Re: [PATCH v4 2/2] rcu: Simplify the code logic of rcu_init_nohz()

["Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>]

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