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Message-ID: <22892254-becb-d6ec-b8c2-a4ceb7fbb6fd@joelfernandes.org>
Date:   Tue, 19 Jul 2022 18:57:26 -0400
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Subject: Re: [PATCH rcu 4/7] rcu/nocb: Add an option to offload all CPUs on
 boot
Content-Language: en-US
To:     paulmck@kernel.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>
References: <20220620224455.GA3840881@paulmck-ThinkPad-P17-Gen-1>
 <20220620224503.3841196-4-paulmck@kernel.org>
 <7a09896b-88dd-040b-86c0-863f8279a04e@quicinc.com>
 <20220719181257.GR1790663@paulmck-ThinkPad-P17-Gen-1>
 <530b3bf4-f790-2973-5bc5-6ccd4797ae2a@joelfernandes.org>
 <20220719225306.GY1790663@paulmck-ThinkPad-P17-Gen-1>
From:   Joel Fernandes <joel@joelfernandes.org>
In-Reply-To: <20220719225306.GY1790663@paulmck-ThinkPad-P17-Gen-1>
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On 7/19/2022 6:53 PM, Paul E. McKenney wrote:
> 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?

Agreed, it would be great if it did not set those bits. I checked other
places in the kernel like kernel/sched/core.c and cannot find that it is
masking the bits after the setall(), so maybe its Ok?

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

Agreed, thanks.

 - Joel

> 
> 							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,