[PATCH rfc] nvme: support io stats on the mpath device

[Max Gurtovoy]

Hi Sagi,

On 9/28/2022 10:55 PM, 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);

Can you please paste the output of the application that shows the 
benefit of this commit ?

>
> Signed-off-by: Sagi Grimberg <sagi at grimberg.me>
> ---
>   drivers/nvme/host/apple.c     |  2 +-
>   drivers/nvme/host/core.c      | 10 ++++++++++
>   drivers/nvme/host/fc.c        |  2 +-
>   drivers/nvme/host/multipath.c | 18 ++++++++++++++++++
>   drivers/nvme/host/nvme.h      | 12 ++++++++++++
>   drivers/nvme/host/pci.c       |  2 +-
>   drivers/nvme/host/rdma.c      |  2 +-
>   drivers/nvme/host/tcp.c       |  2 +-
>   drivers/nvme/target/loop.c    |  2 +-
>   9 files changed, 46 insertions(+), 6 deletions(-)

Several questions:

1. I guess that for the non-mpath case we get this for free from the 
block layer for each bio ?

2. Now we have doubled the accounting, haven't we ?

3. Do you have some performance numbers (we're touching the fast path 
here) ?

4. Should we enable this by default ?


The implementation look good.