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  3. [PATCH v4 0/2] rcu/nocb: Delete local variable 'need_rcu_nocb_mask' in rcu_init_nohz()
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[PATCH v4 2/2] rcu: Simplify the code logic of rcu_init_nohz()

Zhen Lei posted 2 patches 3 years, 7 months ago
There is a newer version of this series
[PATCH v4 2/2] rcu: Simplify the code logic of rcu_init_nohz()
Posted by Zhen Lei 3 years, 7 months ago 
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 ff763e7dc53551f..3c59b12f4465af1 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1209,44 +1209,30 @@ 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 (!cpumask_available(rcu_nocb_mask)) {
-		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);
 	}
 
 	if (!cpumask_available(rcu_nocb_mask))
 		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 v4 2/2] rcu: Simplify the code logic of rcu_init_nohz()
Posted by Paul E. McKenney 3 years, 7 months ago 
On Wed, Aug 17, 2022 at 09:42:53AM +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>

This looks like a nice simplification, but I will wait for your response
on Patch 1/1 before trying it out.

							Thanx, Paul

> ---
>  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 ff763e7dc53551f..3c59b12f4465af1 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -1209,44 +1209,30 @@ 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 (!cpumask_available(rcu_nocb_mask)) {
> -		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);
>  	}
>  
>  	if (!cpumask_available(rcu_nocb_mask))
>  		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 v4 2/2] rcu: Simplify the code logic of rcu_init_nohz()
Posted by Leizhen (ThunderTown) 3 years, 7 months ago 

On 2022/8/23 0:35, Paul E. McKenney wrote:
> On Wed, Aug 17, 2022 at 09:42:53AM +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>
> 
> This looks like a nice simplification, but I will wait for your response
> on Patch 1/1 before trying it out.

How about I post v5 and just do this simplification? Patch 1/2 seems to require
further discussion and in-depth analysis, which may take a long time.

> 
> 							Thanx, Paul
> 
>> ---
>>  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 ff763e7dc53551f..3c59b12f4465af1 100644
>> --- a/kernel/rcu/tree_nocb.h
>> +++ b/kernel/rcu/tree_nocb.h
>> @@ -1209,44 +1209,30 @@ 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 (!cpumask_available(rcu_nocb_mask)) {
>> -		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);
>>  	}
>>  
>>  	if (!cpumask_available(rcu_nocb_mask))
>>  		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
>>
> .
> 

-- 
Regards,
  Zhen Lei
Re: [PATCH v4 2/2] rcu: Simplify the code logic of rcu_init_nohz()
Posted by Paul E. McKenney 3 years, 7 months ago 
On Tue, Aug 23, 2022 at 08:13:00PM +0800, Leizhen (ThunderTown) wrote:
> 
> 
> On 2022/8/23 0:35, Paul E. McKenney wrote:
> > On Wed, Aug 17, 2022 at 09:42:53AM +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>
> > 
> > This looks like a nice simplification, but I will wait for your response
> > on Patch 1/1 before trying it out.
> 
> How about I post v5 and just do this simplification? Patch 1/2 seems to require
> further discussion and in-depth analysis, which may take a long time.

Excellent choice, thank you!


							Thanx, Paul

> >> ---
> >>  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 ff763e7dc53551f..3c59b12f4465af1 100644
> >> --- a/kernel/rcu/tree_nocb.h
> >> +++ b/kernel/rcu/tree_nocb.h
> >> @@ -1209,44 +1209,30 @@ 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 (!cpumask_available(rcu_nocb_mask)) {
> >> -		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);
> >>  	}
> >>  
> >>  	if (!cpumask_available(rcu_nocb_mask))
> >>  		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
> >>
> > .
> > 
> 
> -- 
> Regards,
>   Zhen Lei

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