Re: [PATCH] block: fix iolat timestamp and restore accounting semantics

From: Jens Axboe
Date: Mon Dec 10 2018 - 11:58:20 EST

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]

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

Jens Axboe