[PATCHSET v2 block/for-5.8] iocost: improve use_delay and latency target handling

[Tejun Heo <tj@kernel.org>]

Changes from v1[1]

* Dropped 0002-block-add-request-io_data_len.patch and updated to use
  rq->stats_sectors instead as suggested by Pavel Begunkov.

This patchset improves the following two iocost control behaviors.

* iocost was failing to punish heavy shared IO generators (file metadata, memory
  reclaim) through use_delay mechanism - use_delay automatically decays which
  works well for iolatency but doesn't match how iocost behaves. This led to
  e.g. memory bombs which generate a lot of swap IOs to use over their allotted
  amount. This is fixed by adding non-decaying use_delay mechanism.

* The same latency targets were being applied regardless of the IO sizes. While
  this works fine for loose targets, it gets in the way when trying to tigthen
  them - a latency target adequate for a 4k IO is too short for a 1 meg IO.
  iocost now discounts the size portion of cost when testing whether a given IO
  met or missed its latency target.

While at it, it also makes minor changse to iocost_monitor.py.

This patchset contains the following five patches.

 0001-blk-iocost-switch-to-fixed-non-auto-decaying-use_del.patch
 0002-blk-iocost-account-for-IO-size-when-testing-latencie.patch
 0003-iocost_monitor-exit-successfully-if-interval-is-zero.patch
 0004-iocost_monitor-drop-string-wrap-around-numbers-when-.patch

and is also available in the following git branch.

 git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup.git iocost-delay-latency-v2

diffstat follows. Thanks.

 block/Kconfig                  |    1 
 block/blk-cgroup.c             |    6 ++++
 block/blk-iocost.c             |   56 +++++++++++++++++++++++++++++------------
 include/linux/blk-cgroup.h     |   43 ++++++++++++++++++++++++-------
 tools/cgroup/iocost_monitor.py |   48 +++++++++++++++++++----------------
 5 files changed, 106 insertions(+), 48 deletions(-)

--
tejun

[1] http://lkml.kernel.org/r/20200408201450.3959560-1-tj@kernel.org