Messages in this thread | | | Date | Thu, 24 Oct 2002 02:20:26 +0530 | From | Dipankar Sarma <> | Subject | Re: [PATCH] NMI request/release, version 4 |
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Well, I haven't looked at the whole patch yet, but some quick responses -
On Wed, Oct 23, 2002 at 03:14:52PM -0500, Corey Minyard wrote: > I have noticed that the rcu callback can be delayed a long time, > sometimes up to 3 seconds. That seems odd. I have verified that the > delay happens there.
That kind of latency is really bad. Could you please check the latency with this change in kernel/rcupdate.c -
void rcu_check_callbacks(int cpu, int user) { if (user || - (idle_cpu(cpu) && !in_softirq() && hardirq_count() <= 1)) + (idle_cpu(cpu) && !in_softirq() && + hardirq_count() <= (1 << HARDIRQ_SHIFT))) RCU_qsctr(cpu)++; tasklet_schedule(&RCU_tasklet(cpu));
After local_irq_count() went away, the idle CPU check was broken and that meant that if you had an idle CPU, it could hold up RCU grace period completion.
> +void release_nmi(struct nmi_handler *handler) > +{ > + wait_queue_t q_ent; > + unsigned long flags; > + > + spin_lock_irqsave(&nmi_handler_lock, flags); > + list_del_rcu(&(handler->link)); > + > + /* Wait for handler to finish being freed. This can't be > + interrupted, we must wait until it finished. */ > + init_waitqueue_head(&(handler->wait)); > + init_waitqueue_entry(&q_ent, current); > + add_wait_queue(&(handler->wait), &q_ent); > + call_rcu(&(handler->rcu), free_nmi_handler, handler); > + for (;;) { > + set_current_state(TASK_UNINTERRUPTIBLE); > + if (list_empty(&(handler->link))) > + break; > + spin_unlock_irqrestore(&nmi_handler_lock, flags); > + schedule(); > + spin_lock_irqsave(&nmi_handler_lock, flags); > + } > + remove_wait_queue(&(handler->wait), &q_ent); > + spin_unlock_irqrestore(&nmi_handler_lock, flags); > +}
It might just be simpler to use completions instead -
call_rcu(&(handler->rcu), free_nmi_handler, handler); init_completion(&handler->completion); spin_unlock_irqrestore(&nmi_handler_lock, flags); wait_for_completion(&handler->completion);
and do
complete(&handler->completion);
in the the RCU callback.
Also, now I think your original idea of "Don't do this!" :) was right. Waiting until an nmi handler is seen unlinked could make a task block for a long time if another task re-installs it. You should probably just fail installation of a busy handler (checked under lock).
Thanks Dipankar - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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