipath IO stats using
> REQ_NVME_MPATH_IO_STATS nvme_request flag to avoid queue io stats disable
> in the middle of the request.
> 
> Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
> ---
>   drivers/nvme/host/core.c      |  4 ++++
>   drivers/nvme/host/multipath.c | 25 +++++++++++++++++++++++++
>   drivers/nvme/host/nvme.h      | 12 ++++++++++++
>   3 files changed, 41 insertions(+)
> 
> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
> index 64fd772de817..d5a54ddf73f2 100644
> --- a/drivers/nvme/host/core.c
> +++ b/drivers/nvme/host/core.c
> @@ -384,6 +384,8 @@ static inline void nvme_end_req(struct request *req)
>   		nvme_log_error(req);
>   	nvme_end_req_zoned(req);
>   	nvme_trace_bio_complete(req);
> +	if (req->cmd_flags & REQ_NVME_MPATH)
> +		nvme_mpath_end_request(req);
>   	blk_mq_end_request(req, status);
>   }
>   
> @@ -421,6 +423,8 @@ EXPORT_SYMBOL_GPL(nvme_complete_rq);
>   
>   void nvme_start_request(struct request *rq)
>   {
> +	if (rq->cmd_flags & REQ_NVME_MPATH)
> +		nvme_mpath_start_request(rq);
>   	blk_mq_start_request(rq);
>   }
>   EXPORT_SYMBOL_GPL(nvme_start_request);

Why don't you move the check for REQ_NVME_MPATH into 
nvme_mpath_{start,end}_request?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@suse.de                              +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), Geschäftsführer: Ivo Totev, Andrew
Myers, Andrew McDonald, Martje Boudien Moerman