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

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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