Re: [RFC 4/5] mm, page_alloc: cache pageset high and batch in struct zone

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 4/5] mm, page_alloc: cache pageset high and batch in struct zone
Date: Fri, 18 Sep 2020 14:02:42 +0200	[thread overview]
Message-ID: <08421531-1df0-3cea-fe44-2e4e30808dfd@suse.cz> (raw)
In-Reply-To: <20200910113046.GA5848@linux>

On 9/10/20 1:30 PM, Oscar Salvador wrote:
> On Mon, Sep 07, 2020 at 06:36:27PM +0200, Vlastimil Babka wrote:
>    */
>> -static void setup_pageset(struct per_cpu_pageset *p);
>> +static void pageset_init(struct per_cpu_pageset *p);
> 
> this belongs to the respective patches

Right, thanks.

>> -static void zone_set_pageset_high_and_batch(struct zone *zone)
>> +static void zone_set_pageset_high_and_batch(struct zone *zone, bool force_update)
>>  {
>>  	unsigned long new_high;
>>  	unsigned long new_batch;
>> @@ -6256,6 +6256,14 @@ static void zone_set_pageset_high_and_batch(struct zone *zone)
>>  		new_batch = max(1UL, 1 * new_batch);
>>  	}
>>  
>> +	if (zone->pageset_high != new_high ||
>> +	    zone->pageset_batch != new_batch) {
>> +		zone->pageset_high = new_high;
>> +		zone->pageset_batch = new_batch;
>> +	} else if (!force_update) {
>> +		return;
>> +	}
> 
> I am probably missimg something obvious, so sorry, but why do we need
> force_update here?
> AFAICS, we only want to call pageset_update() in case zone->pageset_high/batch
> and the new computed high/batch differs, so if everything is equal, why do we want
> to call it anyways?

My reasoning is that initially we don't have guarantee that
zone->pageset_high/batch matches the respective pcp->high/batch. So we could
detect no change in the zone values and return, but leave the pcp value
incoherent. But now I think it could be achieved also in a simpler way, so I'll try.