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    SubjectRe: [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@redhat.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@kernel.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

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