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 10:31:20 +0200
From:   Oscar Salvador <osalvador@...e.de>
To:     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>,
        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 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?

(As I said, not important and probably a matter of taste. I just think that
having so many mini functions around is not always cool,
e.g: setup_zone_pageset->zone_pageset_init)

> -/*
> - * 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.
>  
>  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?

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

-- 
Oscar Salvador
SUSE L3