[PATCH] mm/vmscan.c: remove cpu online notification for now

[PATCH] mm/vmscan.c: remove cpu online notification for now

[Wei Yang]

The cpu online notification is used to adjust kswapd cpu affinity when a
NUMA node gains a new CPU.

Since currently we don't see a real runtime configuration like this,
let's drop this online notification for now.

Suggested-by: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>

---
v3:
  * remove the cpu online notification suggested by Michal
v2:
  * rephrase the changelog
---
 mm/vmscan.c | 27 +--------------------------
 1 file changed, 1 insertion(+), 26 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 665f33258cd7..a4fdf3dc8887 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4023,27 +4023,6 @@ unsigned long shrink_all_memory(unsigned long nr_to_reclaim)
 }
 #endif /* CONFIG_HIBERNATION */
 
-/* It's optimal to keep kswapds on the same CPUs as their memory, but
-   not required for correctness.  So if the last cpu in a node goes
-   away, we get changed to run anywhere: as the first one comes back,
-   restore their cpu bindings. */
-static int kswapd_cpu_online(unsigned int cpu)
-{
-	int nid;
-
-	for_each_node_state(nid, N_MEMORY) {
-		pg_data_t *pgdat = NODE_DATA(nid);
-		const struct cpumask *mask;
-
-		mask = cpumask_of_node(pgdat->node_id);
-
-		if (cpumask_any_and(cpu_online_mask, mask) < nr_cpu_ids)
-			/* One of our CPUs online: restore mask */
-			set_cpus_allowed_ptr(pgdat->kswapd, mask);
-	}
-	return 0;
-}
-
 /*
  * This kswapd start function will be called by init and node-hot-add.
  * On node-hot-add, kswapd will moved to proper cpus if cpus are hot-added.
@@ -4083,15 +4062,11 @@ void kswapd_stop(int nid)
 
 static int __init kswapd_init(void)
 {
-	int nid, ret;
+	int nid;
 
 	swap_setup();
 	for_each_node_state(nid, N_MEMORY)
  		kswapd_run(nid);
-	ret = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN,
-					"mm/vmscan:online", kswapd_cpu_online,
-					NULL);
-	WARN_ON(ret < 0);
 	return 0;
 }
 
-- 
2.17.1

Re: [PATCH] mm/vmscan.c: remove cpu online notification for now

[Michal Hocko]

On Tue 18-02-20 08:43:54, Wei Yang wrote:
> The cpu online notification is used to adjust kswapd cpu affinity when a
> NUMA node gains a new CPU.
> 
> Since currently we don't see a real runtime configuration like this,
> let's drop this online notification for now.

This deserves much more explanation IMHO. What would you say about the
following.
"
kswapd kernel thread starts either with a CPU affinity set to the full
cpu mask of its target node or without any affinity at all if the node
is CPUless. There is a cpu hotplug callback (kswapd_cpu_online) that
implements an elaborate way to update this mask when a cpu is onlined.

It is not really clear whether there is any actual benefit from this
scheme. Completely CPU-less NUMA nodes rarely gain a new CPU during
runtime. Drop the code for that reason. If there is a real usecase then
we can resurrect and simplify the code.
"
> 
> Suggested-by: Michal Hocko <mhocko@kernel.org>
> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>

Acked-by: Michal Hocko <mhocko@suse.com>

> 
> ---
> v3:
>   * remove the cpu online notification suggested by Michal
> v2:
>   * rephrase the changelog
> ---
>  mm/vmscan.c | 27 +--------------------------
>  1 file changed, 1 insertion(+), 26 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 665f33258cd7..a4fdf3dc8887 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -4023,27 +4023,6 @@ unsigned long shrink_all_memory(unsigned long nr_to_reclaim)
>  }
>  #endif /* CONFIG_HIBERNATION */
>  
> -/* It's optimal to keep kswapds on the same CPUs as their memory, but
> -   not required for correctness.  So if the last cpu in a node goes
> -   away, we get changed to run anywhere: as the first one comes back,
> -   restore their cpu bindings. */
> -static int kswapd_cpu_online(unsigned int cpu)
> -{
> -	int nid;
> -
> -	for_each_node_state(nid, N_MEMORY) {
> -		pg_data_t *pgdat = NODE_DATA(nid);
> -		const struct cpumask *mask;
> -
> -		mask = cpumask_of_node(pgdat->node_id);
> -
> -		if (cpumask_any_and(cpu_online_mask, mask) < nr_cpu_ids)
> -			/* One of our CPUs online: restore mask */
> -			set_cpus_allowed_ptr(pgdat->kswapd, mask);
> -	}
> -	return 0;
> -}
> -
>  /*
>   * This kswapd start function will be called by init and node-hot-add.
>   * On node-hot-add, kswapd will moved to proper cpus if cpus are hot-added.
> @@ -4083,15 +4062,11 @@ void kswapd_stop(int nid)
>  
>  static int __init kswapd_init(void)
>  {
> -	int nid, ret;
> +	int nid;
>  
>  	swap_setup();
>  	for_each_node_state(nid, N_MEMORY)
>   		kswapd_run(nid);
> -	ret = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN,
> -					"mm/vmscan:online", kswapd_cpu_online,
> -					NULL);
> -	WARN_ON(ret < 0);
>  	return 0;
>  }
>  
> -- 
> 2.17.1

-- 
Michal Hocko
SUSE Labs

Re: [PATCH] mm/vmscan.c: remove cpu online notification for now

[Wei Yang]

On Tue, Feb 18, 2020 at 09:40:23AM +0100, Michal Hocko wrote:
>On Tue 18-02-20 08:43:54, Wei Yang wrote:
>> The cpu online notification is used to adjust kswapd cpu affinity when a
>> NUMA node gains a new CPU.
>> 
>> Since currently we don't see a real runtime configuration like this,
>> let's drop this online notification for now.
>
>This deserves much more explanation IMHO. What would you say about the
>following.
>"
>kswapd kernel thread starts either with a CPU affinity set to the full
>cpu mask of its target node or without any affinity at all if the node
>is CPUless. There is a cpu hotplug callback (kswapd_cpu_online) that
>implements an elaborate way to update this mask when a cpu is onlined.
>
>It is not really clear whether there is any actual benefit from this
>scheme. Completely CPU-less NUMA nodes rarely gain a new CPU during
>runtime. Drop the code for that reason. If there is a real usecase then
>we can resurrect and simplify the code.
>"

More better :-) Thanks

>> 
>> Suggested-by: Michal Hocko <mhocko@kernel.org>
>> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
>
>Acked-by: Michal Hocko <mhocko@suse.com>
>
>> 
>> ---
>> v3:
>>   * remove the cpu online notification suggested by Michal
>> v2:
>>   * rephrase the changelog
>> ---
>>  mm/vmscan.c | 27 +--------------------------
>>  1 file changed, 1 insertion(+), 26 deletions(-)
>> 
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 665f33258cd7..a4fdf3dc8887 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -4023,27 +4023,6 @@ unsigned long shrink_all_memory(unsigned long nr_to_reclaim)
>>  }
>>  #endif /* CONFIG_HIBERNATION */
>>  
>> -/* It's optimal to keep kswapds on the same CPUs as their memory, but
>> -   not required for correctness.  So if the last cpu in a node goes
>> -   away, we get changed to run anywhere: as the first one comes back,
>> -   restore their cpu bindings. */
>> -static int kswapd_cpu_online(unsigned int cpu)
>> -{
>> -	int nid;
>> -
>> -	for_each_node_state(nid, N_MEMORY) {
>> -		pg_data_t *pgdat = NODE_DATA(nid);
>> -		const struct cpumask *mask;
>> -
>> -		mask = cpumask_of_node(pgdat->node_id);
>> -
>> -		if (cpumask_any_and(cpu_online_mask, mask) < nr_cpu_ids)
>> -			/* One of our CPUs online: restore mask */
>> -			set_cpus_allowed_ptr(pgdat->kswapd, mask);
>> -	}
>> -	return 0;
>> -}
>> -
>>  /*
>>   * This kswapd start function will be called by init and node-hot-add.
>>   * On node-hot-add, kswapd will moved to proper cpus if cpus are hot-added.
>> @@ -4083,15 +4062,11 @@ void kswapd_stop(int nid)
>>  
>>  static int __init kswapd_init(void)
>>  {
>> -	int nid, ret;
>> +	int nid;
>>  
>>  	swap_setup();
>>  	for_each_node_state(nid, N_MEMORY)
>>   		kswapd_run(nid);
>> -	ret = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN,
>> -					"mm/vmscan:online", kswapd_cpu_online,
>> -					NULL);
>> -	WARN_ON(ret < 0);
>>  	return 0;
>>  }
>>  
>> -- 
>> 2.17.1
>
>-- 
>Michal Hocko
>SUSE Labs

-- 
Wei Yang
Help you, Help me