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    SubjectRe: regression introduced by - timers: fix itimer/many thread hang
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    On Mon, 2008-11-24 at 13:32 +0100, Petr Tesarik wrote:

    > > Feel like reading the actual spec and trying to come up with a creative
    > > interpretation? :-)
    >
    > Yes, I've just spent a few hours doing that... And I feel very
    > depressed, as expected.

    Thanks for doing that though!

    > > > I really don't think it's a good idea to set a per-process ITIMER_PROF
    > > > to one timer tick on a large machine, but the kernel does allow any
    > > > process to do it, and then it can even cause hard freeze on some
    > > > hardware. This is _not_ acceptable.
    > > >
    > > > What is worse, we can't just limit the granularity of itimers, because
    > > > threads can come into being _after_ the itimer was set.
    > >
    > > Currently it has jiffy granularity, right? And jiffies are different
    > > depending on some compile time constant (HZ), so can't we, for the sake
    > > of per-process itimers, pretend to have a 1 minute jiffie?
    > >
    > > That should be as compliant as we are now, and utterly useless for
    > > everybody, thereby discouraging its use, hmm? :-)
    > 
    > I've got a copy of IEEE Std 10003.1-2004 here, and it suggests that this
    > should be generally possible. In particular, the description for
    > itimer_set says:
    >
    > Implementations may place limitations on the granularity of timer values. For
    > each interval timer, if the requested timer value requires a finer granularity
    > than the implementation supports, the actual timer value shall be rounded up
    > to the next supported value.
    >
    > However, it seems to be vaguely linked to CLOCK_PROCESS_CPUTIME_ID,
    > which is defined as:
    >
    > The identifier of the CPU-time clock associated with the process making a
    > clock ( ) or timer*( ) function call.
    >
    > POSIX does not specify whether this clock is identical to the one used
    > for setitimer et al., or not, but it seems logical that it should. Then,
    > the kernel should probably return the coarse granularity in
    > clock_getres(), too.
    >
    > I tried to find out how this is currently implemented in Linux, and it's
    > broken. How else. :-/
    >
    > 1. clock_getres() always returns a resolution of 1ns
    >
    > This is actually good news, because it means that nobody really cares
    > whether the actual granularity is greater, so I guess we can safely
    > return any bogus number in clock_getres().
    >
    > What about using an actual granularity of NR_CPUS*HZ, which should be
    > safe for any (at least remotely) sane usage?

    nr_cpu_ids * 1/HZ should do I guess, although a cubic function would buy
    us even more slack.

    > 2. clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &ts) returns -EINVAL
    >
    > Should not happen. Looking further into it, I think this line in
    > cpu_clock_sample_group():
    >
    > switch (which_clock) {
    >
    > should look like a similar line in cpu_clock_sample(), ie:
    >
    > switch (CPUCLOCK_WHICH(which_clock)) {
    >
    > Shall I send a patch?

    Feel free - its not an area I'm intimately familiar with, I'll look into
    whipping up a patch removing all the per-cpu crap from there.


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