[RESEND PATCH v2] block: Consider only dispatched requests for inflight statistic

[Gabriel Krisman Bertazi <krisman@collabora.com>]

Hi Jens,

Reviving this, hopefully for the upcoming merge window, following the
suggestion that we let it sit there and see if people complain about the
metric fix.  As I mentioned, I'm also happy to change the documentation
plus create a new in_device metric to track only requests submitted, if
you prefer.

Thanks,

>8

According to Documentation/block/stat.rst, inflight should not include
I/O requests that are in the queue but not yet dispatched to the device,
but blk-mq identifies as inflight any request that has a tag allocated,
which, for queues without elevator, happens at request allocation time
and before it is queued in the ctx (default case in blk_mq_submit_bio).

In addition, current behavior is different for queues with elevator from
queues without it, since for the former the driver tag is allocated at
dispatch time.  A more precise approach would be to only consider
requests with state MQ_RQ_IN_FLIGHT.

This effectively reverts commit 6131837b1de6 ("blk-mq: count allocated
but not started requests in iostats inflight") to consolidate blk-mq
behavior with itself (elevator case) and with original documentation,
but it differs from the behavior used by the legacy path.

This version differs from v1 by using blk_mq_rq_state to access the
state attribute.  Avoid using blk_mq_request_started, which was
suggested, since we don't want to include MQ_RQ_COMPLETE.

Cc: Omar Sandoval <osandov@fb.com>
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
---
 block/blk-mq.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 2e4b3cad2a61..c5fefd39d0c0 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
-- 
2.28.0