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

From: Chengming Zhou
Date: Wed Feb 14 2024 - 04:18:42 EST

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]

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.