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Message-ID: <7a09896b-88dd-040b-86c0-863f8279a04e@quicinc.com>
Date: Tue, 19 Jul 2022 15:04:07 +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>, Joel Fernandes <joel@...lfernandes.org>,
Kalesh Singh <kaleshsingh@...gle.com>,
Uladzislau Rezki <urezki@...il.com>,
kernel test robot <lkp@...el.com>
Subject: Re: [PATCH rcu 4/7] rcu/nocb: Add an option to offload all CPUs on
boot
On 6/21/2022 4:15 AM, Paul E. McKenney wrote:
> From: Joel Fernandes <joel@...lfernandes.org>
>
> Systems built with CONFIG_RCU_NOCB_CPU=y but booted without either
> the rcu_nocbs= or rcu_nohz_full= kernel-boot parameters will not have
> callback offloading on any of the CPUs, nor can any of the CPUs be
> switched to enable callback offloading at runtime. Although this is
> intentional, it would be nice to have a way to offload all the CPUs
> without having to make random bootloaders specify either the rcu_nocbs=
> or the rcu_nohz_full= kernel-boot parameters.
>
> This commit therefore provides a new CONFIG_RCU_NOCB_CPU_DEFAULT_ALL
> Kconfig option that switches the default so as to offload callback
> processing on all of the CPUs. This default can still be overridden
> using the rcu_nocbs= and rcu_nohz_full= kernel-boot parameters.
>
> Reviewed-by: Kalesh Singh <kaleshsingh@...gle.com>
> Reviewed-by: Uladzislau Rezki <urezki@...il.com>
> (In v4.1, fixed issues with CONFIG maze reported by kernel test robot).
> Reported-by: kernel test robot <lkp@...el.com>
> Signed-off-by: Joel Fernandes <joel@...lfernandes.org>
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> ---
Reviewed-by: Neeraj Upadhyay <quic_neeraju@...cinc.com>
One query on cpumask_setall() below
> Documentation/admin-guide/kernel-parameters.txt | 6 ++++++
> kernel/rcu/Kconfig | 13 +++++++++++++
> kernel/rcu/tree_nocb.h | 15 ++++++++++++++-
> 3 files changed, 33 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 2522b11e593f2..34605c275294c 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -3659,6 +3659,9 @@
> just as if they had also been called out in the
> rcu_nocbs= boot parameter.
>
> + Note that this argument takes precedence over
> + the CONFIG_RCU_NOCB_CPU_DEFAULT_ALL option.
> +
> noiotrap [SH] Disables trapped I/O port accesses.
>
> noirqdebug [X86-32] Disables the code which attempts to detect and
> @@ -4557,6 +4560,9 @@
> no-callback mode from boot but the mode may be
> toggled at runtime via cpusets.
>
> + Note that this argument takes precedence over
> + the CONFIG_RCU_NOCB_CPU_DEFAULT_ALL option.
> +
> rcu_nocb_poll [KNL]
> Rather than requiring that offloaded CPUs
> (specified by rcu_nocbs= above) explicitly
> diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
> index 1c630e573548d..27aab870ae4cf 100644
> --- a/kernel/rcu/Kconfig
> +++ b/kernel/rcu/Kconfig
> @@ -262,6 +262,19 @@ config RCU_NOCB_CPU
> Say Y here if you need reduced OS jitter, despite added overhead.
> Say N here if you are unsure.
>
> +config RCU_NOCB_CPU_DEFAULT_ALL
> + bool "Offload RCU callback processing from all CPUs by default"
> + depends on RCU_NOCB_CPU
> + default n
> + help
> + Use this option to offload callback processing from all CPUs
> + by default, in the absence of the rcu_nocbs or nohz_full boot
> + parameter. This also avoids the need to use any boot parameters
> + to achieve the effect of offloading all CPUs on boot.
> +
> + Say Y here if you want offload all CPUs by default on boot.
> + Say N here if you are unsure.
> +
> config TASKS_TRACE_RCU_READ_MB
> bool "Tasks Trace RCU readers use memory barriers in user and idle"
> depends on RCU_EXPERT && TASKS_TRACE_RCU
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index 4cf9a29bba79d..60cc92cc66552 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -1197,11 +1197,21 @@ void __init rcu_init_nohz(void)
> {
> int cpu;
> bool need_rcu_nocb_mask = false;
> + bool offload_all = false;
> struct rcu_data *rdp;
>
> +#if defined(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL)
> + if (!rcu_state.nocb_is_setup) {
> + need_rcu_nocb_mask = true;
> + offload_all = true;
> + }
> +#endif /* #if defined(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL) */
> +
> #if defined(CONFIG_NO_HZ_FULL)
> - if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
> + if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask)) {
> need_rcu_nocb_mask = true;
> + offload_all = false; /* NO_HZ_FULL has its own mask. */
> + }
> #endif /* #if defined(CONFIG_NO_HZ_FULL) */
>
> if (need_rcu_nocb_mask) {
> @@ -1222,6 +1232,9 @@ void __init rcu_init_nohz(void)
> cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask);
> #endif /* #if defined(CONFIG_NO_HZ_FULL) */
>
> + if (offload_all)
> + cpumask_setall(rcu_nocb_mask);
Do we need to do a cpumask_and(rcu_nocb_mask, cpu_possible_mask,
rcu_nocb_mask) after setting all cpus in rcu_nocb_mask (cpumask_subset()
check below takes care of it though)?
Thanks
Neeraj
> +
> if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
> pr_info("\tNote: kernel parameter 'rcu_nocbs=', 'nohz_full', or 'isolcpus=' contains nonexistent CPUs.\n");
> cpumask_and(rcu_nocb_mask, cpu_possible_mask,
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