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    SubjectRe: [PATCH RFC 1/1] mm/swap: queue reclaimable folio to local rotate batch when !folio_test_lru()
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    On 2024/2/14 15:13, Yu Zhao wrote:
    > On Fri, Feb 9, 2024 at 6:00 AM <chengming.zhou@linux.dev> wrote:
    >>
    >> From: Chengming Zhou <zhouchengming@bytedance.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@bytedance.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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