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SubjectRe: [PATCH 2/3] perf tools: Skip MMAP record synthesis for kernel threads
On Fri, Jan 29, 2021 at 02:49:00PM +0900, Namhyung Kim wrote:
> 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. Also 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.

I don't understand this Threads: check, could you please
show example where it's useful for the check?

thanks,
jirka

>
> 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 800522591dde..8b38228c83d8 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;
> @@ -141,12 +147,17 @@ 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;
>
> @@ -157,7 +168,7 @@ 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 {
> @@ -187,8 +198,10 @@ 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)
> @@ -748,6 +761,7 @@ 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)
> @@ -755,7 +769,7 @@ 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,
> @@ -773,7 +787,7 @@ 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
>

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