Re: High lock spin time for zone->lru_lock under extreme conditions

[Ravikiran G Thirumalai <kiran@scalex86.org>]

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.

> 
> It would be interesting to know how long the maximal lru_lock *hold* time 
> is,
> which could give us a better indication of whether it is a hardware problem.
> 
> For example, if the maximum hold time is 10ms, that it might indicate a
> hardware fairness problem.

The maximal hold time was about 3s.
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