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SubjectRe: Linux 2.6.32-rc1
On Mon, 28 Sep 2009 08:39:31 -0700 (PDT)
Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Mon, 28 Sep 2009, Eric Dumazet wrote:
> > Linus Torvalds a écrit :
> > >
> > > Go wild, test it out, and let us know about any regressions you find,
> >
> > Something seems wrong with process time accounting.
> >
> > Following program should consume 10*8 seconds of cpu on a 8 cpu machine, but
> > with 2.6.32-rc1 numbers are crazy.
>
> Ok, so the top-level process time looks sane _while_ the thing is running
> (sum of all threads), but the per-thread times look broken (as if they had
> randomly had the times of the other threads added into them - looks to me
> like they on average had half the other threads' time accounted into
> them).
>
> And then at the end, it looks like the time of the threads (which was
> over-accounted) get re-accounted back into the main process, so the final
> time is indeed wildly inflated.
>
> IOW, looks like we're adding thread times multiple times to other threads
> (and then finally to the parent).
>
> I'm adding the usual suspects to the cc, and leaving your results and
> test-program quoted here for them.. Thomas, Martin, John - if you're not
> the people I should blame for this, let me know.

Uh-oh.. usual suspects, eh?

For starters I run the test program on an s390 with
VIRT_CPU_ACCOUNTING=y:

time ./burn-cpu
PID TTY STAT TIME COMMAND
2979 pts/0 - 0:08 ./burn-cpu
- - Sl+ 0:00 -
- - Rl+ 0:01 -
- - Rl+ 0:01 -
- - Rl+ 0:01 -
- - Rl+ 0:01 -
- - Rl+ 0:01 -
- - Rl+ 0:01 -
- - Rl+ 0:01 -
- - Rl+ 0:01 -
PID TTY STAT TIME COMMAND
2979 pts/0 - 0:16 ./burn-cpu
- - Sl+ 0:00 -
- - Rl+ 0:02 -
- - Rl+ 0:02 -
- - Rl+ 0:02 -
- - Rl+ 0:02 -
- - Rl+ 0:02 -
- - Rl+ 0:02 -
- - Rl+ 0:02 -
- - Rl+ 0:02 -
PID TTY STAT TIME COMMAND
2979 pts/0 - 0:25 ./burn-cpu
- - Sl+ 0:00 -
- - Rl+ 0:03 -
- - Rl+ 0:03 -
- - Rl+ 0:03 -
- - Rl+ 0:03 -
- - Rl+ 0:03 -
- - Rl+ 0:03 -
- - Rl+ 0:03 -
- - Rl+ 0:03 -
PID TTY STAT TIME COMMAND
2979 pts/0 - 0:33 ./burn-cpu
- - Sl+ 0:00 -
- - Rl+ 0:04 -
- - Rl+ 0:04 -
- - Rl+ 0:04 -
- - Rl+ 0:04 -
- - Rl+ 0:04 -
- - Rl+ 0:04 -
- - Rl+ 0:04 -
- - Rl+ 0:04 -
PID TTY STAT TIME COMMAND
2979 pts/0 - 0:41 ./burn-cpu
- - Sl+ 0:00 -
- - Rl+ 0:05 -
- - Rl+ 0:05 -
- - Rl+ 0:05 -
- - Rl+ 0:05 -
- - Rl+ 0:05 -
- - Rl+ 0:05 -
- - Rl+ 0:05 -
- - Rl+ 0:05 -
PID TTY STAT TIME COMMAND
2979 pts/0 - 0:50 ./burn-cpu
- - Sl+ 0:00 -
- - Rl+ 0:06 -
- - Rl+ 0:06 -
- - Rl+ 0:06 -
- - Rl+ 0:06 -
- - Rl+ 0:06 -
- - Rl+ 0:06 -
- - Rl+ 0:06 -
- - Rl+ 0:06 -
PID TTY STAT TIME COMMAND
2979 pts/0 - 0:58 ./burn-cpu
- - Sl+ 0:00 -
- - Rl+ 0:07 -
- - Rl+ 0:07 -
- - Rl+ 0:07 -
- - Rl+ 0:07 -
- - Rl+ 0:07 -
- - Rl+ 0:07 -
- - Rl+ 0:07 -
- - Rl+ 0:07 -
PID TTY STAT TIME COMMAND
2979 pts/0 - 1:07 ./burn-cpu
- - Sl+ 0:00 -
- - Rl+ 0:08 -
- - Rl+ 0:08 -
- - Rl+ 0:08 -
- - Rl+ 0:08 -
- - Rl+ 0:08 -
- - Rl+ 0:08 -
- - Rl+ 0:08 -
- - Rl+ 0:08 -
PID TTY STAT TIME COMMAND
2979 pts/0 - 1:15 ./burn-cpu
- - Sl+ 0:00 -
- - Rl+ 0:09 -
- - Rl+ 0:09 -
- - Rl+ 0:09 -
- - Rl+ 0:09 -
- - Rl+ 0:09 -
- - Rl+ 0:09 -
- - Rl+ 0:09 -
- - Rl+ 0:09 -
PID TTY STAT TIME COMMAND
2979 pts/0 - 1:25 ./burn-cpu
- - S+ 1:25 -

real 0m10.708s
user 1m25.051s
sys 0m0.174s

looks better, gives an average of 10.63 seconds per thread and the per
thread numbers are fine. I'll see what happens with the test case on my
pc@home.

--
blue skies,
Martin.

"Reality continues to ruin my life." - Calvin.

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