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Message-ID: <20220719181257.GR1790663@paulmck-ThinkPad-P17-Gen-1>
Date: Tue, 19 Jul 2022 11:12:57 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Neeraj Upadhyay <quic_neeraju@...cinc.com>
Cc: rcu@...r.kernel.org, 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 Tue, Jul 19, 2022 at 03:04:07PM +0530, Neeraj Upadhyay wrote:
>
>
> 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)?
Without that cpumask_and(), systems with sparse CPU numbering schemes
(for example, 0, 4, 8, 12, ...) will get a pr_info(), and as you noted,
the needed cpumask_and().
I am inclined to see a complaint before we change this. And perhaps if
this is to change, the change should be in cpumask_setall() rather than
in rcu_init_nohz(). But that is an argument for later, if at all. ;-)
And thank you for reviewing this series!
Thanx, Paul
> 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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