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

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Message-Id: <F276BA2C-5401-498D-80CF-D60DFEA71F38@joelfernandes.org>
Date:   Mon, 27 Feb 2023 08:24:13 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Randy Dunlap <rdunlap@...radead.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 Feb 26, 2023, at 7:03 PM, Randy Dunlap <rdunlap@...radead.org> wrote:
> 
> 
> 
>> 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 a lot, I will update the patch with this in the next spin.

 - Joel


> 
> Thanks.
> -- 
> ~Randy