Message-ID: <3f06379a-b9d0-40b7-9ecf-6e4c9a5f51dc@huaweicloud.com>
Date: Fri, 20 Dec 2024 15:48:40 +0800
From: Chen Ridong <chenridong@...weicloud.com>
To: Barry Song <21cnbao@...il.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 2024/12/20 11:09, Barry Song wrote:
> On Fri, Dec 20, 2024 at 3:30 PM Barry Song <21cnbao@...il.com> wrote:
>>
>> 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;
> 
> The name is_retried is a bit confusing. It should be is_retry or
> is_retrying since
> we are currently retrying, not that we have already retried.
> 
>>>
>>>         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.
>>
> 
> By the way, it's completely clear that the trace was added after mglru's retry:
> https://lore.kernel.org/linux-mm/20240105013607.2868-3-cuibixuan@vivo.com/
> 
> Therefore, I don't believe the potential confusion about nr_scanned in the trace
> should prevent Ridong's fix for the missed rotation of written-back folios from
> proceeding.
> 
> If there is an issue with that, we should open a separate thread to address the
> trace.
> 
> Please feel free to add the below in the future version after you fix
> "is_retried".
> 
> Reviewed-by: Barry Song <baohua@...nel.org>
> 

Thank you very much.
I will update.

Best regards
Ridong

>>> +
>>> +       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

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