[PATCH RFC] rcu: Add a minimum time for marking boot as completed

[PATCH RFC] rcu: Add a minimum time for marking boot as completed

[Joel Fernandes (Google)]

On many systems, a great deal of boot happens after the kernel thinks the boot
has completed. It is difficult to determine if the system has really booted
from the kernel side. Some features like lazy-RCU can risk slowing down boot
time if, say, a callback has been added that the boot synchronously depends on.

Further, it is better to boot systems which pass 'rcu_normal_after_boot' to
stay expedited for as long as the system is still booting.

For these reasons, this commit adds a config option
'CONFIG_RCU_BOOT_END_DELAY' and a boot parameter rcupdate.boot_end_delay.

By default, this value is 20s. A system designer can choose to specify a value
here to keep RCU from marking boot completion.  The boot sequence will not be
marked ended until at least boot_end_delay milliseconds have passed.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 .../admin-guide/kernel-parameters.txt         |  4 +++
 kernel/rcu/Kconfig                            | 12 +++++++++
 kernel/rcu/update.c                           | 25 +++++++++++++++++--
 3 files changed, 39 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 2429b5e3184b..0943139fdf01 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5085,6 +5085,10 @@
 	rcutorture.verbose= [KNL]
 			Enable additional printk() statements.
 
+	rcupdate.boot_end_delay= [KNL]
+			Minimum time that must elapse before the boot
+			sequence can be marked as completed.
+
 	rcupdate.rcu_cpu_stall_ftrace_dump= [KNL]
 			Dump ftrace buffer after reporting RCU CPU
 			stall warning.
diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
index 9071182b1284..1033a38bddad 100644
--- a/kernel/rcu/Kconfig
+++ b/kernel/rcu/Kconfig
@@ -217,6 +217,18 @@ config RCU_BOOST_DELAY
 
 	  Accept the default if unsure.
 
+config RCU_BOOT_END_DELAY
+	int "Minimum delay before RCU considers boot has completed"
+	range 0 120000
+	default 20000
+	help
+	  This option specifies the minmum time since boot before which
+	  RCU believes the system is booted. The actual delay can be
+	  higher than this if the kernel takes a long time to initialize
+	  but it will never be smaller than this.
+
+	  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;
+
+		if (boot_ms < boot_end_delay) {
+			schedule_delayed_work(&boot_rcu_work,
+					boot_end_delay - boot_ms);
+			return;
+		}
+	}
+
 	rcu_unexpedite_gp();
 	rcu_async_relax();
 	if (rcu_normal_after_boot)
-- 
2.39.2.637.g21b0678d19-goog

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

[Randy Dunlap]

Hi--

On 2/24/23 19:27, Joel Fernandes (Google) wrote:
> On many systems, a great deal of boot happens after the kernel thinks the boot
> has completed. It is difficult to determine if the system has really booted
> from the kernel side. Some features like lazy-RCU can risk slowing down boot
> time if, say, a callback has been added that the boot synchronously depends on.
> 
> Further, it is better to boot systems which pass 'rcu_normal_after_boot' to
> stay expedited for as long as the system is still booting.
> 
> For these reasons, this commit adds a config option
> 'CONFIG_RCU_BOOT_END_DELAY' and a boot parameter rcupdate.boot_end_delay.
> 
> By default, this value is 20s. A system designer can choose to specify a value
> here to keep RCU from marking boot completion.  The boot sequence will not be
> marked ended until at least boot_end_delay milliseconds have passed.
> 
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
>  .../admin-guide/kernel-parameters.txt         |  4 +++
>  kernel/rcu/Kconfig                            | 12 +++++++++
>  kernel/rcu/update.c                           | 25 +++++++++++++++++--
>  3 files changed, 39 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 2429b5e3184b..0943139fdf01 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -5085,6 +5085,10 @@
>  	rcutorture.verbose= [KNL]
>  			Enable additional printk() statements.
>  
> +	rcupdate.boot_end_delay= [KNL]

Tell units:

> +			Minimum time that must elapse before the boot

+			Minimum time in milliseconds that must elapse before the boot

> +			sequence can be marked as completed.
> +
>  	rcupdate.rcu_cpu_stall_ftrace_dump= [KNL]
>  			Dump ftrace buffer after reporting RCU CPU
>  			stall warning.
> diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
> index 9071182b1284..1033a38bddad 100644
> --- a/kernel/rcu/Kconfig
> +++ b/kernel/rcu/Kconfig
> @@ -217,6 +217,18 @@ config RCU_BOOST_DELAY
>  
>  	  Accept the default if unsure.
>  
> +config RCU_BOOT_END_DELAY
> +	int "Minimum delay before RCU considers boot has completed"
> +	range 0 120000
> +	default 20000
> +	help
> +	  This option specifies the minmum time since boot before which

tpyo:	                            minimum

> +	  RCU believes the system is booted. The actual delay can be
> +	  higher than this if the kernel takes a long time to initialize
> +	  but it will never be smaller than this.

+	  Specified in milliseconds.

> +
> +	  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.)

> +
> +		if (boot_ms < boot_end_delay) {
> +			schedule_delayed_work(&boot_rcu_work,
> +					boot_end_delay - boot_ms);
> +			return;
> +		}
> +	}
> +
>  	rcu_unexpedite_gp();
>  	rcu_async_relax();
>  	if (rcu_normal_after_boot)

-- 
~Randy

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

[Joel Fernandes]

On Fri, Feb 24, 2023 at 07:32:22PM -0800, Randy Dunlap wrote:
> Hi--
> 
> On 2/24/23 19:27, Joel Fernandes (Google) wrote:
> > On many systems, a great deal of boot happens after the kernel thinks the boot
> > has completed. It is difficult to determine if the system has really booted
> > from the kernel side. Some features like lazy-RCU can risk slowing down boot
> > time if, say, a callback has been added that the boot synchronously depends on.
> > 
> > Further, it is better to boot systems which pass 'rcu_normal_after_boot' to
> > stay expedited for as long as the system is still booting.
> > 
> > For these reasons, this commit adds a config option
> > 'CONFIG_RCU_BOOT_END_DELAY' and a boot parameter rcupdate.boot_end_delay.
> > 
> > By default, this value is 20s. A system designer can choose to specify a value
> > here to keep RCU from marking boot completion.  The boot sequence will not be
> > marked ended until at least boot_end_delay milliseconds have passed.
> > 
> > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > ---
> >  .../admin-guide/kernel-parameters.txt         |  4 +++
> >  kernel/rcu/Kconfig                            | 12 +++++++++
> >  kernel/rcu/update.c                           | 25 +++++++++++++++++--
> >  3 files changed, 39 insertions(+), 2 deletions(-)
> > 
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index 2429b5e3184b..0943139fdf01 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -5085,6 +5085,10 @@
> >  	rcutorture.verbose= [KNL]
> >  			Enable additional printk() statements.
> >  
> > +	rcupdate.boot_end_delay= [KNL]
> 
> Tell units:
> 
> > +			Minimum time that must elapse before the boot
> 

Fixed.

> +			Minimum time in milliseconds that must elapse before the boot
> 
> > +			sequence can be marked as completed.
> > +
> >  	rcupdate.rcu_cpu_stall_ftrace_dump= [KNL]
> >  			Dump ftrace buffer after reporting RCU CPU
> >  			stall warning.
> > diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
> > index 9071182b1284..1033a38bddad 100644
> > --- a/kernel/rcu/Kconfig
> > +++ b/kernel/rcu/Kconfig
> > @@ -217,6 +217,18 @@ config RCU_BOOST_DELAY
> >  
> >  	  Accept the default if unsure.
> >  
> > +config RCU_BOOT_END_DELAY
> > +	int "Minimum delay before RCU considers boot has completed"
> > +	range 0 120000
> > +	default 20000
> > +	help
> > +	  This option specifies the minmum time since boot before which
> 
> tpyo:	                            minimum
> 
> > +	  RCU believes the system is booted. The actual delay can be
> > +	  higher than this if the kernel takes a long time to initialize
> > +	  but it will never be smaller than this.

Fixed.

> +	  Specified in milliseconds.
> 
> > +
> > +	  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.)

Ah, maybe. I am not sure if the compiler generates the right stubs. At least
in userspace it does and there's no problem on 32-bit system. I will research
it more, perhaps by building a 32-bit kernel and seeing what the compiler
does.

Will re-spin later after any more feedback.

thanks,

 - Joel

> 
> > +
> > +		if (boot_ms < boot_end_delay) {
> > +			schedule_delayed_work(&boot_rcu_work,
> > +					boot_end_delay - boot_ms);
> > +			return;
> > +		}
> > +	}
> > +
> >  	rcu_unexpedite_gp();
> >  	rcu_async_relax();
> >  	if (rcu_normal_after_boot)
> 
> -- 
> ~Randy

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

[Joel Fernandes]

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

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

[Randy Dunlap]

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

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

[Joel Fernandes]

> On Feb 26, 2023, at 7:03 PM, Randy Dunlap <rdunlap@infradead.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