[PATCH 00/12] unify the interface of the proportional-share policy in blkio/io

[Paolo Valente]

Hi Jens, Tejun, all,
about nine months ago, we agreed on a solution for unifying the
interface of the proportional-share policy in blkio/io [1].  Angelo
and I finally completed it.  Let me briefly recall the problem and the
solution.

The current implementation of cgroups doesn't allow two or more
entities, e.g., I/O schedulers, to share the same files.  So, if CFQ
creates its files for the proportional-share policy, such as, e.g,
weight files for blkio/io groups, BFQ cannot attach somehow to them.
Thus, to enable people to set group weights with BFQ, I resorted to
making BFQ create its own version of these common files, by prepending
a bfq prefix.

Actually, no legacy code uses these different names, or is likely to
do so.  Having these two sets of names is simply a source of
confusion, as pointed out also, e.g., by Lennart Poettering (CCed
here), and acknowledged by Tejun [2].

In [1] we agreed on a solution that solves this problem, by actually
making it possible to share cgroups files.  Both writing to and
reading from a shared file trigger the appropriate operation for each
of the entities that share the file.  In particular, in case of
reading,
- if all entities produce the same output, the this common output is
  shown only once;
- if the outputs differ, then every per-entity output is shown,
  preceded by the name of the entity that produced that output.

With this solution, legacy code that, e.g., sets group weights, just
works, regardless of the I/O scheduler actually implementing
proportional share.

But note that this extension is not restricted to only blkio/io.  The
general group interface now enables files to be shared among multiple
entities of any kind.

(I have also added a patch to fix some clerical errors in bfq doc,
which I found while making the latter consistent with the new
interface.)

Thanks,
Paolo

[1] https://lkml.org/lkml/2018/1/4/667
[2] https://github.com/systemd/systemd/issues/7057

Angelo Ruocco (7):
  kernfs: add function to find kernfs_node without increasing ref
    counter
  cgroup: link cftypes of the same subsystem with the same name
  cgroup: add owner name to cftypes
  block, bfq: align min and default weights with cfq
  cgroup: make all functions of all cftypes be invoked
  block, cfq: allow cgroup files to be shared
  block, throttle: allow sharing cgroup statistic files

Paolo Valente (5):
  cgroup: add hook seq_show_cft with also the owning cftype as parameter
  block, cgroup: pass cftype to functions that need to use it
  block, bfq: use standard file names for the proportional-share policy
  doc, bfq-iosched: fix a few clerical errors
  doc, bfq-iosched: make it consistent with the new cgroup interface

 Documentation/block/bfq-iosched.txt |  31 +++--
 block/bfq-cgroup.c                  | 148 +++++++++++++-------
 block/bfq-iosched.h                 |   4 +-
 block/blk-cgroup.c                  |  22 +--
 block/blk-throttle.c                |  24 ++--
 block/cfq-iosched.c                 | 105 +++++++++++----
 fs/kernfs/dir.c                     |  13 ++
 include/linux/blk-cgroup.h          |  10 +-
 include/linux/cgroup-defs.h         |  14 +-
 include/linux/cgroup.h              |  13 ++
 include/linux/kernfs.h              |   7 +
 kernel/cgroup/cgroup.c              | 262 +++++++++++++++++++++++++++++-------
 12 files changed, 483 insertions(+), 170 deletions(-)

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2.16.1