Re: [PATCH] simplify/improve rcu batch tuning

[Oleg Nesterov <oleg@tv-sign.ru>]

On 09/07, Paul E. McKenney wrote:
>
> Some thoughts for testing...
>
> 1.	Modify rcutorture.c to keep all the rcutorture kernel threads
> 	off of at least one CPU.  Run a CPU-bound user process on that
> 	CPU.  Compare the rate a which grace periods progress in
> 	the following three situations:
>
> 	a.	With your patch.
>
> 	b.	With stock kernel.
>
> 	c.	With the function disabled (e.g., use the
> 		not-CONFIG_SMP version of force_quiescent_state()).
>
> 	You would expect to see fast grace-period progress for (a) and
> 	(b), slow for (c).
>
> 2.	As above, but have another process generating lots of
> 	RCU callbacks, for example, by opening and closing lots
> 	of files, creating and deleting lots of files with long
> 	randomly selected names, thrashing the route cache, or
> 	whatever.

Thanks for review and suggestions. I'll try to run these tests next week.
Afaics, it is enough to just do

	for (;;) close(open(...))

for '2.'.

> > @@ -86,8 +83,8 @@ static void force_quiescent_state(struct
> >  	int cpu;
> >  	cpumask_t cpumask;
> >  	set_need_resched();
>
> Not that it makes a big difference, but why is the above
> set_need_resched() not in the body of the following "if" statement?
> It used to be important, because it could prevent additional IPIs in
> the same grace period, but since the current code will only send one
> IPI per grace period, it seems like it can safely be tucked under the
> "if" statement.

I think there was another reason to do set_need_resched() unconditionally,
but this is only my guess. We are sending IPIs to speedup the flashing of
callbacks we already have in the queue. But set_need_resched() tries to
suppress current process from adding new callbacks (not that it is perfect,
though). Consider the 'for (;;) close(open(...))' loop.

Actually I think it also makes sense to do tasklet_schedule(rcu_tasklet)
in call_rcu(), this way we can detect that we need to start the next batch
earlier.

> > -	if (unlikely(rdp->qlen - rdp->last_rs_qlen > rsinterval)) {
> > -		rdp->last_rs_qlen = rdp->qlen;
> > +	if (unlikely(!rcp->signaled)) {
> > +		rcp->signaled = 1;
> >  		/*
> >  		 * Don't send IPI to itself. With irqs disabled,
> >  		 * rdp->cpu is the current cpu.
> > @@ -297,6 +294,7 @@ static void rcu_start_batch(struct rcu_c
> >  		smp_mb();
> >  		cpus_andnot(rcp->cpumask, cpu_online_map, nohz_cpu_mask);
> >
> > +		rcp->signaled = 0;
>
> Would it make sense to invoke force_quiescent_state() here in the
> case that rdp->qlen is still large?  The disadvantage is that qlen
> still counts the number of callbacks that are already slated for
> invocation.

This is not easy to do. rcu_start_batch() is "global", we need
to scan all per-cpu 'struct rcu_data' and check it's ->qlen.

>              Another approach would be to check rdp->qlen and
> rcp->signaled in rcu_do_batch(), but only once rdp->donlist goes
> NULL.

Agree. Probably we don't need to check !rdp->donlist, it should be
empty after rcu_do_batch() invocation when ->qlen > qhimark, because
in that case ->blimit == INT_MAX.

But first I'd like to do a couple of other cleanups here, I'll send
the patches on weekend.

Thanks!

Oleg.