On 12/10/18 9:35 AM, Dennis Zhou wrote:

The blk-iolatency controller measures the time from rq_qos_throttle() to
rq_qos_done_bio() and attributes this time to the first bio that needs
to create the request. This means if a bio is plug-mergeable or
bio-mergeable, it gets to bypass the blk-iolatency controller.

The recent series, to tag all bios w/ blkgs in [1] changed the timing
incorrectly as well. First, the iolatency controller was tagging bios
and using that information if it should process it in rq_qos_done_bio().
However, now that all bios are tagged, this caused the atomic_t for the
struct rq_wait inflight count to underflow resulting in a stall. Second,
now the timing was using the duration a bio from generic_make_request()
rather than the timing mentioned above.

This patch fixes the errors by accounting time separately in a bio
adding the field bi_start. If this field is set, the bio should be
processed by blk-iolatency in rq_qos_done_bio().

[1] https://lore.kernel.org/lkml/20181205171039.73066-1-dennis@kernel.org/ (local)

Looks reasonable to me, but it needs a Fixes tag as well.

-- 
Jens Axboe