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>
> > +
> > + 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
