linux-kernel - Re: [PATCH 1/3] perf tools: Use /proc/<PID>/task/<TID>/status for synthesis

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Date:   Mon, 1 Feb 2021 13:41:13 +0900
From:   Namhyung Kim <namhyung@...nel.org>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        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>,
        Andi Kleen <ak@...ux.intel.com>,
        Ian Rogers <irogers@...gle.com>
Subject: Re: [PATCH 1/3] perf tools: Use /proc/<PID>/task/<TID>/status for synthesis

On Mon, Feb 1, 2021 at 8:00 AM Jiri Olsa <jolsa@...hat.com> wrote:
>
> On Fri, Jan 29, 2021 at 02:48:59PM +0900, Namhyung Kim wrote:
> > To save memory usage, it needs to reduce number of entries in the proc
> > filesystem.  It's using /proc/<PID>/task directory to traverse threads
> > in the process and then kernel creates /proc/<PID>/task/<TID> entries.
> >
> > After that it checks the thread info using the /proc/<TID>/status file
> > rather than /proc/<PID>/task/<TID>/status.  As far as I can see, they
> > are the same and contain all the info we need.
> >
> > Using the latter eliminates the unnecessary /proc/<TID> entry.  This
> > can be useful especially a large number of threads are used in the
> > system.  In my experiment around 1KB of memory on average was saved
> > for each thread (which is not a thread group leader).
> >
> > To do this, pass both pid and tid to perf_event_prepare_comm() if it
> > knows them.  In case it doesn't know, passing 0 as pid will do the old
> > way.
> >
> > Signed-off-by: Namhyung Kim <namhyung@...nel.org>
> > ---
> >  tools/perf/util/synthetic-events.c | 30 +++++++++++++++++++-----------
> >  1 file changed, 19 insertions(+), 11 deletions(-)
> >
> > diff --git a/tools/perf/util/synthetic-events.c b/tools/perf/util/synthetic-events.c
> > index 3a898520f05c..800522591dde 100644
> > --- a/tools/perf/util/synthetic-events.c
> > +++ b/tools/perf/util/synthetic-events.c
> > @@ -69,7 +69,7 @@ int perf_tool__process_synth_event(struct perf_tool *tool,
> >   * Assumes that the first 4095 bytes of /proc/pid/stat contains
> >   * the comm, tgid and ppid.
> >   */
> > -static int perf_event__get_comm_ids(pid_t pid, char *comm, size_t len,
> > +static int perf_event__get_comm_ids(pid_t pid, pid_t tid, char *comm, size_t len,
> >                                   pid_t *tgid, pid_t *ppid)
> >  {
> >       char bf[4096];
> > @@ -81,7 +81,10 @@ static int perf_event__get_comm_ids(pid_t pid, char *comm, size_t len,
> >       *tgid = -1;
> >       *ppid = -1;
> >
> > -     snprintf(bf, sizeof(bf), "/proc/%d/status", pid);
> > +     if (pid)
> > +             snprintf(bf, sizeof(bf), "/proc/%d/task/%d/status", pid, tid);
> > +     else
> > +             snprintf(bf, sizeof(bf), "/proc/%d/status", tid);
> >
> >       fd = open(bf, O_RDONLY);
> >       if (fd < 0) {
> > @@ -93,7 +96,7 @@ static int perf_event__get_comm_ids(pid_t pid, char *comm, size_t len,
> >       close(fd);
> >       if (n <= 0) {
> >               pr_warning("Couldn't get COMM, tigd and ppid for pid %d\n",
> > -                        pid);
> > +                        tid);
> >               return -1;
> >       }
> >       bf[n] = '\0';
> > @@ -116,27 +119,32 @@ static int perf_event__get_comm_ids(pid_t pid, char *comm, size_t len,
> >               memcpy(comm, name, size);
> >               comm[size] = '\0';
> >       } else {
> > -             pr_debug("Name: string not found for pid %d\n", pid);
> > +             pr_debug("Name: string not found for pid %d\n", tid);
> >       }
> >
> >       if (tgids) {
> >               tgids += 5;  /* strlen("Tgid:") */
> >               *tgid = atoi(tgids);
> > +
> > +             if (pid && pid != *tgid) {
> > +                     pr_debug("Tgid: not match to given pid: %d vs %d\n",
> > +                              pid, *tgid);
>
> hm, could this actually happen in our case?

Probably not.  I'll remove it in the next version if you want.

Thanks,
Namhyung