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Date: Tue, 19 Apr 2022 16:56:26 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Joel Fernandes <joel@...lfernandes.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
Josh Triplett <josh@...htriplett.org>,
Lai Jiangshan <jiangshanlai@...il.com>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
rcu <rcu@...r.kernel.org>, Steven Rostedt <rostedt@...dmis.org>,
Rushikesh S Kadam <rushikesh.s.kadam@...el.com>,
Vineeth Pillai <vineethrp@...il.com>,
"Uladzislau Rezki (Sony)" <urezki@...il.com>
Subject: Re: [PATCH v4 rcu/dev] rcu/nocb: Add an option to offload all CPUs
on boot
On Tue, Apr 19, 2022 at 04:10:33PM -0400, Joel Fernandes wrote:
> On Tue, Apr 19, 2022 at 4:08 PM Joel Fernandes (Google)
> <joel@...lfernandes.org> wrote:
> >
> > From: Joel Fernandes <joel@...lfernandes.org>
> >
> > 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 and about how to provide
> > the mask.
> >
> > With the new option enabled, all CPUs will be offloaded on boot unless
> > rcu_nocbs= or rcu_nohz_full= kernel parameters provide a CPU list.
> >
> > Signed-off-by: Joel Fernandes <joel@...lfernandes.org>
>
> Oops, forgot to add Reviewed-by tags:
> Reviewed-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> Reviewed-by: Kalesh Singh <kaleshsingh@...gle.com>
>
> Paul, would you mind doing so, so I don't have to resend? Apologies.
OK, but only because it passed an allmodconfig build this time. ;-)
Thanx, Paul
> Thanks,
>
> Joel
>
>
> > ---
> > v4: Rebased on rcu/dev, fixed conflict with Frederick's patch changing
> > location of nocb_is_setup variable.
> >
> > Documentation/admin-guide/kernel-parameters.txt | 6 ++++++
> > kernel/rcu/Kconfig | 13 +++++++++++++
> > kernel/rcu/tree_nocb.h | 13 +++++++++++++
> > 3 files changed, 32 insertions(+)
> >
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index 789ef586009b..1e82ecb7a649 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -3572,6 +3572,9 @@
> > 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
> > @@ -4475,6 +4478,9 @@
> > 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 1c630e573548..27aab870ae4c 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 4cf9a29bba79..9f29f25a0cbc 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))
> > 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);
> > +
> > 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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