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

From: Vlastimil Babka <vbabka@suse.cz>
To: Oscar Salvador <osalvador@suse.de>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Michal Hocko <mhocko@kernel.org>,
	Pavel Tatashin <pasha.tatashin@soleen.com>,
	David Hildenbrand <david@redhat.com>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>
Subject: Re: [RFC 1/5] mm, page_alloc: clean up pageset high and batch update
Date: Thu, 17 Sep 2020 18:05:33 +0200	[thread overview]
Message-ID: <78f43f31-4388-923f-83aa-dc9831de6e18@suse.cz> (raw)
In-Reply-To: <20200910083116.GA2285@linux>

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@suse.de> 

Thanks!

> Thanks
> 