Message-ID: <CAGsJ_4yg8ih2ZJsqoZG5__axw1hB40keFsbeykjURd=zK2uEhA@mail.gmail.com>
Date: Fri, 20 Dec 2024 15:30:38 +1300
From: Barry Song <21cnbao@...il.com>
To: Chen Ridong <chenridong@...weicloud.com>, cuibixuan@...o.com
Cc: akpm@...ux-foundation.org, mhocko@...e.com, hannes@...xchg.org, 
	yosryahmed@...gle.com, yuzhao@...gle.com, david@...hat.com, 
	willy@...radead.org, ryan.roberts@....com, wangkefeng.wang@...wei.com, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, chenridong@...wei.com, 
	wangweiyang2@...wei.com, xieym_ict@...mail.com
Subject: Re: [PATCH -next v5] mm: vmscan: retry folios written back while
 isolated for traditional LRU

On Fri, Dec 20, 2024 at 2:19 PM Chen Ridong <chenridong@...weicloud.com> wrote:
>
> From: Chen Ridong <chenridong@...wei.com>
>
> The page reclaim isolates a batch of folios from the tail of one of the
> LRU lists and works on those folios one by one.  For a suitable
> swap-backed folio, if the swap device is async, it queues that folio for
> writeback.  After the page reclaim finishes an entire batch, it puts back
> the folios it queued for writeback to the head of the original LRU list.
>
> In the meantime, the page writeback flushes the queued folios also by
> batches.  Its batching logic is independent from that of the page reclaim.
> For each of the folios it writes back, the page writeback calls
> folio_rotate_reclaimable() which tries to rotate a folio to the tail.
>
> folio_rotate_reclaimable() only works for a folio after the page reclaim
> has put it back.  If an async swap device is fast enough, the page
> writeback can finish with that folio while the page reclaim is still
> working on the rest of the batch containing it.  In this case, that folio
> will remain at the head and the page reclaim will not retry it before
> reaching there.
>
> The commit 359a5e1416ca ("mm: multi-gen LRU: retry folios written back
> while isolated") only fixed the issue for mglru. However, this issue
> also exists in the traditional active/inactive LRU. This issue will be
> worse if THP is split, which makes the list longer and needs longer time
> to finish a batch of folios reclaim.
>
> This issue should be fixed in the same way for the traditional LRU.
> Therefore, the common logic was extracted to the 'find_folios_written_back'
> function firstly, which is then reused in the 'shrink_inactive_list'
> function. Finally, retry reclaiming those folios that may have missed the
> rotation for traditional LRU.
>
> Link: https://lore.kernel.org/linux-kernel/20241010081802.290893-1-chenridong@huaweicloud.com/
> Link: https://lore.kernel.org/linux-kernel/CAGsJ_4zqL8ZHNRZ44o_CC69kE7DBVXvbZfvmQxMGiFqRxqHQdA@mail.gmail.com/
> Signed-off-by: Chen Ridong <chenridong@...wei.com>
> ---
>  mm/vmscan.c | 108 ++++++++++++++++++++++++++++++++++------------------
>  1 file changed, 70 insertions(+), 38 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 39886f435ec5..e67e446540ba 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -283,6 +283,39 @@ static void set_task_reclaim_state(struct task_struct *task,
>         task->reclaim_state = rs;
>  }
>
> +/**
> + * find_folios_written_back - Find and move the written back folios to a new list.
> + * @list: filios list
> + * @clean: the written back folios list
> + * @is_retried: whether the list has already been retried.
> + */
> +static inline void find_folios_written_back(struct list_head *list,
> +               struct list_head *clean, bool is_retried)
> +{
> +       struct folio *folio;
> +       struct folio *next;
> +
> +       list_for_each_entry_safe_reverse(folio, next, list, lru) {
> +               if (!folio_evictable(folio)) {
> +                       list_del(&folio->lru);
> +                       folio_putback_lru(folio);
> +                       continue;
> +               }
> +
> +               /* retry folios that may have missed folio_rotate_reclaimable() */
> +               if (!is_retried && !folio_test_active(folio) && !folio_mapped(folio) &&
> +                   !folio_test_dirty(folio) && !folio_test_writeback(folio)) {
> +                       list_move(&folio->lru, clean);
> +                       continue;
> +               }
> +
> +               /* don't add rejected folios to the oldest generation */
> +               if (lru_gen_enabled() && !lru_gen_distance(folio, false))
> +                       set_mask_bits(&folio->flags, LRU_REFS_FLAGS, BIT(PG_active));
> +       }
> +
> +}
> +
>  /*
>   * flush_reclaim_state(): add pages reclaimed outside of LRU-based reclaim to
>   * scan_control->nr_reclaimed.
> @@ -1959,14 +1992,18 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan,
>                 enum lru_list lru)
>  {
>         LIST_HEAD(folio_list);
> +       LIST_HEAD(clean_list);
>         unsigned long nr_scanned;
> -       unsigned int nr_reclaimed = 0;
> +       unsigned int nr_reclaimed, total_reclaimed = 0;
> +       unsigned int nr_pageout = 0;
> +       unsigned int nr_unqueued_dirty = 0;
>         unsigned long nr_taken;
>         struct reclaim_stat stat;
>         bool file = is_file_lru(lru);
>         enum vm_event_item item;
>         struct pglist_data *pgdat = lruvec_pgdat(lruvec);
>         bool stalled = false;
> +       bool is_retried = false;
>
>         while (unlikely(too_many_isolated(pgdat, file, sc))) {
>                 if (stalled)
> @@ -2000,22 +2037,47 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan,
>         if (nr_taken == 0)
>                 return 0;
>
> +retry:
>         nr_reclaimed = shrink_folio_list(&folio_list, pgdat, sc, &stat, false);
>
> +       sc->nr.dirty += stat.nr_dirty;
> +       sc->nr.congested += stat.nr_congested;
> +       sc->nr.unqueued_dirty += stat.nr_unqueued_dirty;
> +       sc->nr.writeback += stat.nr_writeback;
> +       sc->nr.immediate += stat.nr_immediate;
> +       total_reclaimed += nr_reclaimed;
> +       nr_pageout += stat.nr_pageout;
> +       nr_unqueued_dirty += stat.nr_unqueued_dirty;
> +
> +       trace_mm_vmscan_lru_shrink_inactive(pgdat->node_id,
> +                       nr_scanned, nr_reclaimed, &stat, sc->priority, file);

This is a bit odd, as nr_scanned during a retry still uses the
previous nr_scanned
value. However, I find that mglru shows no difference.

retry:
        reclaimed = shrink_folio_list(&list, pgdat, sc, &stat, false);
        sc->nr.unqueued_dirty += stat.nr_unqueued_dirty;
        sc->nr_reclaimed += reclaimed;
        trace_mm_vmscan_lru_shrink_inactive(pgdat->node_id,
                        scanned, reclaimed, &stat, sc->priority,
                        type ? LRU_INACTIVE_FILE : LRU_INACTIVE_ANON);

Currently, the active/inactive state aligns with mglru in this trace.
It seems that
the userspace BPF should recognize that the nr_scanned during a retry doesn't
indicate we are isolating new nr_scanned folios. Ideally, the is_retry
flag should
be passed to the trace, allowing userspace to identify that it's a retry and
disregard the nr_scanned value.

It might be worth addressing this in a separate patch. Add Bixuan to clarify
how userspace depends on this trace and if "retry" will break his userspace
BPF for both MGLRU and active/inactive cases.

Otherwise, the patch looks good to me.

> +
> +       find_folios_written_back(&folio_list, &clean_list, is_retried);
> +
>         spin_lock_irq(&lruvec->lru_lock);
>         move_folios_to_lru(lruvec, &folio_list);
>
>         __mod_lruvec_state(lruvec, PGDEMOTE_KSWAPD + reclaimer_offset(),
>                                         stat.nr_demoted);
> -       __mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, -nr_taken);
>         item = PGSTEAL_KSWAPD + reclaimer_offset();
>         if (!cgroup_reclaim(sc))
>                 __count_vm_events(item, nr_reclaimed);
>         __count_memcg_events(lruvec_memcg(lruvec), item, nr_reclaimed);
>         __count_vm_events(PGSTEAL_ANON + file, nr_reclaimed);
> +
> +       if (!list_empty(&clean_list)) {
> +               list_splice_init(&clean_list, &folio_list);
> +               is_retried = true;
> +               spin_unlock_irq(&lruvec->lru_lock);
> +               goto retry;
> +       }
> +       __mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, -nr_taken);
>         spin_unlock_irq(&lruvec->lru_lock);
> +       sc->nr.taken += nr_taken;
> +       if (file)
> +               sc->nr.file_taken += nr_taken;
>
> -       lru_note_cost(lruvec, file, stat.nr_pageout, nr_scanned - nr_reclaimed);
> +       lru_note_cost(lruvec, file, nr_pageout, nr_scanned - total_reclaimed);
>
>         /*
>          * If dirty folios are scanned that are not queued for IO, it
> @@ -2028,7 +2090,7 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan,
>          * the flushers simply cannot keep up with the allocation
>          * rate. Nudge the flusher threads in case they are asleep.
>          */
> -       if (stat.nr_unqueued_dirty == nr_taken) {
> +       if (nr_unqueued_dirty == nr_taken) {
>                 wakeup_flusher_threads(WB_REASON_VMSCAN);
>                 /*
>                  * For cgroupv1 dirty throttling is achieved by waking up
> @@ -2043,18 +2105,7 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan,
>                         reclaim_throttle(pgdat, VMSCAN_THROTTLE_WRITEBACK);
>         }
>
> -       sc->nr.dirty += stat.nr_dirty;
> -       sc->nr.congested += stat.nr_congested;
> -       sc->nr.unqueued_dirty += stat.nr_unqueued_dirty;
> -       sc->nr.writeback += stat.nr_writeback;
> -       sc->nr.immediate += stat.nr_immediate;
> -       sc->nr.taken += nr_taken;
> -       if (file)
> -               sc->nr.file_taken += nr_taken;
> -
> -       trace_mm_vmscan_lru_shrink_inactive(pgdat->node_id,
> -                       nr_scanned, nr_reclaimed, &stat, sc->priority, file);
> -       return nr_reclaimed;
> +       return total_reclaimed;
>  }
>
>  /*
> @@ -4585,12 +4636,10 @@ static int evict_folios(struct lruvec *lruvec, struct scan_control *sc, int swap
>         int reclaimed;
>         LIST_HEAD(list);
>         LIST_HEAD(clean);
> -       struct folio *folio;
> -       struct folio *next;
>         enum vm_event_item item;
>         struct reclaim_stat stat;
>         struct lru_gen_mm_walk *walk;
> -       bool skip_retry = false;
> +       bool is_retried = false;
>         struct lru_gen_folio *lrugen = &lruvec->lrugen;
>         struct mem_cgroup *memcg = lruvec_memcg(lruvec);
>         struct pglist_data *pgdat = lruvec_pgdat(lruvec);
> @@ -4616,24 +4665,7 @@ static int evict_folios(struct lruvec *lruvec, struct scan_control *sc, int swap
>                         scanned, reclaimed, &stat, sc->priority,
>                         type ? LRU_INACTIVE_FILE : LRU_INACTIVE_ANON);
>
> -       list_for_each_entry_safe_reverse(folio, next, &list, lru) {
> -               if (!folio_evictable(folio)) {
> -                       list_del(&folio->lru);
> -                       folio_putback_lru(folio);
> -                       continue;
> -               }
> -
> -               /* retry folios that may have missed folio_rotate_reclaimable() */
> -               if (!skip_retry && !folio_test_active(folio) && !folio_mapped(folio) &&
> -                   !folio_test_dirty(folio) && !folio_test_writeback(folio)) {
> -                       list_move(&folio->lru, &clean);
> -                       continue;
> -               }
> -
> -               /* don't add rejected folios to the oldest generation */
> -               if (!lru_gen_distance(folio, false))
> -                       set_mask_bits(&folio->flags, LRU_REFS_FLAGS, BIT(PG_active));
> -       }
> +       find_folios_written_back(&list, &clean, is_retried);
>
>         spin_lock_irq(&lruvec->lru_lock);
>
> @@ -4656,7 +4688,7 @@ static int evict_folios(struct lruvec *lruvec, struct scan_control *sc, int swap
>         list_splice_init(&clean, &list);
>
>         if (!list_empty(&list)) {
> -               skip_retry = true;
> +               is_retried = true;
>                 goto retry;
>         }
>
> --
> 2.34.1
>

Thanks
barry

Powered by blists - more mailing lists