linux-kernel - Re: [PATCH rcu 3/3] doc: Document rcutree.nocb_nobypass_lim_per

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Date:   Tue, 21 Jun 2022 12:03:20 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Neeraj Upadhyay <quic_neeraju@...cinc.com>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, rostedt@...dmis.org,
        Frederic Weisbecker <frederic@...nel.org>,
        Uladzislau Rezki <urezki@...il.com>
Subject: Re: [PATCH rcu 3/3] doc: Document
 rcutree.nocb_nobypass_lim_per_jiffy kernel parameter

On Tue, Jun 21, 2022 at 10:41:54AM +0530, Neeraj Upadhyay wrote:
> 
> 
> On 6/21/2022 3:47 AM, Paul E. McKenney wrote:
> > This commit provides documentation for the kernel parameter controlling
> > RCU's handling of callback floods on offloaded (rcu_nocbs) CPUs.
> > This parameter might be obscure, but it is always there when you need it.
> > 
> > Reported-by: Frederic Weisbecker <frederic@...nel.org>
> > Reported-by: Uladzislau Rezki <urezki@...il.com>
> > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > ---
> 
> Reviewed-by: Neeraj Upadhyay <quic_neeraju@...cinc.com>
> 
> >   Documentation/admin-guide/kernel-parameters.txt | 13 +++++++++++++
> >   1 file changed, 13 insertions(+)
> > 
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index bdf431bdbfdc4..a30890141b1a5 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -4681,6 +4681,19 @@
> >   			invoked in the context of an rcuoc kthread, which
> >   			scheduler will preempt as it does any other task.
> > +	rcutree.nocb_nobypass_lim_per_jiffy= [KNL]
> > +			On callback-offloaded (rcu_nocbs) CPUs,
> > +			RCU reduces the lock contention that would
> > +			otherwise be caused callback floods through
> 
> Minor : "caused by" ?

Good catch, fixed, thank you!

I applied your Reviewed-by to this series as well, and thank you for
that as well.

							Thanx, Paul

> Thanks
> Neeraj
> 
> > +			use of the ->nocb_bypass list.	However, in the
> > +			common non-flooded case, RCU queues directly to
> > +			the main ->cblist in order to avoid the extra
> > +			overhead of the ->nocb_bypass list and its lock.
> > +			But if there are too many callbacks queued during
> > +			a single jiffy, RCU pre-queues the callbacks into
> > +			the ->nocb_bypass queue.  The definition of "too
> > +			many" is supplied by this kernel boot parameter.
> > +
> >   	rcutree.rcu_nocb_gp_stride= [KNL]
> >   			Set the number of NOCB callback kthreads in
> >   			each group, which defaults to the square root