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SubjectRe: [PATCH 1/3] perf tools: Use /proc/<PID>/task/<TID>/status for synthesis
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 <namhyung@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?

jirka

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