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SubjectRe: [PATCH rcu 3/3] doc: Document rcutree.nocb_nobypass_lim_per_jiffy kernel parameter
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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@kernel.org>
> Reported-by: Uladzislau Rezki <urezki@gmail.com>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> ---

Reviewed-by: Neeraj Upadhyay <quic_neeraju@quicinc.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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