Hi, As of 5.11 kernel (pid,pid_start_time) is not unique /monotonic even though the underlying counters are . I chose start_boottime because i wanted the counter to increase during suspend as well. 1. Is there any case where task->start_boottime or ktime_get_boottime_ns doesn't become monotonic i.e increasing ? 2. If start_boottime is not monotonic which counter to use ? 3. If i create a new field in task_struct , then i can use a atomic_add_return(1,&v) to fill in the task->new_field. Will this also work ? By doing this <pid,pid_start_time> becomes unique. In linux/fs/proc/array.c at line 566 we have /* apply timens offset for boottime and convert nsec -> ticks */ start_time = nsec_to_clock_t(timens_add_boottime_ns(task->start_boottime)); task->start_boottime is a monotonic increasing counter fetched from ktime_get_boottime_ns in fork.c nsec_to_clock_t contains div_u64 due to which we loose some lower bits/digits on divison and is not unique unless the divisor is 1. if CONFIG_HZ = 250 and nsec_to_clock_t x=4000001 , then #if (NSEC_PER_SEC % USER_HZ) == 0 return div_u64(x, NSEC_PER_SEC / USER_HZ); becomes div_u64(x, 4000000) then return value is 1 when x=4000002, return value is 1 until x=8000000 which returns 2. The value shown in /proc/[pid]/stat is actually the truncated value. Hence i'm planning to display a counter at the end of /proc/[pid]/stat as the 53rd field. I've prepared a patch as inlined. >From a2c6b5d6435394f015d38700008ff74f16dfa5fd Mon Sep 17 00:00:00 2001 From: Navin P <navinp0304@xxxxxxxxx> Date: Thu, 25 Mar 2021 15:27:30 +0530 Subject: [PATCH] Display task->start_boottime as 53rd field in /proc/[pid]/stat. The 22nd field start_time currently shown in /proc/[pid]/stat as start_time is truncated by division.Hence it is not unique . Signed-off-by: Navin P <navinp0304@xxxxxxxxx> --- Documentation/filesystems/proc.rst | 1 + fs/proc/array.c | 1 + 2 files changed, 2 insertions(+) diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst index 48fbfc336ebf..3b7a1543b2c0 100644 --- a/Documentation/filesystems/proc.rst +++ b/Documentation/filesystems/proc.rst @@ -381,6 +381,7 @@ It's slow but very precise. env_end address below which program environment is placed exit_code the thread's exit_code in the form reported by the waitpid system call + start_boottime the process start time in nanoseconds since boot ============= =============================================================== The /proc/PID/maps file contains the currently mapped memory regions and diff --git a/fs/proc/array.c b/fs/proc/array.c index bb87e4d89cd8..74389aaefa9c 100644 --- a/fs/proc/array.c +++ b/fs/proc/array.c @@ -645,6 +645,7 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns, else seq_puts(m, " 0"); + seq_put_decimal_ull(m, " ", task->start_boottime); seq_putc(m, '\n'); if (mm) mmput(mm); -- 2.25.1 _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
