Date:   Mon, 12 Nov 2018 16:45:39 +0100
From:   Paolo Valente <paolo.valente@...aro.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Oleksandr Natalenko <oleksandr@...alenko.name>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
        Angelo Ruocco <angeloruocco90@...il.com>,
        Dennis Zhou <dennis@...nel.org>,
        Josef Bacik <josef@...icpanda.com>,
        Liu Bo <bo.liu@...ux.alibaba.com>,
        Bart Van Assche <bvanassche@....org>,
        Johannes Weiner <hannes@...xchg.org>,
        linux-block <linux-block@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Mark Brown <broonie@...nel.org>,
        'Paolo Valente' via bfq-iosched 
        <bfq-iosched@...glegroups.com>, cgroups@...r.kernel.org,
        linux-doc@...r.kernel.org, Jonathan Corbet <corbet@....net>,
        lennart@...ttering.net
Subject: Re: [PATCH 00/12] unify the interface of the proportional-share
 policy in blkio/io



> Il giorno 12 nov 2018, alle ore 16:35, Jens Axboe <axboe@...nel.dk> ha scritto:
> 
> On 11/12/18 3:17 AM, Paolo Valente wrote:
>> 
>> 
>>> Il giorno 12 nov 2018, alle ore 11:00, Oleksandr Natalenko <oleksandr@...alenko.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).
> 
> There's no way this series can get accepted, since you've made the
> mistake of basing it on something that won't apply to the block
> tree for 4.21.

Of course, sorry :(

We'll rebase V2.

BTW, since this patch series is probably even more useful for older
than for future kernels, might it make sense to also propose it for
stable/longterm kernels (provided that such a possibility exists)?

Thanks,
Paolo

> I've outlined these rules before, but here they are
> again:
> 
> 1) Patches destined for the CURRENT kernel version should be
>   against my for-linus branch. That means that right now, any
>   patches that should to into 4.20 should be against that.
> 
> 2) Patches destined for the NEXT kernel version should be against
>   my for-x.y/block branch, where x.y is the next version. As of
>   right now, patches for 4.21 should be against my for-4.21/bloc
>   branch.
> 
> I'd encourage you to respin against that, particularly in this case
> since we've both got a lot of churn, and also removal of various
> items that you are patching here.
> 
> -- 
> Jens Axboe

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