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

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Date:   Thu, 10 Sep 2020 12:47:49 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Oscar Salvador <osalvador@...e.de>,
        Vlastimil Babka <vbabka@...e.cz>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Michal Hocko <mhocko@...nel.org>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [RFC 1/5] mm, page_alloc: clean up pageset high and batch update

On 10.09.20 10:31, Oscar Salvador wrote:
> On Mon, Sep 07, 2020 at 06:36:24PM +0200, Vlastimil Babka wrote:
>> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> 
>>  	for_each_possible_cpu(cpu)
>> -		setup_pageset(&per_cpu(boot_pageset, cpu), 0);
>> +		setup_pageset(&per_cpu(boot_pageset, cpu));
> 
> This is not really anything important but I realized we have like 7 functions
> messing with pcp lists, and everytime I try to follow them my head spins.
> 
> Since setup_pageset is only being called here, could we replace it by the
> pageset_init and pageset_update?

Had the same thought, so +1.

>> -/*
>> - * 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.

At least I didn't really understand what "pageset_set_high() sets the
high water mark for hot per_cpu_pagelist to the value high for the
pageset p." was trying to tell me.

I think the only valuable information is the "hot", meaning it is in use
and we have to be careful when updating, right?

>>  
>>  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?

Agreed, this is an actual change in the code. If this is a fix, separate
patch?

Apart from that, looks much better to me!

-- 
Thanks,

David / dhildenb