Date: Wed, 14 Feb 2024 17:18:22 +0800
From: Chengming Zhou <chengming.zhou@...ux.dev>
To: Yu Zhao <yuzhao@...gle.com>
Cc: willy@...radead.org, hannes@...xchg.org, yosryahmed@...gle.com,
 nphamcs@...il.com, akpm@...ux-foundation.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Chengming Zhou <zhouchengming@...edance.com>
Subject: Re: [PATCH RFC 1/1] mm/swap: queue reclaimable folio to local rotate
 batch when !folio_test_lru()

On 2024/2/14 15:13, Yu Zhao wrote:
> On Fri, Feb 9, 2024 at 6:00 AM <chengming.zhou@...ux.dev> wrote:
>>
>> From: Chengming Zhou <zhouchengming@...edance.com>
>>
>> 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@...edance.com>
> 
> 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@google.com/

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.

Thanks.

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