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

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Date:   Mon, 7 Oct 2019 07:16:51 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     Song Liu <liu.song.a23@...il.com>, 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

Hello,

On Wed, Oct 02, 2019 at 03:28:00PM +0900, Namhyung Kim wrote:
> On Sat, Sep 21, 2019 at 6:04 AM Tejun Heo <tj@...nel.org> wrote:
> >
> > On Fri, Sep 20, 2019 at 05:47:45PM +0900, Namhyung Kim wrote:
> > > 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
> >
> > It's not.  gen gets incremented on every allocation, so it has not
> > high but still realistic chance of collisions.
> 
> In __kernfs_new_node(), gen gets increased only if idr_alloc_cyclic()
> returns lower than the cursor...  I'm not sure you talked about it.

Ah, I forgot that it's using cyclic idr, so yeah, it's not as bad in
terms of recycling although cyclic allocation on idr is pretty
inefficient.  I still think it'd be better to switch to rbtree and so
that 64bit can simply use monotonically increasing numbers but that
definitely isn't a must and we can juse continue with the current
allocation method.

Thanks.

-- 
tejun