On Mon, Feb 01, 2021 at 01:41:13PM +0900, Namhyung Kim wrote:
> On Mon, Feb 1, 2021 at 8:00 AM Jiri Olsa <jo...@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 <namhy...@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.
IMO if there's no reason let's remove it
jirka
