linux-kernel - Re: [PATCH 1/2] cgroup: Add generation number with cgroup id

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Message-ID: <20191025110623.GH3622521@devbig004.ftw2.facebook.com>
Date:   Fri, 25 Oct 2019 04:06:23 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Li Zefan <lizefan@...wei.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Song Liu <liu.song.a23@...il.com>
Subject: Re: [PATCH 1/2] cgroup: Add generation number with cgroup id

Hello,

On Fri, Oct 25, 2019 at 05:30:35PM +0900, Namhyung Kim wrote:
> > Any chance I can persuade you into making this conversion?  idr is
> > exactly the wrong data structure to use for cyclic allocations.  We've
> > been doing it mostly for historical reasons but I really hope we can
> > move away from it.  These lookups aren't in super hot paths and doing
> > locked lookups should be fine.
> 
> As you know, it entails change in kernfs id and its users.
> And I really want to finish the perf cgroup sampling work first.
> Can I work on this after the perf work is done?

Sure, but I think we should get the userland visible behaviors right.
Ignoring implementation details:

* cgroup vs. css IDs doesn't matter for now.  css IDs aren't visible
  to userland anyway and it could be that keeping using idr as-is or
  always using 64bit IDs is the better solution for them.

* On 32bit ino setups, 32bit ino + gen as cgroup and export fs IDs.

* On 64bit ino setups, 64bit unique ino (allocated whichever way) + 0
  gen as cgroup and export fs IDs.

Thanks.

-- 
tejun