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Message-ID: <3988b203-0a65-526e-b445-c2ee9ea0f4e8@quicinc.com>
Date: Tue, 21 Jun 2022 10:41:54 +0530
From: Neeraj Upadhyay <quic_neeraju@...cinc.com>
To: "Paul E. McKenney" <paulmck@...nel.org>, <rcu@...r.kernel.org>
CC: <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 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" ?
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
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