Re: [patch 1/2] rcu batch tuning

[Oleg Nesterov]

"Paul E. McKenney" wrote:
> 
> On Fri, Jan 27, 2006 at 10:57:59PM +0300, Oleg Nesterov wrote:
> >
> > When ->qlen exceeds qhimark for the first time we send reschedule IPI to
> > other CPUs and force_quiescent_state() records ->last_rs_qlen = ->qlen.
> > But we don't reset ->last_rs_qlen when ->qlen goes to 0, this means that
> > next time we need ++rdp->qlen > qhimark + rsinterval to force other CPUS
> > to pass quiescent state, no?
> 
> Good catch -- this could well explain Lee's continuing to hit
> latency problems.  Although this would not cause the first
> latency event, only subsequent ones, it seems to me that ->last_rs_qlen
> should be reset whenever ->blimit is reset.

May be it's better to do it in other way?

struct rcu_ctrlblk {
	...
	int signaled;
	...
};

void force_quiescent_state(rdp, rcp)
{
	if (!rcp->signaled) {
		// racy, but tolerable
		rcp->signaled = 1;

		for_each_cpu_mask(cpu, cpumask)
			smp_send_reschedule(cpu);
	}
}

void rcu_start_batch(rcp, rdp)
{
	if (->next_pending && ->completed == ->cur) {
		...
		rcp->signaled = 0;
		...
	}
}

Probably it is also makes sense to tasklet_schedule(rcu_tasklet)
in call_rcu() when ++rdp->qlen > qhimark, this way we can detect
that we need to start the next batch earlier.

> > Also, it seems to me it's better to have 2 counters, one for length(->donelist)
> > and another for length(->curlist + ->nxtlist). I think we don't need
> > force_quiescent_state() when all rcu callbacks are placed in ->donelist,
> > we only need to increase rdp->blimit in this case.
>
> True, currently the patch keeps the sum of the length of all three lists,
> and takes both actions when the sum gets too large.  But the only way
> you would get unneeded IPIs would be if callback processing was
> stalled, but callback generation and grace-period processing was
> still proceeding.  Seems at first glance to be an unusual corner
> case, with the only downside being some extra IPIs.  Or am I missing
> some aspect?

Yes, it is probably not worth to complicate the code.

Oleg.
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