linux-kernel - Re: [PATCH RFC] rcu/nocb: Provide default all-CPUs mask for RCU_NOCB

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Message-ID: <CAEXW_YRu8z0tHn0nOpzd4PtsL0vKpgSuN86CgwXeWCEEiXbaog@mail.gmail.com>
Date:   Mon, 11 Apr 2022 11:17:56 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Paul E. McKenney" <paulmck@...nel.org>, rcu <rcu@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH RFC] rcu/nocb: Provide default all-CPUs mask for RCU_NOCB_CPU=y

On Mon, Apr 11, 2022 at 9:49 AM Uladzislau Rezki <urezki@...il.com> wrote:
>
> Hello, Joel.
>
> > On systems with CONFIG_RCU_NOCB_CPU=y, there is no default mask provided
> > which ends up not offloading any CPU. This patch removes yet another
> > dependency from the bootloader having to know about RCU, about how many
> > CPUs the system has, and about how to provide the mask. Basically, I
> > think we should stop pretending that the user knows what they are doing :).
> > In other words, if NO_CB_CPU is enabled, lets make use of it.
> >
> Could you also please modify the documentation accordingly and send v2?

Yes good point, I can go clean that up in v2.

 - Joel

> <snip>
> urezki@...38:~/data/raid0/coding/linux-rcu.git$ grep -rn RCU_NOCB_CPU ./Documentation/
> ./Documentation/timers/no_hz.rst:188:using the CONFIG_RCU_NOCB_CPU=y Kconfig option.  The specific CPUs to
> ./Documentation/admin-guide/kernel-parameters.txt:4380:                 In kernels built with CONFIG_RCU_NOCB_CPU=y,
> ./Documentation/admin-guide/kernel-parameters.txt:4507:                 When RCU_NOCB_CPU is set, also adjust the
> ./Documentation/admin-guide/kernel-per-CPU-kthreads.rst:311:3.  Build with CONFIG_RCU_NOCB_CPU=y and boot with the rcu_nocbs=
> ./Documentation/admin-guide/kernel-per-CPU-kthreads.rst:331:2.  Build with CONFIG_RCU_NOCB_CPU=n, which will prevent these
> ./Documentation/RCU/Design/Requirements/Requirements.rst:1424:``CONFIG_RCU_NOCB_CPU=y`` and booted with ``rcu_nocbs=1-63``, where
> urezki@...38:~/data/raid0/coding/linux-rcu.git$ grep -rn RCU_NOCB_CPU ./kernel/rcu/
> ...
> ./kernel/rcu/Kconfig:198:config RCU_NOCB_CPU
> ..
> <snip>
>
> --
> Uladzislau Rezki