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Message-ID: <20220415173515.GL4285@paulmck-ThinkPad-P17-Gen-1>
Date: Fri, 15 Apr 2022 10:35:15 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc: linux-kernel@...r.kernel.org,
Josh Triplett <josh@...htriplett.org>,
Lai Jiangshan <jiangshanlai@...il.com>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
rcu@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
rushikesh.s.kadam@...el.com, vineethrp@...il.com, urezki@...il.com
Subject: Re: [PATCH v3] rcu/nocb: Add an option to offload all CPUs on boot
On Fri, Apr 15, 2022 at 04:02:24PM +0000, Joel Fernandes (Google) wrote:
> From: Joel Fernandes <joel@...lfernandes.org>
Much better, thank you!
> On systems with CONFIG_RCU_NOCB_CPU=y, there is no default mask provided
> which ends up not offloading any CPU. This patch removes a dependency
> from the bootloader having to know about RCU, about how many CPUs the
> system has, and about how to provide the mask.
The "about how many CPUs the system has" does not apply to current
mainline, in which "rcu_nocbs=0-N" says to offload all CPUs. It can be
added back in for a backport to v5.10. ;-)
> With the new option enabled, all CPUs will be offloaded on boot.
>
> Signed-off-by: Joel Fernandes <joel@...lfernandes.org>
I second Joel's call for testing and add to that a call for additional
review.
My thought is to queue this after some independent testing.
> ---
> v2 was forcing the option to override no_cbs=
> v3 is back to v1 but with a config option defaulting to 'n'.
>
> Documentation/admin-guide/kernel-parameters.txt | 3 +++
> kernel/rcu/Kconfig | 13 +++++++++++++
> kernel/rcu/tree_nocb.h | 16 ++++++++++++++--
> 3 files changed, 30 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index f5a27f067db9..7648a7dd335e 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -4398,6 +4398,9 @@
> 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.
Very good, thank you!
Do we want to say the same about nohz_full? Or am I misreading the
code below?
> +
> 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 bf8e341e75b4..2f8bd694ed85 100644
> --- a/kernel/rcu/Kconfig
> +++ b/kernel/rcu/Kconfig
> @@ -223,6 +223,19 @@ 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
The needed dependency on RCU_EXPERT is provided transitively via
RCU_NOCB_CPU, so this should be OK. (To check, build a .config file,
queue this patch, and do "make oldconfig". If any questions are asked,
a change is needed.)
> + default n
> + help
> + Use this option to offload callback processing from all CPUs
> + by default, in the absence of the rcu_nocbs boot parameter.
And also in the absence of the nohz_full boot parameter, correct?
> + 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
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index eeafb546a7a0..673fa0d1f801 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -1165,12 +1165,21 @@ EXPORT_SYMBOL_GPL(rcu_nocb_cpu_offload);
> void __init rcu_init_nohz(void)
> {
> int cpu;
> - bool need_rcu_nocb_mask = false;
> + bool need_rcu_nocb_mask = false, offload_all = false;
Please use the extra line to perserve alphabetical order. (I do know
about inverse tree, though I have no idea why it is considered to be a
good thing.)
> struct rcu_data *rdp;
>
> +#if defined(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL)
> + if (!rcu_nocb_is_setup) {
> + need_rcu_nocb_mask = true;
> + offload_all = true;
> + }
> +#endif
It would be nice to be able to use IS_ENABLED() here and below, but that
is a separate issue. For one thing, need_rcu_nocb_mask would need to
be defined unconditionally. So this is fine as it is for this patch.
> +
> #if defined(CONFIG_NO_HZ_FULL)
> - if (tick_nohz_full_running && cpumask_weight(tick_nohz_full_mask))
> + if (tick_nohz_full_running && cpumask_weight(tick_nohz_full_mask)) {
> need_rcu_nocb_mask = true;
> + offload_all = false; /* NO_HZ_FULL has its own mask. */
This is good, and is also why I am asking for the addition of nohz_full
above.
> + }
> #endif /* #if defined(CONFIG_NO_HZ_FULL) */
>
> if (need_rcu_nocb_mask) {
> @@ -1191,6 +1200,9 @@ 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);
> +
> 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,
> --
> 2.36.0.rc0.470.gd361397f0d-goog
>
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