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

[Wei Yang <richardw.yang@linux.intel.com>]

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
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