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 21:41:17 +0100
From:   Jiri Olsa <jolsa@...hat.com>
To:     Namhyung Kim <namhyung@...nel.org>
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 01, 2021 at 01:41:13PM +0900, Namhyung Kim wrote:
> 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.

IMO if there's no reason let's remove it

jirka