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Date:   Tue, 19 Jul 2022 15:53:06 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     Neeraj Upadhyay <quic_neeraju@quicinc.com>, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org, kernel-team@fb.com,
        rostedt@goodmis.org, Kalesh Singh <kaleshsingh@google.com>,
        Uladzislau Rezki <urezki@gmail.com>,
        kernel test robot <lkp@intel.com>
Subject: Re: [PATCH rcu 4/7] rcu/nocb: Add an option to offload all CPUs on
 boot
Message-ID: <20220719225306.GY1790663@paulmck-ThinkPad-P17-Gen-1>
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On Tue, Jul 19, 2022 at 06:42:00PM -0400, Joel Fernandes wrote:
> 
> 
> On 7/19/2022 2:12 PM, Paul E. McKenney wrote:
> > 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@joelfernandes.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@google.com>
> >>> Reviewed-by: Uladzislau Rezki <urezki@gmail.com>
> >>> (In v4.1, fixed issues with CONFIG maze reported by kernel test robot).
> >>> Reported-by: kernel test robot <lkp@intel.com>
> >>> Signed-off-by: Joel Fernandes <joel@joelfernandes.org>
> >>> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> >>> ---
> >>
> >>
> >> Reviewed-by: Neeraj Upadhyay <quic_neeraju@quicinc.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.  ;-)
> > 
> >>> +
> >>>   	if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
> 
> We could also suppress the pr_info() by making it conditional.
> 
> like:
> 
> if (!CONFIG_RCU_NOCB_CPU_DEFAULT_ALL) {
> 	pr_info(...);
> }
> 
> In other words, we could make the cpumask_and() as expected/normal on
> systems with sparse CPU numbering schemes. Would that work?

That would be a good within-RCU workaround if we get an urgent complaint,
but if this requires a change, shouldn't cpumask_setall() refrain from
setting bits for non-existent CPUs?  It does refrain from setting any
bits beyond the largest-numbered CPU.

But perhaps there is an early boot reason why cpumask_setall() cannot
do this?

Either way, we are just doing a pr_info(), not a WARN_ON() or similar,
so the current state is probably fine.

							Thanx, Paul

> Thanks,
> 
>  - Joel
> 
> 
> >>>   		pr_info("\tNote: kernel parameter 'rcu_nocbs=', 'nohz_full', or 'isolcpus=' contains nonexistent CPUs.\n");
> 
> 
> 
> >>>   		cpumask_and(rcu_nocb_mask, cpu_possible_mask,