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From:   Joel Fernandes <joel@joelfernandes.org>
Date:   Fri, 8 Apr 2022 06:21:03 -0400
Message-ID: <CAEXW_YSqY2nFZrn4AjpUzJ+dwZc7jaVMG9RG5gvTyb3zFYWtQA@mail.gmail.com>
Subject: Re: [PATCH v2] EXP rcu: Move expedited grace period (GP) work to RT kthread_worker
To:     Kalesh Singh <kaleshsingh@google.com>
Cc:     Suren Baghdasaryan <surenb@google.com>,
        kernel-team <kernel-team@android.com>,
        "Paul E. McKenney" <paulmck@kernel.org>, Tejun Heo <tj@kernel.org>,
        Tim Murray <timmurray@google.com>, Wei Wang <wvw@google.com>,
        Kyle Lin <kylelin@google.com>,
        Chunwei Lu <chunweilu@google.com>,
        Lulu Wang <luluw@google.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Neeraj Upadhyay <quic_neeraju@quicinc.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        rcu <rcu@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>
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On Fri, Apr 8, 2022 at 12:57 AM Kalesh Singh <kaleshsingh@google.com> wrote:
[..]
> +config RCU_EXP_KTHREAD
> +       bool "Perform RCU expedited work in a real-time kthread"
> +       depends on RCU_BOOST && RCU_EXPERT

Doesn't this disable the fix if a system is not RCU_EXPERT ?  Please
see the definition of RCU_EXPERT:
"This option needs to be enabled if you wish to make expert-level
adjustments to RCU configuration."

I don't think a bug fix counts as an expert-level adjustment.

> +       default !PREEMPT_RT && NR_CPUS <= 32

What is the benefit of turning it off on PREEMPT_RT, even if
PREEMPT_RT does not use expedited GPs much post-boot? I would think in
the future if PREEMPT_RT ever uses expedited GPs, they would want this
feature even more. I'd rather be future-proof now as I don't see any
advantages of disabling it on !PREEMPT_RT (And a drawback that the fix
won't apply to those systems). Also will keep the config simple.

> +       help
> +         Use this option to further reduce the latencies of expedited
> +         grace periods at the expense of being more disruptive.
> +
> +         Accept the default if unsure.
> +
>  config RCU_NOCB_CPU
>         bool "Offload RCU callback processing from boot-selected CPUs"
>         depends on TREE_RCU
[...]

Thanks,

 - Joel