Messages in this thread | | | Date | Fri, 21 Nov 2014 21:01:38 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH -mm 1/3] exit: reparent: avoid find_new_reaper() if no children |
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On 11/20, Andrew Morton wrote: > > On Thu, 20 Nov 2014 19:34:23 +0100 Oleg Nesterov <oleg@redhat.com> wrote: > > > $ time ./test 16 16536 shows: > > > > real user sys > > - 5m37.628s 0m4.437s 8m5.560s > > + 0m50.032s 0m7.130s 1m4.927s > > Is that the best you can do?
Unfortunately these changes do not even try to solve the main problem, tasklist_lock doesn't scale simply because it is global. These changes make sense (I hope) anyway, even if/when we redesign the locking. But so far I do not have a good plan.
> (I assume the increase in user time was a glitch?)
To be honest, I didn't even notice this change. I repeated the testing before/after this patch and (to my surprize) the "user" numbers are more or less stable, and /usr/bin/time reports the increase.
1. First of all: this is impossible ;)
Note that this test-case uses SIGTRAP to trigger the coredumping. This means that exit_notify() can only be called when all threads are already in kernel mode, the coredumping thread sleeps until they all are parked in exit_mm(). Until then this patch has no effect.
2. With this patch applied, I added mdelay(2) into forget_original_parent(), right after find_child_reaper(). And yes, this changes the numbers too:
real user sys 10m1.225s 0m5.443s 17m25.797s
note that "user time" goes down.
3. So I think that this just reminds that utime/stime accounting isn't precise. sum_exec_runtime is accurate and thus we can more or less trust utime + stime, but utime/stime is random. Plus scale_stime() doesn't look very accurate too.
4. In this particular case the accounting is even more impresize, this test-case spends a lot of time in kernel mode with irqs disabled and this "freezes" task->stime.
5. That said, I still can't really understand why "user" grows. If I understand the calculations in cputime_adjust() correctly (probably I don't), it should not.
In short, I am a bit confused but I still don't think that this increase is real.
Oleg.
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