On Tue, 1 Jul 2008, Wolfgang Grandegger wrote:
>
> I continue this thread because it's still not understood why enabling
> CONFIG_RCU_TRACE is necessary to get reasonable latencies on the
> MPC5200. It might also explain, why I get much worse latencies with
> 2.6.25.8-rt7.
Have you tried turning on ftrace?
>
> To recapitulate, with CONFIG_RCU_TRACE enabled, cyclictest reports max.
> latencies of approx. 130 us with 2.6.24.4-rt4 on my MPC5200 PowerPC
> board. If I disable it, the latency goes up to 600 us. Obviously, the
> trace_mark() calls in rcupreempt*.c have some positive impact on the
> latency. I narrowed down, that the 2 calls in __rcu_preempt_boost() in
> rcupreempt-boost.c are the important one:
>
> void __rcu_preempt_unboost(void)
> {
> struct task_struct *curr = current;
> struct rcu_boost_dat *rbd;
> int prio;
> unsigned long flags;
>
> trace_mark(unboost_called, "NULL");
>
> /* if not boosted, then ignore */
> if (likely(!rcu_is_boosted(curr)))
> return;
I wonder if the "likely" is faulty on the PPC code generation. Have you
tried removing that "likely" statement.
>
> /*
> * Need to be very careful with NMIs.
> * If we take the lock and an NMI comes in
> * and it may try to unboost us if curr->rcub_rbdp
> * is still set. So we zero it before grabbing the lock.
> * But this also means that we might be boosted again
> * so the boosting code needs to be aware of this.
> */
> rbd = curr->rcub_rbdp;
> curr->rcub_rbdp = NULL;
>
> /*
> * Now an NMI might have came in after we grab
> * the below lock. This check makes sure that
> * the NMI doesn't try grabbing the lock
> * while we already have it.
> */
> if (unlikely(!rbd))
> return;
Actually, remove all "likely" and "unlikely". The marker code could be
making it work better. But still, this shouldn't cause 600us latencies.
>
> spin_lock_irqsave(&rbd->rbs_lock, flags);
> /*
> * It is still possible that an NMI came in
> * between the "is_boosted" check and setting
> * the rcu_rbdp to NULL. This would mean that
> * the NMI already dequeued us.
> */
> if (unlikely(!rcu_is_boosted(curr)))
> goto out;
>
> list_del_init(&curr->rcub_entry);
>
> trace_mark(unboosted, "NULL");
>
> curr->rcu_prio = MAX_PRIO;
>
> spin_lock(&curr->pi_lock);
> prio = rt_mutex_getprio(curr);
> task_setprio(curr, prio);
>
> curr->rcub_rbdp = NULL;
>
> spin_unlock(&curr->pi_lock);
> out:
> spin_unlock_irqrestore(&rbd->rbs_lock, flags);
> }
>
> With them and all other trace_mark() calls commented out, the latency is
> still OK. The first one has a bigger impact.
>
> In 2.6.25.8-rt7, trace_mark() is not used any more but a function
> incrementing the corresponding counter directly and I suspect that's the
> reason why I'm seeing high latencies with both, CONFIG_RCU_TRACE enabled
> and disabled.
>
> I hope this observation sheds some light on the issue.
It is still a mystery to me. Maybe looking at the different assembly
outputs with the different configurations.
-- Steve
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