linux-kernel - Re: [PATCH 2/9] perf/core: Add PERF_SAMPLE

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Message-ID: <CAPhsuW6zxkKd3ExXj5GCq+4yEWv85qmM9_Weatv4PoVd+aWKDQ@mail.gmail.com>
Date:   Fri, 20 Sep 2019 09:13:04 -0700
From:   Song Liu <liu.song.a23@...il.com>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     Tejun Heo <tj@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Stephane Eranian <eranian@...gle.com>,
        Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH 2/9] perf/core: Add PERF_SAMPLE_CGROUP feature

Hi Namhyung,

On Fri, Sep 20, 2019 at 1:47 AM Namhyung Kim <namhyung@...nel.org> wrote:
>
> Hello Song,
>
> On Thu, Sep 19, 2019 at 3:43 PM Song Liu <liu.song.a23@...il.com> wrote:
>
> > Sharing some offline discussions with Tejun.
> >
> > ino in current kernfs is not a good unique ID for cgroup, because it doesn't
> > increase monotonically. So we need to improve kernfs.
> >
> > For 64-bit, we can make the ino monotonic, and use it as the ID.
> > For 32-bit, we need to make the ino monotonic. and use <ino> and <gen>
> > as the 64-bit ID.
>
> Thanks for the sharing information!  For 32-bit, while the ino itself is not
> monotonic, gen << 32 + ino is monotonic right?  I think we can use the
> same logic of kernfs id allocation, but not sure what the problem Tejun
> mentioned before is.

How would we manage gen here? One way that works is:
1. make ino monotonic,
2. increase gen when 32-bit ino overflows

I think current kernfs id is not monotonic, so it can be reused before overflow.

Thanks,
Song