linux-kernel - Re: [PATCH v7 02/11] rcu: Make call

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Date:   Tue, 4 Oct 2022 08:58:14 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        rushikesh.s.kadam@...el.com, neeraj.iitr10@...il.com,
        frederic@...nel.org, rostedt@...dmis.org, youssefesmat@...gle.com,
        surenb@...gle.com
Subject: Re: [PATCH v7 02/11] rcu: Make call_rcu() lazy to save power

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?

							Thanx, Paul