[PATCH v5 0/1] rcu/nocb: Delete local variable 'need_rcu_nocb_mask' in rcu_init_nohz()

[PATCH v5 0/1] rcu/nocb: Delete local variable 'need_rcu_nocb_mask' in rcu_init_nohz()

[Zhen Lei]

v4 --> v5:
1. Cancel eliminate rcu_state.nocb_is_setup. It is useful for case 1) below.

Test cases and test results as below:
 ----------------------------------------------------------------------
| CONFIG_NO_HZ_FULL  | CONFIG_RCU_NOCB_CPU_DEFAULT_ALL |    cmdline    |
 ----------------------------------------------------------------------
1)         N         |                 N               |     none      |
2)         N         |                 N               | rcu_nocbs=1-2 |
3)         Y         |                 N               | nohz_full=1-2 |
4)         Y         |                 Y               |     none      |
 ----------------------------------------------------------------------
 
1)
[ 1018.082417] rcu-torture:--- End of test: SUCCESS (default)
root@genericarmv8:~# dmesg | grep "Offload RCU callbacks"

root@genericarmv8:~# zcat /proc/config.gz | grep NOCB
CONFIG_RCU_NOCB_CPU=y
# CONFIG_RCU_NOCB_CPU_DEFAULT_ALL is not set
root@genericarmv8:~# zcat /proc/config.gz | grep NO_HZ_FULL
# CONFIG_NO_HZ_FULL is not set
root@genericarmv8:~# cat /proc/cmdline
console=ttyS0 earlyprintk=serial root=/dev/sda rw

2) 3) 4)
The same to v4.


v3 --> v4:
Fix a build warning.
-       struct cpumask *cpumask = NULL;
+       const struct cpumask *cpumask = NULL;

v2 --> v3:
1. Eliminate rcu_state.nocb_is_setup
2. Update the code based on the above patch and commit b37a667c6242
   ("rcu/nocb: Add an option to offload all CPUs on boot").

Test cases and test results as below:

 ----------------------------------------------------------------------
| CONFIG_NO_HZ_FULL  | CONFIG_RCU_NOCB_CPU_DEFAULT_ALL |    cmdline    |
 ----------------------------------------------------------------------
1)         N         |                 N               |     none      |
2)         N         |                 N               | rcu_nocbs=1-2 |
3)         Y         |                 N               | nohz_full=1-2 |
4)         Y         |                 Y               |     none      |
 ----------------------------------------------------------------------

1)
[ 1085.691770] rcu-torture:--- End of test: SUCCESS: (default)
root@genericarmv8:~# dmesg | grep "Offload RCU callbacks"
[    0.000000] rcu:     Offload RCU callbacks from CPUs: (none).
root@genericarmv8:~# zcat /proc/config.gz | grep NOCB
CONFIG_RCU_NOCB_CPU=y
# CONFIG_RCU_NOCB_CPU_DEFAULT_ALL is not set
root@genericarmv8:~# zcat /proc/config.gz | grep NO_HZ_FULL
# CONFIG_NO_HZ_FULL is not set
root@genericarmv8:~# cat /proc/cmdline
console=ttyAMA0 root=/dev/vda2 crashkernel=256M@ee000000

2)
[ 1022.532198] rcu-torture:--- End of test: SUCCESS: (default)
root@genericarmv8:~# dmesg | grep "Offload RCU callbacks"
[    0.000000] rcu:     Offload RCU callbacks from CPUs: 1-2.
root@genericarmv8:~# zcat /proc/config.gz | grep NOCB
CONFIG_RCU_NOCB_CPU=y
# CONFIG_RCU_NOCB_CPU_DEFAULT_ALL is not set
root@genericarmv8:~# zcat /proc/config.gz | grep NO_HZ_FULL
# CONFIG_NO_HZ_FULL is not set
root@genericarmv8:~# cat /proc/cmdline
console=ttyAMA0 root=/dev/vda2 crashkernel=256M@ee000000 rcu_nocbs=1-2

3)
[ 1011.714712] rcu-torture:--- End of test: SUCCESS: (default)
root@genericarmv8:~# dmesg | grep "Offload RCU callbacks"
[    0.000000] rcu:     Offload RCU callbacks from CPUs: 1-2.
root@genericarmv8:~# zcat /proc/config.gz | grep NOCB
CONFIG_RCU_NOCB_CPU=y
# CONFIG_RCU_NOCB_CPU_DEFAULT_ALL is not set
root@genericarmv8:~# zcat /proc/config.gz | grep NO_HZ_FULL
CONFIG_NO_HZ_FULL=y
root@genericarmv8:~# cat /proc/cmdline
console=ttyAMA0 root=/dev/vda2 crashkernel=256M@ee000000 nohz_full=1-2

4)
[ 1031.047094] rcu-torture:--- End of test: SUCCESS: (default)
root@genericarmv8:~# dmesg | grep "Offload RCU callbacks"
[    0.000000] rcu:     Offload RCU callbacks from CPUs: 0-3.
root@genericarmv8:~# zcat /proc/config.gz | grep NOCB
CONFIG_RCU_NOCB_CPU=y
CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y
root@genericarmv8:~# zcat /proc/config.gz | grep NO_HZ_FULL
CONFIG_NO_HZ_FULL=y
root@genericarmv8:~# cat /proc/cmdline
console=ttyAMA0 root=/dev/vda2 crashkernel=256M@ee000000


v1 --> v2:
Update commit message.


Zhen Lei (1):
  rcu: Simplify the code logic of rcu_init_nohz()

 kernel/rcu/tree_nocb.h | 32 +++++++++-----------------------
 1 file changed, 9 insertions(+), 23 deletions(-)

-- 
2.25.1

[PATCH v5 1/1] rcu: Simplify the code logic of rcu_init_nohz()

[Zhen Lei]

When CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y or CONFIG_NO_HZ_FULL=y, additional
CPUs need to be added to 'rcu_nocb_mask'. But 'rcu_nocb_mask' may be not
available now, due to 'rcu_nocbs' is not specified. Check and initialize
'rcu_nocb_mask' before using it. This code simplification strictly follows
this logic, compared with old implementations, unnecessary crossovers are
avoided and easy to understand.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
---
 kernel/rcu/tree_nocb.h | 32 +++++++++-----------------------
 1 file changed, 9 insertions(+), 23 deletions(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 0a5f0ef41484518..c8167be2288fa2c 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1210,45 +1210,31 @@ EXPORT_SYMBOL_GPL(rcu_nocb_cpu_offload);
 void __init rcu_init_nohz(void)
 {
 	int cpu;
-	bool need_rcu_nocb_mask = false;
-	bool offload_all = false;
 	struct rcu_data *rdp;
+	const struct cpumask *cpumask = NULL;
 
 #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)) {
-		need_rcu_nocb_mask = true;
-		offload_all = false; /* NO_HZ_FULL has its own mask. */
-	}
-#endif /* #if defined(CONFIG_NO_HZ_FULL) */
+	cpumask = cpu_possible_mask;
+#elif defined(CONFIG_NO_HZ_FULL)
+	if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
+		cpumask = tick_nohz_full_mask;
+#endif
 
-	if (need_rcu_nocb_mask) {
+	if (cpumask) {
 		if (!cpumask_available(rcu_nocb_mask)) {
 			if (!zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL)) {
 				pr_info("rcu_nocb_mask allocation failed, callback offloading disabled.\n");
 				return;
 			}
 		}
+
+		cpumask_or(rcu_nocb_mask, rcu_nocb_mask, cpumask);
 		rcu_state.nocb_is_setup = true;
 	}
 
 	if (!rcu_state.nocb_is_setup)
 		return;
 
-#if defined(CONFIG_NO_HZ_FULL)
-	if (tick_nohz_full_running)
-		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);
-
 	if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
 		pr_info("\tNote: kernel parameter 'rcu_nocbs=', 'nohz_full', or 'isolcpus=' contains nonexistent CPUs.\n");
 		cpumask_and(rcu_nocb_mask, cpu_possible_mask,
-- 
2.25.1

Re: [PATCH v5 1/1] rcu: Simplify the code logic of rcu_init_nohz()

[Paul E. McKenney]

On Thu, Aug 25, 2022 at 05:23:11PM +0800, Zhen Lei wrote:
> When CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y or CONFIG_NO_HZ_FULL=y, additional
> CPUs need to be added to 'rcu_nocb_mask'. But 'rcu_nocb_mask' may be not
> available now, due to 'rcu_nocbs' is not specified. Check and initialize
> 'rcu_nocb_mask' before using it. This code simplification strictly follows
> this logic, compared with old implementations, unnecessary crossovers are
> avoided and easy to understand.
> 
> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>

Much nicer, thank you!

As usual, I could not resist the urge to wordsmith.  Could you please
check to make sure that I did not mess anything up?

							Thanx, Paul

------------------------------------------------------------------------

commit 4ac3b3d1a19943b1522c0b1d0895aefbb80ec294
Author: Zhen Lei <thunder.leizhen@huawei.com>
Date:   Thu Aug 25 17:23:11 2022 +0800

    rcu: Simplify rcu_init_nohz() cpumask handling
    
    In kernels built with either CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y or
    CONFIG_NO_HZ_FULL=y, additional CPUs must be added to rcu_nocb_mask.
    Except that kernels booted without the rcu_nocb_mask= will not have
    allocated rcu_nocb_mask.  And the current rcu_init_nohz() function uses
    its need_rcu_nocb_mask and offload_all local variables to track the
    rcu_nocb and nohz_full state.
    
    But there is a much simpler approach, namely creating a cpumask pointer
    to track the default and then using cpumask_available() to check the
    rcu_nocb_mask state.  This commit takes this approach, thereby simplifying
    and shortening the rcu_init_nohz() function.
    
    Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
    Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 0a5f0ef414845..c8167be2288fa 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1210,45 +1210,31 @@ EXPORT_SYMBOL_GPL(rcu_nocb_cpu_offload);
 void __init rcu_init_nohz(void)
 {
 	int cpu;
-	bool need_rcu_nocb_mask = false;
-	bool offload_all = false;
 	struct rcu_data *rdp;
+	const struct cpumask *cpumask = NULL;
 
 #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)) {
-		need_rcu_nocb_mask = true;
-		offload_all = false; /* NO_HZ_FULL has its own mask. */
-	}
-#endif /* #if defined(CONFIG_NO_HZ_FULL) */
+	cpumask = cpu_possible_mask;
+#elif defined(CONFIG_NO_HZ_FULL)
+	if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
+		cpumask = tick_nohz_full_mask;
+#endif
 
-	if (need_rcu_nocb_mask) {
+	if (cpumask) {
 		if (!cpumask_available(rcu_nocb_mask)) {
 			if (!zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL)) {
 				pr_info("rcu_nocb_mask allocation failed, callback offloading disabled.\n");
 				return;
 			}
 		}
+
+		cpumask_or(rcu_nocb_mask, rcu_nocb_mask, cpumask);
 		rcu_state.nocb_is_setup = true;
 	}
 
 	if (!rcu_state.nocb_is_setup)
 		return;
 
-#if defined(CONFIG_NO_HZ_FULL)
-	if (tick_nohz_full_running)
-		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);
-
 	if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
 		pr_info("\tNote: kernel parameter 'rcu_nocbs=', 'nohz_full', or 'isolcpus=' contains nonexistent CPUs.\n");
 		cpumask_and(rcu_nocb_mask, cpu_possible_mask,

Re: [PATCH v5 1/1] rcu: Simplify the code logic of rcu_init_nohz()

[Joel Fernandes]

On Thu, Aug 25, 2022 at 10:26:54AM -0700, Paul E. McKenney wrote:
> On Thu, Aug 25, 2022 at 05:23:11PM +0800, Zhen Lei wrote:
> > When CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y or CONFIG_NO_HZ_FULL=y, additional
> > CPUs need to be added to 'rcu_nocb_mask'. But 'rcu_nocb_mask' may be not
> > available now, due to 'rcu_nocbs' is not specified. Check and initialize
> > 'rcu_nocb_mask' before using it. This code simplification strictly follows
> > this logic, compared with old implementations, unnecessary crossovers are
> > avoided and easy to understand.
> > 
> > Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
> 
> Much nicer, thank you!
> 
> As usual, I could not resist the urge to wordsmith.  Could you please
> check to make sure that I did not mess anything up?
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> commit 4ac3b3d1a19943b1522c0b1d0895aefbb80ec294
> Author: Zhen Lei <thunder.leizhen@huawei.com>
> Date:   Thu Aug 25 17:23:11 2022 +0800
> 
>     rcu: Simplify rcu_init_nohz() cpumask handling
>     
>     In kernels built with either CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y or
>     CONFIG_NO_HZ_FULL=y, additional CPUs must be added to rcu_nocb_mask.
>     Except that kernels booted without the rcu_nocb_mask= will not have
>     allocated rcu_nocb_mask.  And the current rcu_init_nohz() function uses
>     its need_rcu_nocb_mask and offload_all local variables to track the
>     rcu_nocb and nohz_full state.
>     
>     But there is a much simpler approach, namely creating a cpumask pointer
>     to track the default and then using cpumask_available() to check the
>     rcu_nocb_mask state.  This commit takes this approach, thereby simplifying
>     and shortening the rcu_init_nohz() function.
>     
>     Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
>     Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>

thanks,

 - Joel


> 
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index 0a5f0ef414845..c8167be2288fa 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -1210,45 +1210,31 @@ EXPORT_SYMBOL_GPL(rcu_nocb_cpu_offload);
>  void __init rcu_init_nohz(void)
>  {
>  	int cpu;
> -	bool need_rcu_nocb_mask = false;
> -	bool offload_all = false;
>  	struct rcu_data *rdp;
> +	const struct cpumask *cpumask = NULL;
>  
>  #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)) {
> -		need_rcu_nocb_mask = true;
> -		offload_all = false; /* NO_HZ_FULL has its own mask. */
> -	}
> -#endif /* #if defined(CONFIG_NO_HZ_FULL) */
> +	cpumask = cpu_possible_mask;
> +#elif defined(CONFIG_NO_HZ_FULL)
> +	if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
> +		cpumask = tick_nohz_full_mask;
> +#endif
>  
> -	if (need_rcu_nocb_mask) {
> +	if (cpumask) {
>  		if (!cpumask_available(rcu_nocb_mask)) {
>  			if (!zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL)) {
>  				pr_info("rcu_nocb_mask allocation failed, callback offloading disabled.\n");
>  				return;
>  			}
>  		}
> +
> +		cpumask_or(rcu_nocb_mask, rcu_nocb_mask, cpumask);
>  		rcu_state.nocb_is_setup = true;
>  	}
>  
>  	if (!rcu_state.nocb_is_setup)
>  		return;
>  
> -#if defined(CONFIG_NO_HZ_FULL)
> -	if (tick_nohz_full_running)
> -		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);
> -
>  	if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
>  		pr_info("\tNote: kernel parameter 'rcu_nocbs=', 'nohz_full', or 'isolcpus=' contains nonexistent CPUs.\n");
>  		cpumask_and(rcu_nocb_mask, cpu_possible_mask,

Re: [PATCH v5 1/1] rcu: Simplify the code logic of rcu_init_nohz()

[Paul E. McKenney]

On Thu, Aug 25, 2022 at 07:24:05PM +0000, Joel Fernandes wrote:
> On Thu, Aug 25, 2022 at 10:26:54AM -0700, Paul E. McKenney wrote:
> > On Thu, Aug 25, 2022 at 05:23:11PM +0800, Zhen Lei wrote:
> > > When CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y or CONFIG_NO_HZ_FULL=y, additional
> > > CPUs need to be added to 'rcu_nocb_mask'. But 'rcu_nocb_mask' may be not
> > > available now, due to 'rcu_nocbs' is not specified. Check and initialize
> > > 'rcu_nocb_mask' before using it. This code simplification strictly follows
> > > this logic, compared with old implementations, unnecessary crossovers are
> > > avoided and easy to understand.
> > > 
> > > Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
> > 
> > Much nicer, thank you!
> > 
> > As usual, I could not resist the urge to wordsmith.  Could you please
> > check to make sure that I did not mess anything up?
> > 
> > 							Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> > commit 4ac3b3d1a19943b1522c0b1d0895aefbb80ec294
> > Author: Zhen Lei <thunder.leizhen@huawei.com>
> > Date:   Thu Aug 25 17:23:11 2022 +0800
> > 
> >     rcu: Simplify rcu_init_nohz() cpumask handling
> >     
> >     In kernels built with either CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y or
> >     CONFIG_NO_HZ_FULL=y, additional CPUs must be added to rcu_nocb_mask.
> >     Except that kernels booted without the rcu_nocb_mask= will not have
> >     allocated rcu_nocb_mask.  And the current rcu_init_nohz() function uses
> >     its need_rcu_nocb_mask and offload_all local variables to track the
> >     rcu_nocb and nohz_full state.
> >     
> >     But there is a much simpler approach, namely creating a cpumask pointer
> >     to track the default and then using cpumask_available() to check the
> >     rcu_nocb_mask state.  This commit takes this approach, thereby simplifying
> >     and shortening the rcu_init_nohz() function.
> >     
> >     Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
> >     Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> 
> Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>

Thank you, and I will apply this on the next rebase.

							Thanx, Paul

> thanks,
> 
>  - Joel
> 
> 
> > 
> > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> > index 0a5f0ef414845..c8167be2288fa 100644
> > --- a/kernel/rcu/tree_nocb.h
> > +++ b/kernel/rcu/tree_nocb.h
> > @@ -1210,45 +1210,31 @@ EXPORT_SYMBOL_GPL(rcu_nocb_cpu_offload);
> >  void __init rcu_init_nohz(void)
> >  {
> >  	int cpu;
> > -	bool need_rcu_nocb_mask = false;
> > -	bool offload_all = false;
> >  	struct rcu_data *rdp;
> > +	const struct cpumask *cpumask = NULL;
> >  
> >  #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)) {
> > -		need_rcu_nocb_mask = true;
> > -		offload_all = false; /* NO_HZ_FULL has its own mask. */
> > -	}
> > -#endif /* #if defined(CONFIG_NO_HZ_FULL) */
> > +	cpumask = cpu_possible_mask;
> > +#elif defined(CONFIG_NO_HZ_FULL)
> > +	if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
> > +		cpumask = tick_nohz_full_mask;
> > +#endif
> >  
> > -	if (need_rcu_nocb_mask) {
> > +	if (cpumask) {
> >  		if (!cpumask_available(rcu_nocb_mask)) {
> >  			if (!zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL)) {
> >  				pr_info("rcu_nocb_mask allocation failed, callback offloading disabled.\n");
> >  				return;
> >  			}
> >  		}
> > +
> > +		cpumask_or(rcu_nocb_mask, rcu_nocb_mask, cpumask);
> >  		rcu_state.nocb_is_setup = true;
> >  	}
> >  
> >  	if (!rcu_state.nocb_is_setup)
> >  		return;
> >  
> > -#if defined(CONFIG_NO_HZ_FULL)
> > -	if (tick_nohz_full_running)
> > -		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);
> > -
> >  	if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
> >  		pr_info("\tNote: kernel parameter 'rcu_nocbs=', 'nohz_full', or 'isolcpus=' contains nonexistent CPUs.\n");
> >  		cpumask_and(rcu_nocb_mask, cpu_possible_mask,

Re: [PATCH v5 1/1] rcu: Simplify the code logic of rcu_init_nohz()

[Leizhen (ThunderTown)]

On 2022/8/26 1:26, Paul E. McKenney wrote:
> On Thu, Aug 25, 2022 at 05:23:11PM +0800, Zhen Lei wrote:
>> When CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y or CONFIG_NO_HZ_FULL=y, additional
>> CPUs need to be added to 'rcu_nocb_mask'. But 'rcu_nocb_mask' may be not
>> available now, due to 'rcu_nocbs' is not specified. Check and initialize
>> 'rcu_nocb_mask' before using it. This code simplification strictly follows
>> this logic, compared with old implementations, unnecessary crossovers are
>> avoided and easy to understand.
>>
>> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
> 
> Much nicer, thank you!
> 
> As usual, I could not resist the urge to wordsmith.  Could you please
> check to make sure that I did not mess anything up?

Okay, thank you very much. The boot option name "rcu_nocb_mask=" is
incorrect, should be "rcu_nocbs=".

> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> commit 4ac3b3d1a19943b1522c0b1d0895aefbb80ec294
> Author: Zhen Lei <thunder.leizhen@huawei.com>
> Date:   Thu Aug 25 17:23:11 2022 +0800
> 
>     rcu: Simplify rcu_init_nohz() cpumask handling
>     
>     In kernels built with either CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y or
>     CONFIG_NO_HZ_FULL=y, additional CPUs must be added to rcu_nocb_mask.
>     Except that kernels booted without the rcu_nocb_mask= will not have

rcu_nocb_mask=   ---->  rcu_nocbs=

>     allocated rcu_nocb_mask.  And the current rcu_init_nohz() function uses
>     its need_rcu_nocb_mask and offload_all local variables to track the
>     rcu_nocb and nohz_full state.
>     
>     But there is a much simpler approach, namely creating a cpumask pointer
>     to track the default and then using cpumask_available() to check the
>     rcu_nocb_mask state.  This commit takes this approach, thereby simplifying
>     and shortening the rcu_init_nohz() function.
>     
>     Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
>     Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> 
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index 0a5f0ef414845..c8167be2288fa 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -1210,45 +1210,31 @@ EXPORT_SYMBOL_GPL(rcu_nocb_cpu_offload);
>  void __init rcu_init_nohz(void)
>  {
>  	int cpu;
> -	bool need_rcu_nocb_mask = false;
> -	bool offload_all = false;
>  	struct rcu_data *rdp;
> +	const struct cpumask *cpumask = NULL;
>  
>  #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)) {
> -		need_rcu_nocb_mask = true;
> -		offload_all = false; /* NO_HZ_FULL has its own mask. */
> -	}
> -#endif /* #if defined(CONFIG_NO_HZ_FULL) */
> +	cpumask = cpu_possible_mask;
> +#elif defined(CONFIG_NO_HZ_FULL)
> +	if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
> +		cpumask = tick_nohz_full_mask;
> +#endif
>  
> -	if (need_rcu_nocb_mask) {
> +	if (cpumask) {
>  		if (!cpumask_available(rcu_nocb_mask)) {
>  			if (!zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL)) {
>  				pr_info("rcu_nocb_mask allocation failed, callback offloading disabled.\n");
>  				return;
>  			}
>  		}
> +
> +		cpumask_or(rcu_nocb_mask, rcu_nocb_mask, cpumask);
>  		rcu_state.nocb_is_setup = true;
>  	}
>  
>  	if (!rcu_state.nocb_is_setup)
>  		return;
>  
> -#if defined(CONFIG_NO_HZ_FULL)
> -	if (tick_nohz_full_running)
> -		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);
> -
>  	if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
>  		pr_info("\tNote: kernel parameter 'rcu_nocbs=', 'nohz_full', or 'isolcpus=' contains nonexistent CPUs.\n");
>  		cpumask_and(rcu_nocb_mask, cpu_possible_mask,
> .
> 

-- 
Regards,
  Zhen Lei

Re: [PATCH v5 1/1] rcu: Simplify the code logic of rcu_init_nohz()

[Paul E. McKenney]

On Fri, Aug 26, 2022 at 09:42:11AM +0800, Leizhen (ThunderTown) wrote:
> 
> 
> On 2022/8/26 1:26, Paul E. McKenney wrote:
> > On Thu, Aug 25, 2022 at 05:23:11PM +0800, Zhen Lei wrote:
> >> When CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y or CONFIG_NO_HZ_FULL=y, additional
> >> CPUs need to be added to 'rcu_nocb_mask'. But 'rcu_nocb_mask' may be not
> >> available now, due to 'rcu_nocbs' is not specified. Check and initialize
> >> 'rcu_nocb_mask' before using it. This code simplification strictly follows
> >> this logic, compared with old implementations, unnecessary crossovers are
> >> avoided and easy to understand.
> >>
> >> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
> > 
> > Much nicer, thank you!
> > 
> > As usual, I could not resist the urge to wordsmith.  Could you please
> > check to make sure that I did not mess anything up?
> 
> Okay, thank you very much. The boot option name "rcu_nocb_mask=" is
> incorrect, should be "rcu_nocbs=".

Good eyes, thank you!  I will fix this on the next rebase.

							Thanx, Paul

> > ------------------------------------------------------------------------
> > 
> > commit 4ac3b3d1a19943b1522c0b1d0895aefbb80ec294
> > Author: Zhen Lei <thunder.leizhen@huawei.com>
> > Date:   Thu Aug 25 17:23:11 2022 +0800
> > 
> >     rcu: Simplify rcu_init_nohz() cpumask handling
> >     
> >     In kernels built with either CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y or
> >     CONFIG_NO_HZ_FULL=y, additional CPUs must be added to rcu_nocb_mask.
> >     Except that kernels booted without the rcu_nocb_mask= will not have
> 
> rcu_nocb_mask=   ---->  rcu_nocbs=
> 
> >     allocated rcu_nocb_mask.  And the current rcu_init_nohz() function uses
> >     its need_rcu_nocb_mask and offload_all local variables to track the
> >     rcu_nocb and nohz_full state.
> >     
> >     But there is a much simpler approach, namely creating a cpumask pointer
> >     to track the default and then using cpumask_available() to check the
> >     rcu_nocb_mask state.  This commit takes this approach, thereby simplifying
> >     and shortening the rcu_init_nohz() function.
> >     
> >     Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
> >     Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > 
> > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> > index 0a5f0ef414845..c8167be2288fa 100644
> > --- a/kernel/rcu/tree_nocb.h
> > +++ b/kernel/rcu/tree_nocb.h
> > @@ -1210,45 +1210,31 @@ EXPORT_SYMBOL_GPL(rcu_nocb_cpu_offload);
> >  void __init rcu_init_nohz(void)
> >  {
> >  	int cpu;
> > -	bool need_rcu_nocb_mask = false;
> > -	bool offload_all = false;
> >  	struct rcu_data *rdp;
> > +	const struct cpumask *cpumask = NULL;
> >  
> >  #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)) {
> > -		need_rcu_nocb_mask = true;
> > -		offload_all = false; /* NO_HZ_FULL has its own mask. */
> > -	}
> > -#endif /* #if defined(CONFIG_NO_HZ_FULL) */
> > +	cpumask = cpu_possible_mask;
> > +#elif defined(CONFIG_NO_HZ_FULL)
> > +	if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
> > +		cpumask = tick_nohz_full_mask;
> > +#endif
> >  
> > -	if (need_rcu_nocb_mask) {
> > +	if (cpumask) {
> >  		if (!cpumask_available(rcu_nocb_mask)) {
> >  			if (!zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL)) {
> >  				pr_info("rcu_nocb_mask allocation failed, callback offloading disabled.\n");
> >  				return;
> >  			}
> >  		}
> > +
> > +		cpumask_or(rcu_nocb_mask, rcu_nocb_mask, cpumask);
> >  		rcu_state.nocb_is_setup = true;
> >  	}
> >  
> >  	if (!rcu_state.nocb_is_setup)
> >  		return;
> >  
> > -#if defined(CONFIG_NO_HZ_FULL)
> > -	if (tick_nohz_full_running)
> > -		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);
> > -
> >  	if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
> >  		pr_info("\tNote: kernel parameter 'rcu_nocbs=', 'nohz_full', or 'isolcpus=' contains nonexistent CPUs.\n");
> >  		cpumask_and(rcu_nocb_mask, cpu_possible_mask,
> > .
> > 
> 
> -- 
> Regards,
>   Zhen Lei