Re: [PATCH v7 02/11] rcu: Make call_rcu() lazy to save power

[Uladzislau Rezki <urezki@gmail.com>]

On Tue, Oct 04, 2022 at 08:58:14AM -0700, Paul E. McKenney wrote:
> On Tue, Oct 04, 2022 at 04:53:09PM +0200, Uladzislau Rezki wrote:
> > On Tue, Oct 04, 2022 at 06:30:04AM -0700, Paul E. McKenney wrote:
> > > On Tue, Oct 04, 2022 at 01:41:38PM +0200, Uladzislau Rezki wrote:
> > > > >  		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("Check"));
> > > > >  		rcu_nocb_lock_irqsave(rdp, flags);
> > > > >  		lockdep_assert_held(&rdp->nocb_lock);
> > > > >  		bypass_ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);
> > > > > -		if (bypass_ncbs &&
> > > > > +		lazy_ncbs = READ_ONCE(rdp->lazy_len);
> > > > > +
> > > > > +		if (bypass_ncbs && (lazy_ncbs == bypass_ncbs) &&
> > > > > +		    (time_after(j, READ_ONCE(rdp->nocb_bypass_first) + jiffies_till_flush) ||
> > > > > +		     bypass_ncbs > 2 * qhimark)) {
> > > > Do you know why we want double "qhimark" threshold? It is not only this
> > > > place, there are several. I am asking because it is not expected by the
> > > > user.
> > > 
> > > OK, I will bite...  What does the user expect?  Or, perhaps a better
> > > question, how is this choice causing the user problems?
> > > 
> > Yesterday when i was checking the lazy-v6 on Android i noticed the following:
> > 
> > <snip>
> > ...
> >      rcuop/4-48      [006] d..1   184.780328: rcu_batch_start: rcu_preempt CBs=15572 bl=121
> >      rcuop/6-62      [000] d..1   184.796939: rcu_batch_start: rcu_preempt CBs=21503 bl=167
> >      rcuop/6-62      [003] d..1   184.800706: rcu_batch_start: rcu_preempt CBs=24677 bl=192
> >      rcuop/6-62      [005] d..1   184.803773: rcu_batch_start: rcu_preempt CBs=27117 bl=211
> >      rcuop/6-62      [005] d..1   184.805732: rcu_batch_start: rcu_preempt CBs=22391 bl=174
> >      rcuop/6-62      [005] d..1   184.809083: rcu_batch_start: rcu_preempt CBs=12554 bl=98
> >      rcuop/6-62      [005] d..1   184.824228: rcu_batch_start: rcu_preempt CBs=16177 bl=126
> >      rcuop/4-48      [006] d..1   184.836193: rcu_batch_start: rcu_preempt CBs=24129 bl=188
> >      rcuop/4-48      [006] d..1   184.844147: rcu_batch_start: rcu_preempt CBs=25854 bl=201
> >      rcuop/4-48      [006] d..1   184.847257: rcu_batch_start: rcu_preempt CBs=21328 bl=166
> >      rcuop/4-48      [006] d..1   184.852128: rcu_batch_start: rcu_preempt CBs=21710 bl=169
> > ...
> > <snip>
> > 
> > On my device the "qhimark" is set to:
> > 
> > <snip>
> > XQ-CT54:/sys/module/rcutree/parameters # cat qhimark
> > 10000
> > XQ-CT54:/sys/module/rcutree/parameters #
> > <snip>
> > 
> > so i expect that once we pass 10 000 callbacks threshold the flush 
> > should occur. This parameter gives us an opportunity to control a
> > memory that should be reclaimed sooner or later.
> 
> I did understand that you were surprised.
> 
> But what problem did this cause other than you being surprised?
> 
It is not about surprising. It is about expectation. So if i set a
threshold to 100 i expect it that around 100 callbacks my memory will
be reclaimed. But the resolution is 2 * 100 in fact.

I am not aware about any issues with it. I just noticed such behaviour
during testing.

--
Uladzislau Rezki