linux-kernel - Re: [RFC 1/5] mm, page_alloc: clean up pageset high and batch update

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Date:   Thu, 17 Sep 2020 18:05:33 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Michal Hocko <mhocko@...nel.org>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        David Hildenbrand <david@...hat.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [RFC 1/5] mm, page_alloc: clean up pageset high and batch update

On 9/10/20 10:31 AM, Oscar Salvador wrote:
> On Mon, Sep 07, 2020 at 06:36:24PM +0200, Vlastimil Babka wrote:
> 
>> -/*
>> - * pageset_set_high() sets the high water mark for hot per_cpu_pagelist
>> - * to the value high for the pageset p.
>> - */
>> -static void pageset_set_high(struct per_cpu_pageset *p,
>> -				unsigned long high)
>> -{
>> -	unsigned long batch = max(1UL, high / 4);
>> -	if ((high / 4) > (PAGE_SHIFT * 8))
>> -		batch = PAGE_SHIFT * 8;
>> -
>> -	pageset_update(&p->pcp, high, batch);
>> +	pageset_update(&p->pcp, 0, 1);
>>  }
> 
> Could we restore the comment we had in pageset_set_high, and maybe
> update it to match this new function? I think it would be useful.

Same as David, I didn't find the comment useful at all. But I can try writing a
better one instead.

>>  
>>  static void pageset_set_high_and_batch(struct zone *zone,
>> -				       struct per_cpu_pageset *pcp)
>> +				       struct per_cpu_pageset *p)
>>  {
>> -	if (percpu_pagelist_fraction)
>> -		pageset_set_high(pcp,
>> -			(zone_managed_pages(zone) /
>> -				percpu_pagelist_fraction));
>> -	else
>> -		pageset_set_batch(pcp, zone_batchsize(zone));
>> +	unsigned long new_high;
>> +	unsigned long new_batch;
>> +	int fraction = READ_ONCE(percpu_pagelist_fraction);
> 
> Why the READ_ONCE? In case there is a parallel update so things to get
> messed up?

Yeah, I think online of a new zone can race with sysctl update to
percpu_pagelist_fraction, because online doesn't take pcp_batch_high_lock...
until patch 5. But you're right this should be separate.

> as I said, I'd appreciate a comment in pageset_set_high_and_batch to be
> restored and updated, otherwise:
> 
> Reviewed-by: Oscar Salvador <osalvador@...e.de> 

Thanks!

> Thanks
>