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

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Date:   Wed, 5 Oct 2022 13:21:08 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Uladzislau Rezki <urezki@...il.com>, paulmck@...nel.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 4, 2022 at 7:41 AM Uladzislau Rezki <urezki@...il.com> 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.
> 
> I am following the qhimark conventions in existing code. However
> qhimark does not mean that your callbacks cannot exceed these many or
> something, it is not a hard limit on queued callbacks.
> 
> qhimark (And Paul can correct me) was introduced to reduce the number
> of callbacks after which RCU will not limit execution of callbacks to
> a batch of them. That has nothing to do with limiting the maximum
> number of callbacks, per-se. However, its usage certainly seems to
> have grown since that introduction.
> 
> Maybe you are confusing it with blimit:
> 
"blimit" controls how many/long callbacks are executed by the rcu_do_batch().
Whereas the "qhimark" controls when the bypass list is flushed to a regular one
to initiate gp and start executing.

--
Uladzislau Rezki