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
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This archive was generated by hypermail 2b29 : Wed Oct 23 2002 - 22:01:06 EST