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

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Message-ID: <20221004133004.GD4196@paulmck-ThinkPad-P17-Gen-1>
Date:   Tue, 4 Oct 2022 06:30:04 -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 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?

							Thanx, Paul