Re: [RFC][PATCH 09/10] taskstats: Fix exit CPU time accounting

[Roland McGrath <roland@redhat.com>]

> Thanks! That information was missing! Although still for me it not seems
> to be a good decision to do it that way. Because of that it currently is
> not possible to evaluate all consumed CPU time by looking at the current
> processes. Time can simply disappear.

I agree that it seems dubious.  I don't know why that decision was made in
POSIX, but that's how it is.  Anyway, POSIX only constrains what we report
in the POSIX calls, i.e. getrusage, times, waitid, SIGCHLD siginfo_t.
Nothing says we can't track more information and make it accessible in
other ways on Linux.

> What about adding a new set of CPU time fields (e.g. cr-times) for the
> cumulative "autoreap" children times to the signal struct and export
> them via taskstats?

I don't have a particular opinion about the details of how you export the
information.  Something generally along those lines certainly sounds
reasonable to me.

> Then the following set of CPU times will give a complete picture (I also
> added steal time (st) that is currently not accounted in Linux per
> task):
> 
> * task->(u/s/st-time):
>   Time that has been consumed by task itself

There is also "gtime", "guest time" when the task is a kvm vcpu.

There is also "sched time" (task->se.sum_exec_runtime), which is
all states of task time, tracked by a different method than [usg]time.

> * task->signal->(c-u/s/st-time):
>   Time that has been consumed by dead children of process where parent
>   has done a sys_wait()

Also cgtime (to gtime as cutime is to utime).

> * task->signal->(u/s/st-time):
>   Time that has been consumed by dead threads of thread group of process
>   - NEW: Has to be exported via taskstats

Also gtime here.  These are reported as part of the aggregate process times
that include both live and dead threads, but not distinguished.

> * task->signal->(cr-u/s/st-time):
>   Time that has been consumed by dead children that reaped 
>   themselves, because parent ignored SIGCHLD or has set SA_NOCLDWAIT
>   - NEW: Fields have to be added to signal struct
>   - NEW: Has to be exported via taskstats

Note that there are other stats aside from times that are treated the same
way (c{min,maj}_flt, cn{v,iv}csw, c{in,ou}block, cmaxrss, and io accounting).

What probably makes sense is to move all those cfoo fields from
signal_struct into foo fields in a new struct, and then signal_struct can
have "struct child_stats reaped_children, ignored_children" or whatnot.


Thanks,
Roland