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

[Jiri Olsa]

On Mon, Dec 21, 2020 at 04:00:28PM +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.

how much does this save? for kernel threads the maps file is empty
and we just open the file and close

also perhaps just stat(".....maps") and check the size would be easier?

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@xxxxxxxxxx>
> ---
>  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 515d145a4303..153a822f411a 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)
> @@ -703,6 +716,7 @@ 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)
> @@ -710,7 +724,7 @@ 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,
> @@ -728,7 +742,7 @@ 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);
> -- 
> 2.29.2.684.gfbc64c5ab5-goog
>