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SubjectRe: [PATCH 00/12] unify the interface of the proportional-share policy in blkio/io
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> Il giorno 12 nov 2018, alle ore 11:00, Oleksandr Natalenko <oleksandr@natalenko.name> 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 completely…
>

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)

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