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

[Paolo Valente]

> Il giorno 12 nov 2018, alle ore 11:00, Oleksandr Natalenko <oleksandr@xxxxxxxxxxxxxx> ha scritto:
> 
> On 12.11.2018 10:56, Paolo Valente wrote:
>> 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(-)
>> --
>> 2.16.1
> 
> I thought all the legacy stuff including CFS et al. is going to be removed in v4.21 completelyâ
> 

Thanks for pointing this out.

People with a lower kernel version than the future 4.21 just cannot
and will not be able to use the proportional share policy on blk-mq
(with legacy code), because of the name issue highlighted in this
email.  If this patch series gets accepted, a backport will solve the
problem.  In this respect, such a backport might even happen
'automatically', as most bfq commit seem to get backported to older,
stable kernels.

In addition, this extension
- extends the whole cgroups interface, in a seamless and
  backward-compatible way, to prevent future issues like these;
- solves a similar issue with throttle (which AFAIK won't go away
  with 4.21).

Thanks,
Paolo

> -- 
>  Oleksandr Natalenko (post-factum)