Re: [PATCH 3/9] mm, page_alloc: remove setup_pageset()

[Vlastimil Babka <vbabka@suse.cz>]

On 10/5/20 2:59 PM, Michal Hocko wrote:
> On Tue 22-09-20 16:37:06, Vlastimil Babka wrote:
>> We initialize boot-time pagesets with setup_pageset(), which sets high and
>> batch values that effectively disable pcplists.
>> 
>> We can remove this wrapper if we just set these values for all pagesets in
>> pageset_init(). Non-boot pagesets then subsequently update them to the proper
>> values.
>> 
>> No functional change.
>> 
>> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
>> ---
>>  mm/page_alloc.c | 17 ++++++++++-------
>>  1 file changed, 10 insertions(+), 7 deletions(-)
>> 
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 26069c8d1b19..76c2b4578723 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -5823,7 +5823,7 @@ static void build_zonelists(pg_data_t *pgdat)
>>   * not check if the processor is online before following the pageset pointer.
>>   * Other parts of the kernel may not check if the zone is available.
>>   */
>> -static void setup_pageset(struct per_cpu_pageset *p);
>> +static void pageset_init(struct per_cpu_pageset *p);
>>  static DEFINE_PER_CPU(struct per_cpu_pageset, boot_pageset);
>>  static DEFINE_PER_CPU(struct per_cpu_nodestat, boot_nodestats);
>>  
>> @@ -5891,7 +5891,7 @@ build_all_zonelists_init(void)
>>  	 * (a chicken-egg dilemma).
>>  	 */
>>  	for_each_possible_cpu(cpu)
>> -		setup_pageset(&per_cpu(boot_pageset, cpu));
>> +		pageset_init(&per_cpu(boot_pageset, cpu));
>>  
>>  	mminit_verify_zonelist();
>>  	cpuset_init_current_mems_allowed();
>> @@ -6210,12 +6210,15 @@ static void pageset_init(struct per_cpu_pageset *p)
>>  	pcp = &p->pcp;
>>  	for (migratetype = 0; migratetype < MIGRATE_PCPTYPES; migratetype++)
>>  		INIT_LIST_HEAD(&pcp->lists[migratetype]);
>> -}
>>  
>> -static void setup_pageset(struct per_cpu_pageset *p)
>> -{
>> -	pageset_init(p);
>> -	pageset_update(&p->pcp, 0, 1);
>> +	/*
>> +	 * Set batch and high values safe for a boot pageset. A true percpu
>> +	 * pageset's initialization will update them subsequently. Here we don't
>> +	 * need to be as careful as pageset_update() as nobody can access the
>> +	 * pageset yet.
> 
> Isn't this slightly misleading? pageset_init is called from setup_zone_pageset
> which is called from the memory hotplug as well. Isn't this more about

It is.

> early zone initialization rather than boot pagesets? Or am I misreading
> the patch?

It's for boot pagesets (call from build_all_zonelists_init()) that we need these 
particular values (well, just high = 0). For early zone initialization during 
hotplug the values don't matter as the caller is setup_zone_pageset() who 
immediately replaces the values by zone_set_pageset_high_and_batch(). Which is 
what the comment tries to say.

>> +	 */
>> +	pcp->high = 0;
>> +	pcp->batch = 1;
>>  }
>>  
>>  /*
>> -- 
>> 2.28.0
>