linux-kernel - Re: [PATCH 3/3] tools/lib/fs: Cache cgroupfs mount point

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Date:   Fri, 8 Jan 2021 14:51:44 +0900
From:   Namhyung Kim <namhyung@...nel.org>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     Jiri Olsa <jolsa@...hat.com>, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Stephane Eranian <eranian@...gle.com>,
        Ian Rogers <irogers@...gle.com>,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 3/3] tools/lib/fs: Cache cgroupfs mount point

On Wed, Jan 6, 2021 at 10:33 AM Namhyung Kim <namhyung@...nel.org> wrote:
>
> Hi Arnaldo,
>
> On Tue, Dec 29, 2020 at 8:51 PM Arnaldo Carvalho de Melo
> <acme@...nel.org> wrote:
> >
> > Em Wed, Dec 16, 2020 at 06:05:56PM +0900, Namhyung Kim escreveu:
> > > Currently it parses the /proc file everytime it opens a file in the
> > > cgroupfs.  Save the last result to avoid it (assuming it won't be
> > > changed between the accesses).
> >
> > Which is the most likely case, but can't we use something like inotify
> > to detect that and bail out or warn the user?
>
> Hmm.. looks doable.  Will check.

So I've played with inotify a little bit, and it seems it needs to monitor
changes on the file or the directory.  I didn't get any notification from
the /proc/mounts file even if I did some mount/umount.

Instead, I could get IN_UNMOUNT when the cgroup filesystem was
unmounted.  But for the monitoring, we need to do one of a) select-like
syscall to wait for the events, b) signal-driven IO notification or c) read
the inotify file with non-block mode everytime.

In a library code, I don't think we can do a) or b) since it can affect
user program behaviors.  Then we should go with c) but I think
it's opposite to the purpose of this patch. :)

As you said, I think mostly we don't care as the accesses will happen
in a short period of time.  But if you really care, maybe for the upcoming
perf daemon changes, I think we can add an API to invalidate the cache
or internal time-based invalidation logic (like remove it after 10 sec.).

Thoughts?

Thanks,
Namhyung