Re: [PATCH v2] mm, page_alloc: enable pcpu_drain with zone capability

[Wei Yang <richard.weiyang@gmail.com>]

On Wed, Dec 12, 2018 at 06:00:49PM +0100, David Hildenbrand wrote:
>On 12.12.18 15:25, Wei Yang wrote:
>> drain_all_pages is documented to drain per-cpu pages for a given zone (if
>> non-NULL). The current implementation doesn't match the description though.
>> It will drain all pcp pages for all zones that happen to have cached pages
>> on the same cpu as the given zone. This will leave to premature pcp cache
>> draining for zones that are not of an interest for the caller - e.g.
>> compaction, hwpoison or memory offline.
>> 
>> This would force the page allocator to take locks and potential lock
>> contention as a result.
>> 
>> There is no real reason for this sub-optimal implementnation. Replace
>> per-cpu work item with a dedicated structure which contains a pointer to
>> zone and pass it over to the worker. This will get the zone information all
>> the way down to the worker function and do the right job.
>> 
>> [mhocko@suse.com: refactor the whole changelog]
>> 
>> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
>> Acked-by: Michal Hocko <mhocko@suse.com>
>> ---
>> v2:
>>    * refactor changelog from Michal's suggestion
>> ---
>>  mm/page_alloc.c | 20 ++++++++++++++------
>>  1 file changed, 14 insertions(+), 6 deletions(-)
>> 
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 65db26995466..eb4df3f63f5e 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -96,8 +96,12 @@ int _node_numa_mem_[MAX_NUMNODES];
>>  #endif
>>  
>>  /* work_structs for global per-cpu drains */
>
>s/work_structs/work_struct/ ?

Maybe the original comment wants to use plural form to mean there are
totally several work_structs since each cpu gets one?

>
>> +struct pcpu_drain {
>> +	struct zone *zone;
>> +	struct work_struct work;
>> +};
>>  DEFINE_MUTEX(pcpu_drain_mutex);
>> -DEFINE_PER_CPU(struct work_struct, pcpu_drain);
>> +DEFINE_PER_CPU(struct pcpu_drain, pcpu_drain);
>>  
>>  #ifdef CONFIG_GCC_PLUGIN_LATENT_ENTROPY
>>  volatile unsigned long latent_entropy __latent_entropy;
>> @@ -2596,6 +2600,8 @@ void drain_local_pages(struct zone *zone)
>>  
>>  static void drain_local_pages_wq(struct work_struct *work)
>>  {
>> +	struct pcpu_drain *drain =
>> +		container_of(work, struct pcpu_drain, work);
>>  	/*
>>  	 * drain_all_pages doesn't use proper cpu hotplug protection so
>>  	 * we can race with cpu offline when the WQ can move this from
>> @@ -2604,7 +2610,7 @@ static void drain_local_pages_wq(struct work_struct *work)
>>  	 * a different one.
>>  	 */
>>  	preempt_disable();
>> -	drain_local_pages(NULL);
>> +	drain_local_pages(drain->zone);
>>  	preempt_enable();
>>  }
>>  
>> @@ -2675,12 +2681,14 @@ void drain_all_pages(struct zone *zone)
>>  	}
>>  
>>  	for_each_cpu(cpu, &cpus_with_pcps) {
>> -		struct work_struct *work = per_cpu_ptr(&pcpu_drain, cpu);
>> -		INIT_WORK(work, drain_local_pages_wq);
>> -		queue_work_on(cpu, mm_percpu_wq, work);
>> +		struct pcpu_drain *drain = per_cpu_ptr(&pcpu_drain, cpu);
>> +
>> +		drain->zone = zone;
>> +		INIT_WORK(&drain->work, drain_local_pages_wq);
>> +		queue_work_on(cpu, mm_percpu_wq, &drain->work);
>>  	}
>>  	for_each_cpu(cpu, &cpus_with_pcps)
>> -		flush_work(per_cpu_ptr(&pcpu_drain, cpu));
>> +		flush_work(&per_cpu_ptr(&pcpu_drain, cpu)->work);
>>  
>>  	mutex_unlock(&pcpu_drain_mutex);
>>  }
>> 
>
>Looks good to me!
>
>Reviewed-by: David Hildenbrand <david@redhat.com>
>
>
>-- 
>
>Thanks,
>
>David / dhildenb

-- 
Wei Yang
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