[PATCH 2/3] perf tools: Skip MMAP record synthesis for kernel threads

Namhyung Kim
2021-02-02T09:02:35Z

To synthesize information to resolve sample IPs, it needs to scan task
and mmap info from the /proc filesystem. For each process, it
opens (and reads) status and maps file respectively. But as kernel
threads don't have memory maps so we can skip the maps file.

To find kernel threads, check "VmPeak:" line in /proc/<PID>/status
file. It's about the peak virtual memory usage so only user-level
tasks have that. Note that it's possible to miss the line due to
partial reads. So we should double-check if it's a really kernel
thread when there's no VmPeak line.

Thus check "Threads:" line (which follows the VmPeak line whether or
not it exists) to be sure it's read enough data - just in case of
deeply nested pid namespaces or large number of supplementary groups
are involved.

This is for user process:

$ head -40 /proc/1/status
Name: systemd
Umask: 0000
State: S (sleeping)
Tgid: 1
Ngid: 0
Pid: 1
PPid: 0
TracerPid: 0
Uid: 0 0 0 0
Gid: 0 0 0 0
FDSize: 256
Groups:
NStgid: 1
NSpid: 1
NSpgid: 1
NSsid: 1
VmPeak: 234192 kB <-- here
VmSize: 169964 kB
VmLck: 0 kB
VmPin: 0 kB
VmHWM: 29528 kB
VmRSS: 6104 kB
RssAnon: 2756 kB
RssFile: 3348 kB
RssShmem: 0 kB
VmData: 19776 kB
VmStk: 1036 kB
VmExe: 784 kB
VmLib: 9532 kB
VmPTE: 116 kB
VmSwap: 2400 kB
HugetlbPages: 0 kB
CoreDumping: 0
THP_enabled: 1
Threads: 1 <-- and here
SigQ: 1/62808
SigPnd: 0000000000000000
ShdPnd: 0000000000000000
SigBlk: 7be3c0fe28014a03
SigIgn: 0000000000001000

And this is for kernel thread:

$ head -20 /proc/2/status
Name: kthreadd
Umask: 0000
State: S (sleeping)
Tgid: 2
Ngid: 0
Pid: 2
PPid: 0
TracerPid: 0
Uid: 0 0 0 0
Gid: 0 0 0 0
FDSize: 64
Groups:
NStgid: 2
NSpid: 2
NSpgid: 0
NSsid: 0
Threads: 1 <-- here
SigQ: 1/62808
SigPnd: 0000000000000000
ShdPnd: 0000000000000000

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
---
tools/perf/util/synthetic-events.c | 32 +++++++++++++++++++++---------
1 file changed, 23 insertions(+), 9 deletions(-)

diff --git a/tools/perf/util/synthetic-events.c b/tools/perf/util/synthetic-events.c
index 0cc998663b03..abea6885f94b 100644
--- a/tools/perf/util/synthetic-events.c
+++ b/tools/perf/util/synthetic-events.c
@@ -70,13 +70,13 @@ int perf_tool__process_synth_event(struct perf_tool *tool,
* the comm, tgid and ppid.
*/
static int perf_event__get_comm_ids(pid_t pid, pid_t tid, char *comm, size_t len,
- pid_t *tgid, pid_t *ppid)
+ pid_t *tgid, pid_t *ppid, bool *kernel)
{
char bf[4096];
int fd;
size_t size = 0;
ssize_t n;
- char *name, *tgids, *ppids;
+ char *name, *tgids, *ppids, *vmpeak, *threads;

*tgid = -1;
*ppid = -1;
@@ -102,8 +102,14 @@ static int perf_event__get_comm_ids(pid_t pid, pid_t tid, char *comm, size_t len
bf[n] = '\0';

name = strstr(bf, "Name:");
- tgids = strstr(bf, "Tgid:");
- ppids = strstr(bf, "PPid:");
+ tgids = strstr(name ?: bf, "Tgid:");
+ ppids = strstr(tgids ?: bf, "PPid:");
+ vmpeak = strstr(ppids ?: bf, "VmPeak:");
+
+ if (vmpeak)
+ threads = NULL;
+ else
+ threads = strstr(ppids ?: bf, "Threads:");

if (name) {
char *nl;
@@ -136,12 +142,17 @@ static int perf_event__get_comm_ids(pid_t pid, pid_t tid, char *comm, size_t len
pr_debug("PPid: string not found for pid %d\n", tid);
}

+ if (!vmpeak && threads)
+ *kernel = true;
+ else
+ *kernel = false;
+
return 0;
}

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)
+ pid_t *tgid, pid_t *ppid, bool *kernel)
{
size_t size;

@@ -152,7 +163,7 @@ static int perf_event__prepare_comm(union perf_event *event, pid_t pid, pid_t ti
if (machine__is_host(machine)) {
if (perf_event__get_comm_ids(pid, tid, event->comm.comm,
sizeof(event->comm.comm),
- tgid, ppid) != 0) {
+ tgid, ppid, kernel) != 0) {
return -1;
}
} else {
@@ -182,8 +193,10 @@ pid_t perf_event__synthesize_comm(struct perf_tool *tool,
struct machine *machine)
{
pid_t tgid, ppid;
+ bool kernel_thread;

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

if (perf_tool__process_synth_event(tool, event, machine, process) != 0)
@@ -743,6 +756,7 @@ static int __event__synthesize_thread(union perf_event *comm_event,
while ((dirent = readdir(tasks)) != NULL) {
char *end;
pid_t _pid;
+ bool kernel_thread;

_pid = strtol(dirent->d_name, &end, 10);
if (*end)
@@ -750,7 +764,7 @@ static int __event__synthesize_thread(union perf_event *comm_event,

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

if (perf_event__synthesize_fork(tool, fork_event, _pid, tgid,
@@ -768,7 +782,7 @@ static int __event__synthesize_thread(union perf_event *comm_event,
break;

rc = 0;
- if (_pid == pid) {
+ if (_pid == pid && !kernel_thread) {
/* process the parent's maps too */
rc = perf_event__synthesize_mmap_events(tool, mmap_event, pid, tgid,
process, machine, mmap_data);
--
2.30.0.365.g02bc693789-goog