linux-kernel - Re: [PATCH RFC] rcu: Add a minimum time for marking boot as completed

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Date:   Sun, 26 Feb 2023 16:03:24 -0800
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org,
        Frederic Weisbecker <frederic@...nel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-doc@...r.kernel.org, "Paul E. McKenney" <paulmck@...nel.org>,
        rcu@...r.kernel.org
Subject: Re: [PATCH RFC] rcu: Add a minimum time for marking boot as completed



On 2/24/23 19:59, Joel Fernandes wrote:
> On Fri, Feb 24, 2023 at 07:32:22PM -0800, Randy Dunlap wrote:
> [..] 
>>> +
>>> +	  Accept the default if unsure.
>>> +
>>>  config RCU_EXP_KTHREAD
>>>  	bool "Perform RCU expedited work in a real-time kthread"
>>>  	depends on RCU_BOOST && RCU_EXPERT
>>> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
>>> index 19bf6fa3ee6a..5b73341d9b89 100644
>>> --- a/kernel/rcu/update.c
>>> +++ b/kernel/rcu/update.c
>>> @@ -62,6 +62,10 @@ module_param(rcu_normal_after_boot, int, 0444);
>>>  #endif
>>>  #endif /* #ifndef CONFIG_TINY_RCU */
>>>  
>>> +/* Minimum time until RCU considers boot as completed. */
>>> +static int boot_end_delay = CONFIG_RCU_BOOT_END_DELAY;
>>> +module_param(boot_end_delay, int, 0444);
>>> +
>>>  #ifdef CONFIG_DEBUG_LOCK_ALLOC
>>>  /**
>>>   * rcu_read_lock_held_common() - might we be in RCU-sched read-side critical section?
>>> @@ -225,12 +229,29 @@ void rcu_unexpedite_gp(void)
>>>  EXPORT_SYMBOL_GPL(rcu_unexpedite_gp);
>>>  
>>>  static bool rcu_boot_ended __read_mostly;
>>> -
>>>  /*
>>> - * Inform RCU of the end of the in-kernel boot sequence.
>>> + * Inform RCU of the end of the in-kernel boot sequence. The boot sequence will
>>> + * not be marked ended until at least boot_end_delay milliseconds have passed.
>>>   */
>>> +void rcu_end_inkernel_boot(void);
>>> +static void boot_rcu_work_fn(struct work_struct *work)
>>> +{
>>> +	rcu_end_inkernel_boot();
>>> +}
>>> +static DECLARE_DELAYED_WORK(boot_rcu_work, boot_rcu_work_fn);
>>> +
>>>  void rcu_end_inkernel_boot(void)
>>>  {
>>> +	if (boot_end_delay) {
>>> +		u64 boot_ms = ktime_get_boot_fast_ns() / 1000000UL;
>>
>> Is that division OK on 32-bit?  Might have to use a helper macro. (I dunno.)
> 
> I believe the below should work on 32-bit, but I will test it more. It does
> on 64-bit.
> 
> Thanks!
> 
> ---8<-----------------------
> 
> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> index cbdad7b46841..2f539c18b310 100644
> --- a/kernel/rcu/update.c
> +++ b/kernel/rcu/update.c
> @@ -243,7 +243,7 @@ static DECLARE_DELAYED_WORK(boot_rcu_work, boot_rcu_work_fn);
>  void rcu_end_inkernel_boot(void)
>  {
>  	if (boot_end_delay) {
> -		u64 boot_ms = ktime_get_boot_fast_ns() / 1000000UL;
> +		u64 boot_ms = div_u64(ktime_get_boot_fast_ns(), 1000000UL);
>  
>  		if (boot_ms < boot_end_delay) {
>  			schedule_delayed_work(&boot_rcu_work,

Joel, this now builds cleanly on 32-bit. Before the patch:

ld: kernel/rcu/update.o: in function `boot_rcu_work_fn':
update.c:(.text+0x4623): undefined reference to `__udivdi3'
ld: kernel/rcu/update.o: in function `rcu_end_inkernel_boot':
update.c:(.text+0x46b3): undefined reference to `__udivdi3'

I don't know what the second one refers to, but it didn't show up
after the patch above (using div_u64()).

Thanks.
-- 
~Randy