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    SubjectRe: High lock spin time for zone->lru_lock under extreme conditions
    On Sat, Jan 13, 2007 at 12:00:17AM -0800, Andrew Morton wrote:
    > > On Fri, 12 Jan 2007 23:36:43 -0800 Ravikiran G Thirumalai <kiran@scalex86.org> wrote:
    > > > >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...)

    Well, that is exactly what I described above as CS time. The
    instrumentation goes like this:

    void __lockfunc _spin_lock_irq(spinlock_t *lock)
    {
    unsigned long long t1,t2;
    local_irq_disable();
    t1 = get_cycles_sync();
    preempt_disable();
    spin_acquire(&lock->dep_map, 0, 0, _RET_IP_);
    _raw_spin_lock(lock);
    t2 = get_cycles_sync();
    lock->raw_lock.htsc = t2;
    if (lock->spin_time < (t2 - t1))
    lock->spin_time = t2 - t1;
    }
    ...

    void __lockfunc _spin_unlock_irq(spinlock_t *lock)
    {
    unsigned long long t1 ;
    spin_release(&lock->dep_map, 1, _RET_IP_);
    t1 = get_cycles_sync();
    if (lock->cs_time < (t1 - lock->raw_lock.htsc))
    lock->cs_time = t1 - lock->raw_lock.htsc;
    _raw_spin_unlock(lock);
    local_irq_enable();
    preempt_enable();
    }

    Am I missing something? Is this not what you just described? (The
    synchronizing rdtsc might not be really required at all locations, but I
    doubt if it would contribute a significant fraction to 33s or even
    the 3s hold time on a 2.6 GHZ opteron).
    -
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