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SubjectRe: [PATCH] simplify/improve rcu batch tuning
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.

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