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    SubjectRe: High lock spin time for zone->lru_lock under extreme conditions
    > On Fri, 12 Jan 2007 23:36:43 -0800 Ravikiran G Thirumalai <kiran@scalex86.org> wrote:
    > On Sat, Jan 13, 2007 at 03:39:45PM +1100, Nick Piggin wrote:
    > > Ravikiran G Thirumalai wrote:
    > > >Hi,
    > > >We noticed high interrupt hold off times while running some memory
    > > >intensive
    > > >tests on a Sun x4600 8 socket 16 core x86_64 box. We noticed softlockups,
    > >
    > > [...]
    > >
    > > >We did not use any lock debugging options and used plain old rdtsc to
    > > >measure cycles. (We disable cpu freq scaling in the BIOS). All we did was
    > > >this:
    > > >
    > > >void __lockfunc _spin_lock_irq(spinlock_t *lock)
    > > >{
    > > > local_irq_disable();
    > > > ------------------------> rdtsc(t1);
    > > > preempt_disable();
    > > > spin_acquire(&lock->dep_map, 0, 0, _RET_IP_);
    > > > _raw_spin_lock(lock);
    > > > ------------------------> rdtsc(t2);
    > > > if (lock->spin_time < (t2 - t1))
    > > > lock->spin_time = t2 - t1;
    > > >}
    > > >
    > > >On some runs, we found that the zone->lru_lock spun for 33 seconds or more
    > > >while the maximal CS time was 3 seconds or so.
    > >
    > > What is the "CS time"?
    >
    > Critical Section :). This is the maximal time interval I measured from
    > t2 above to the time point we release the spin lock. This is the hold
    > time I guess.

    By no means. The theory here is that CPUA is taking and releasing the
    lock at high frequency, but CPUB never manages to get in and take it. In
    which case the maximum-acquisition-time is much larger than the
    maximum-hold-time.

    I'd suggest that you use a similar trick to measure the maximum hold time:
    start the timer after we got the lock, stop it just before we release the
    lock (assuming that the additional rdtsc delay doesn't "fix" things, of
    course...)


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