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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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