[PATCH v10 06/14] mm: multi-gen LRU: minimal implementation

[Chen Wandun]

在 2022/4/15 14:44, Yu Zhao 写道:
> On Fri, Apr 15, 2022 at 02:31:37PM +0800, Chen Wandun wrote:
>>
>> 在 2022/4/15 13:25, Yu Zhao 写道:
>>> On Fri, Apr 15, 2022 at 10:23:18AM +0800, Chen Wandun wrote:
>>>> 在 2022/4/15 4:53, Yu Zhao 写道:
>>>>> On Thu, Apr 14, 2022 at 07:47:54PM +0800, Chen Wandun wrote:
>>>>>> On 2022/4/7 11:15, Yu Zhao wrote:
>>>>>>> +static void inc_min_seq(struct lruvec *lruvec)
>>>>>>> +{
>>>>>>> +	int type;
>>>>>>> +	struct lru_gen_struct *lrugen = &lruvec->lrugen;
>>>>>>> +
>>>>>>> +	VM_BUG_ON(!seq_is_valid(lruvec));
>>>>>>> +
>>>>>>> +	for (type = 0; type < ANON_AND_FILE; type++) {
>>>>>>> +		if (get_nr_gens(lruvec, type) != MAX_NR_GENS)
>>>>>>> +			continue;
>>>>>> I'm confused about relation between aging and LRU list operation.
>>>>>>
>>>>>> In function inc_max_seq,  both min_seq and max_seq will increase，
>>>>>> the lrugen->lists[] indexed by lru_gen_from_seq(max_seq + 1) may
>>>>>> be non-empty?
>>>>> Yes.
>>>>>
>>>>>> for example,
>>>>>> before inc_max_seq:
>>>>>> min_seq == 0, lrugen->lists[0][type][zone]
>>>>>> max_seq ==3, lrugen->lists[3][type][zone]
>>>>>>
>>>>>> after inc_max_seq:
>>>>>> min_seq ==1, lrugen->lists[1][type][zone]
>>>>>> max_seq ==4, lrugen->lists[0][type][zone]
>>>>>>
>>>>>> If lrugen->lists[0][type][zone] is not empty before inc_max_seq and it is
>>>>>> the most inactive list，however lurgen->lists[0][type][zone] will become
>>>>>> the most active list after inc_max_seq.
>>>>> Correct.
>>>>>
>>>>>> So,  in this place,
>>>>>>
>>>>>> if (get_nr_gens(lruvec, type) != MAX_NR_GENS)
>>>>>> 	continue;
>>>>>>
>>>>>> should change to
>>>>>>
>>>>>> if (get_nr_gens(lruvec, type) == MAX_NR_GENS)
>>>>>> 	continue;
>>>>> No, because max/min_seq will overlap if we do so.
>>>>>
>>>>> lrugen->lists[max_seq+1] can only be non-empty for anon LRU, for a
>>>>> couple of reasons:
>>>>> 1. We can't swap at all.
>>>>> 2. Swapping is constrained, e.g., swapfile is full.
>>>>>
>>>>> Both cases are similar to a producer (the aging) overrunning a
>>>>> consumer (the eviction). We used to handle them, but I simplified the
>>>>> code because I don't feel they are worth handling [1].
>>>> Can lrugen->lists[max_seq+1]  also be non-empty for file LRU？
>>> On reclaim path, no. But it can be forced to do so via debugfs.
>>>
>>>> such as in dont reclaim mapped file page case(isolation will fail).
>>> You mean may_unmap=false? Pages stays in the same generation if
>>> isolation fails. So lrugen->lists[min_seq] won't be empty in this
>>> case.
>>>
>>>> If so, after aging, eviction will reclaim memory start from
>>>> lrugen->lists[min_seq+1], but some oldest file page still
>>>> remain in lrugen->lists[max_seq+1].
>>>>
>>>> sort_folio can help to put misplaced pages to the right
>>>> LRU list, but in this case, it does't help, because sort_folio
>>>> only sort lrugen->lists[min_seq+1].
>>> On reclaim path, inc_max_seq() is only called when need_aging=true,
>>> and this guarantees max_seq-min_seq[LRU_GEN_FILE]+1 < MAX_NR_GENS.
>> yes, I think so, but I did't find the logical in function get_nr_evictable,
>> or am I missing something
>>
>>          if (min_seq[LRU_GEN_FILE] + MIN_NR_GENS > max_seq)
>>                  *need_aging = true;
>>          else if (min_seq[LRU_GEN_FILE] + MIN_NR_GENS < max_seq)
>>                  *need_aging = false;
> This branch.
>
> And the following is also relavent:
>
>      static int __init init_lru_gen(void)
>      {
>          BUILD_BUG_ON(MIN_NR_GENS + 1 >= MAX_NR_GENS);
Got it, many thanks.
Wandun
> .