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

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Date:   Fri, 19 Feb 2021 08:37:00 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Namhyung Kim <namhyung@...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

Em Fri, Feb 19, 2021 at 07:05:59PM +0900, Namhyung Kim escreveu:
> On Wed, Feb 17, 2021 at 9:58 PM Arnaldo Carvalho de Melo <acme@...nel.org> wrote:
> > Em Fri, Jan 08, 2021 at 02:51:44PM +0900, Namhyung Kim escreveu:
> > > On Wed, Jan 6, 2021 at 10:33 AM Namhyung Kim <namhyung@...nel.org> wrote:

> > > 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.).

> > Ok, we can have something in 'perf daemon' to periodically invalidate
> > this, maybe do a poor man inotify and when asking for the cgroup
> > mountpoint, check some characteristic of that file that changes when it
> > is modified, or plain use a timestamp and have some threshold.
 
> I thought about this again.
 
> We don't directly access the cgroups in the perf daemon.  It just
> creates new record processes so they'll see a new mountpoint whenever
> they started since this cache is shared within the process only.
 
> That means we don't need to care about the invalidate in the daemon
> but each perf record and perf stat should do it when they are required
> to do the work repeatedly.
 
> But looking at the code, the cgroup is set during event parsing (-G
> option) or early in the command (--for-each-cgroup option).  So cgroup
> info would not be changed even if the command runs repeatedly.
 
> So I think you can take the patch as is.

Its in perf/core branch on its way to Linus soon :-)

Thanks for checking it.

- Arnaldo