On 2024/12/20 11:09, Barry Song wrote:
> On Fri, Dec 20, 2024 at 3:30 PM Barry Song <21cnbao@xxxxxxxxx> wrote:
>>
>> On Fri, Dec 20, 2024 at 2:19 PM Chen Ridong <chenridong@xxxxxxxxxxxxxxx> wrote:
>>>
>>> From: Chen Ridong <chenridong@xxxxxxxxxx>
>>>
>>> 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@xxxxxxxxxxxxxxx/
>>> Link: https://lore.kernel.org/linux-kernel/CAGsJ_4zqL8ZHNRZ44o_CC69kE7DBVXvbZfvmQxMGiFqRxqHQdA@xxxxxxxxxxxxxx/
>>> Signed-off-by: Chen Ridong <chenridong@xxxxxxxxxx>
>>> ---
>>> 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@xxxxxxxx/
>
> 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@xxxxxxxxxx>
>
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
