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

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Date:   Sat, 25 Feb 2023 03:59:32 +0000
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 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,
-- 
2.39.2.637.g21b0678d19-goog