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    SubjectRe: [GIT PULL] cputime patch for 2.6.30-rc6
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    On Tue, 2009-05-19 at 11:00 +0200, Martin Schwidefsky wrote:
    > On Mon, 18 May 2009 17:28:53 +0100 (BST)
    > Michael Abbott <michael@araneidae.co.uk> wrote:
    >
    > > > > + for_each_possible_cpu(i)
    > > > > + idletime = cputime64_add(idletime, kstat_cpu(i).cpustat.idle);
    > > > > + idletime = cputime64_to_clock_t(idletime);
    > > > >
    > > > > do_posix_clock_monotonic_gettime(&uptime);
    > > > > monotonic_to_bootbased(&uptime);
    > > >
    > > > This is a world readable proc file, adding a for_each_possible_cpu() in
    > > > there scares me a little (this wouldn't be the first and only such case
    > > > though).
    > > >
    > > > Suppose you have lots of cpus, and all those cpus are dirtying those
    > > > cachelines (who's updating idle time when they're idle?), then this loop
    > > > can cause a massive cacheline bounce fest.
    > > >
    > > > Then think about userspace doing:
    > > > while :; do cat /proc/uptime > /dev/null; done
    > >
    > > Well, the offending code derives pretty well directly from /proc/stat,
    > > which is used, for example, by top. So if there is an issue then I guess
    > > it already exists.
    > >
    > > There is a pending problem in this code: for a multiple cpu system we'll
    > > end up with more idle time than elapsed time, which is not really very
    > > nice. Unfortunately *something* has to be done here, as it looks as if
    > > .utime and .stime (at least for init_task) have lost any meaning. I sort
    > > of though of dividing by number of cpus, but that's not going to work very
    > > well..
    >
    > I don't see a problem here. In an idle multiple cpu system there IS
    > more idle time than elapsed time. What would makes sense is to compare
    > elapsed time * #cpus with the idle time. But then there is cpu hotplug
    > which forces you to look at the delta of two measuring points where the
    > number of cpus did not change.

    Sure, this one case isn't that bad, esp. as you note its about idle
    time. However, see for example /proc/stat and fs/proc/stat.c:

    for_each_possible_cpu(i) {
    user = cputime64_add(user, kstat_cpu(i).cpustat.user);
    nice = cputime64_add(nice, kstat_cpu(i).cpustat.nice);
    system = cputime64_add(system, kstat_cpu(i).cpustat.system);
    idle = cputime64_add(idle, kstat_cpu(i).cpustat.idle);
    idle = cputime64_add(idle, arch_idle_time(i));
    iowait = cputime64_add(iowait, kstat_cpu(i).cpustat.iowait);
    irq = cputime64_add(irq, kstat_cpu(i).cpustat.irq);
    softirq = cputime64_add(softirq, kstat_cpu(i).cpustat.softirq);
    steal = cputime64_add(steal, kstat_cpu(i).cpustat.steal);
    guest = cputime64_add(guest, kstat_cpu(i).cpustat.guest);
    for_each_irq_nr(j) {
    sum += kstat_irqs_cpu(j, i);
    }
    sum += arch_irq_stat_cpu(i);
    }

    If that isn't a problem on a large machine, then I don't know what is.



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