[PATCH 1/3] perf tools: Use /proc//task//status for synthesis

Namhyung Kim
2021-02-02T09:03:03Z

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 | 25 ++++++++++++++-----------
1 file changed, 14 insertions(+), 11 deletions(-)

diff --git a/tools/perf/util/synthetic-events.c b/tools/perf/util/synthetic-events.c
index 3a898520f05c..0cc998663b03 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 +119,27 @@ 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);
} else {
- pr_debug("Tgid: string not found for pid %d\n", pid);
+ pr_debug("Tgid: string not found for pid %d\n", tid);
}

if (ppids) {
ppids += 5; /* strlen("PPid:") */
*ppid = atoi(ppids);
} else {
- pr_debug("PPid: string not found for pid %d\n", pid);
+ pr_debug("PPid: string not found for pid %d\n", tid);
}

return 0;
}

-static int perf_event__prepare_comm(union perf_event *event, pid_t pid,
+static int perf_event__prepare_comm(union perf_event *event, pid_t pid, pid_t tid,
struct machine *machine,
pid_t *tgid, pid_t *ppid)
{
@@ -147,7 +150,7 @@ static int perf_event__prepare_comm(union perf_event *event, pid_t pid,
memset(&event->comm, 0, sizeof(event->comm));

if (machine__is_host(machine)) {
- if (perf_event__get_comm_ids(pid, event->comm.comm,
+ if (perf_event__get_comm_ids(pid, tid, event->comm.comm,
sizeof(event->comm.comm),
tgid, ppid) != 0) {
return -1;
@@ -168,7 +171,7 @@ static int perf_event__prepare_comm(union perf_event *event, pid_t pid,
event->comm.header.size = (sizeof(event->comm) -
(sizeof(event->comm.comm) - size) +
machine->id_hdr_size);
- event->comm.tid = pid;
+ event->comm.tid = tid;

return 0;
}
@@ -180,7 +183,7 @@ pid_t perf_event__synthesize_comm(struct perf_tool *tool,
{
pid_t tgid, ppid;

- if (perf_event__prepare_comm(event, pid, machine, &tgid, &ppid) != 0)
+ if (perf_event__prepare_comm(event, 0, pid, machine, &tgid, &ppid) != 0)
return -1;

if (perf_tool__process_synth_event(tool, event, machine, process) != 0)
@@ -746,7 +749,7 @@ static int __event__synthesize_thread(union perf_event *comm_event,
continue;

rc = -1;
- if (perf_event__prepare_comm(comm_event, _pid, machine,
+ if (perf_event__prepare_comm(comm_event, pid, _pid, machine,
&tgid, &ppid) != 0)
break;

--
2.30.0.365.g02bc693789-goog