Re: [PATCH] mm/vmscan.c: only adjust related kswapd cpu affinity when online cpu

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

On Sun, Jan 26, 2020 at 02:44:31PM -0800, David Rientjes wrote:
>On Sun, 26 Jan 2020, Wei Yang wrote:
>
>> When onlining a cpu, kswapd_cpu_online() is called to adjust kswapd cpu
>> affinity.
>> 
>> Current routine does like this:
>> 
>>   * Iterate all the numa node
>>   * Adjust cpu affinity when node has an online cpu
>> 
>> Actually we could improve this by:
>> 
>>   * Just adjust the numa node to which current online cpu belongs
>> 
>> Because we know which numa node the cpu belongs to and this cpu would
>> not affect other node.
>> 
>
>Is there ever a situation where the cpu to be onlined would have appeared 
>in the cpumask of another node and so a different kswapd's cpumask would 
>now include an off-node cpu?

No, I don't think so.

Per my understanding, kswapd_cpu_online() will be invoked when a cpu is
onlined. And the particular cpu belongs to a particular numa node. So it is
not necessary to iterate on every nodes.

And current code use cpumask_and_any() to do the check. If my understanding is
correct, the check would return true if this node has any online cpu. This is
likely to be true.

This is why I want to make the logic clear.

>
>> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
>> ---
>>  mm/vmscan.c | 19 ++++++++++---------
>>  1 file changed, 10 insertions(+), 9 deletions(-)
>> 
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 572fb17c6273..19c92d35045c 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -4049,18 +4049,19 @@ unsigned long shrink_all_memory(unsigned long nr_to_reclaim)
>>     restore their cpu bindings. */
>>  static int kswapd_cpu_online(unsigned int cpu)
>>  {
>> -	int nid;
>> +	int nid = cpu_to_node(cpu);
>> +	pg_data_t *pgdat;
>> +	const struct cpumask *mask;
>>  
>> -	for_each_node_state(nid, N_MEMORY) {
>> -		pg_data_t *pgdat = NODE_DATA(nid);
>> -		const struct cpumask *mask;
>> +	if (!node_state(nid, N_MEMORY))
>> +		return 0;
>>  
>> -		mask = cpumask_of_node(pgdat->node_id);
>> +	pgdat = NODE_DATA(nid);
>> +	mask = cpumask_of_node(nid);
>> +
>> +	/* One of our CPUs online: restore mask */
>> +	set_cpus_allowed_ptr(pgdat->kswapd, mask);
>>  
>> -		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;
>>  }
>>  
>> -- 
>> 2.17.1
>> 
>> 
>> 

-- 
Wei Yang
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