Re: [PATCH v2] mm/page_alloc: optimize find_suitable_fallback() and fallbacks array

[Yajun Deng]

February 10, 2023 3:58 PM, "Vlastimil Babka" <vbabka@xxxxxxx> wrote:

> On 2/10/23 03:51, Yajun Deng wrote:
> 
>> February 10, 2023 10:33 AM, "Yajun Deng" <yajun.deng@xxxxxxxxx> wrote:
>> 
>>> February 10, 2023 10:14 AM, "Zi Yan" <ziy@xxxxxxxxxx> wrote:
>> 
>> On 9 Feb 2023, at 20:57, Yajun Deng wrote:
>> 
>> February 9, 2023 11:50 PM, "Zi Yan" <ziy@xxxxxxxxxx> wrote:
>> 
>> On 9 Feb 2023, at 5:11, Yajun Deng wrote:
>> There is no need to execute the next loop if it not return in the first
>> loop. So add a break at the end of the loop.
>> 
>> Can you explain why? If it is the case, MIGRATE_UNMOVABLE cannot fall back
>> to MIGRATE_MOVABLE? And MIGRATE_MOVABLE cannot fall back to MIGRATE_UNMOVABLE?
>> And MIGRATE_RECLAIMABLE cannot fall back to MIGRATE_MOVABLE?
>> The return in the loop is only related to 'order', 'migratetype' and 'only_stealable'
>> variables. Even if it execute the next loop, it can't change the result. So the loop
>> can be broken if the first loop can't return.
>> 
>> OK. Got it. Would the code below look better?
>> 
>> for (i = 0; i < MIGRATE_PCPTYPES - 1 ; i++) {
>> fallback_mt = fallbacks[migratetype][i];
>> if (free_area_empty(area, fallback_mt))
>> continue;
>> }
>> 
>> if (can_steal_fallback(order, migratetype))
>> *can_steal = true;
>> 
>> if (!only_stealable || *can_steal)
>> return fallback_mt;
>> 
>> return -1;
>>> Yes, I'll submit a v3 patch.
>>> Thanks.
>> 
>> I found a logical error in your code. It should be like this:
>> 
>> for (i = 0; i < MIGRATE_PCPTYPES - 1 ; i++) {
>> fallback_mt = fallbacks[migratetype][i];
>> if (!free_area_empty(area, fallback_mt))
>> break;
>> }
>> 
>> if (can_steal_fallback(order, migratetype))
>> *can_steal = true;
>> 
>> if (!only_stealable || *can_steal)
>> return fallback_mt;
>> 
>> return -1;
>> 
>> This code will modify the logic to the opposite.
> 
> It's still wrong, IMHO. If all fallbacks have free_area_empty(), it will
> return the last one and not -1. Also will set *can_steal in such case.
> 

Yes, you are right.

>> So can anyone tell me if I should use this code or the v2 patch?
> 
> Once that bugs are fixed, the result will probably not look much better than
> v2, so I don't mind keeping v2.

I agree with that.