Re: [PATCH RFC 1/1] mm/swap: queue reclaimable folio to local rotate batch when !folio_test_lru()

[Chengming Zhou]

On 2024/2/15 15:06, Yu Zhao wrote:
> On Wed, Feb 14, 2024 at 4:18 AM Chengming Zhou <chengming.zhou@xxxxxxxxx> wrote:
>>
>> On 2024/2/14 15:13, Yu Zhao wrote:
>>> On Fri, Feb 9, 2024 at 6:00 AM <chengming.zhou@xxxxxxxxx> wrote:
>>>>
>>>> From: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
>>>>
>>>> All LRU move interfaces have a problem that it has no effect if the
>>>> folio is isolated from LRU (in cpu batch or isolated by shrinker).
>>>> Since it can't move/change folio LRU status when it's isolated, mostly
>>>> just clear the folio flag and do nothing in this case.
>>>>
>>>> In our case, a written back and reclaimable folio won't be rotated to
>>>> the tail of inactive list, since it's still in cpu lru_add batch. It
>>>> may cause the delayed reclaim of this folio and evict other folios.
>>>>
>>>> This patch changes to queue the reclaimable folio to cpu rotate batch
>>>> even when !folio_test_lru(), hoping it will likely be handled after
>>>> the lru_add batch which will put folio on the LRU list first, so
>>>> will be rotated to the tail successfully when handle rotate batch.
>>>>
>>>> Signed-off-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
>>>
>>> I don't think the analysis is correct. IIRC, writeback from non
>>> reclaim paths doesn't require isolation and the reclaim path doesn't
>>> use struct folio_batch lru_add.
>>
>> Ah, my bad, I forgot to mention the important context in the message:
>>
>> This is not from the normal reclaim context, it's from zswap writeback
>> reclaim context, which will first set PG_reclaim flag, then submit the
>> async writeback io.
>>
>> If the writeback io complete fast enough, folio_rotate_reclaimable()
>> will be called before that folio put on LRU list (it still in the local
>> lru_add batch, so it's somewhat like isolated too)
>>
>>>
>>> Did you see any performance improvements with this patch? In general,
>>> this kind of patches should have performance numbers to show it really
>>> helps (not just in theory).
>>
>> Right, there are some improvements, the numbers are put in cover letter.
>> But this solution is not good enough, just RFC for discussion. :)
>>
>>                            mm-unstable-hot   zswap-lru-reclaim
>> real                       63.34             62.72
>> user                       1063.20           1060.30
>> sys                        272.04            256.14
>> workingset_refault_anon    2103297.00        1788155.80
>> workingset_refault_file    28638.20          39249.40
>> workingset_activate_anon   746134.00         695435.40
>> workingset_activate_file   4344.60           4255.80
>> workingset_restore_anon    653163.80         605315.60
>> workingset_restore_file    1079.00           883.00
>> workingset_nodereclaim     0.00              0.00
>> pgscan                     12971305.60       12730331.20
>> pgscan_kswapd              0.00              0.00
>> pgscan_direct              12971305.60       12730331.20
>> pgscan_khugepaged          0.00              0.00
>>
>>>
>>> My guess is that you are hitting this problem [1].
>>>
>>> [1] https://lore.kernel.org/linux-mm/20221116013808.3995280-1-yuzhao@xxxxxxxxxx/
>>
>> Right, I just see it, it's the same problem. The only difference is that
>> in your case the folio is isolated by shrinker, in my case, the folio is
>> in cpu lru_add batch. Anyway, the result is the same, that folio can't be
>> rotated successfully when writeback complete.
> 
> In that case, a better solution would be to make lru_add add
> (_reclaim() && !_dirty() && !_writeback()) folios at the tail.
> (_rotate() needs to leave _reclaim() set if it fails to rotate.)

Right, this is a solution. But PG_readahead is alias of PG_reclaim,
I'm afraid this would rotate readahead folio to the inactive tail.