Re: [v6 PATCH 2/2] mm: vmscan: correct some vmscan counters for THP swapout
From: Yang Shi
Date: Mon May 27 2019 - 03:44:13 EST
On 5/27/19 3:06 PM, Huang, Ying wrote:
Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> writes:
Since commit bd4c82c22c36 ("mm, THP, swap: delay splitting THP after
swapped out"), THP can be swapped out in a whole. But, nr_reclaimed
and some other vm counters still get inc'ed by one even though a whole
THP (512 pages) gets swapped out.
This doesn't make too much sense to memory reclaim. For example, direct
reclaim may just need reclaim SWAP_CLUSTER_MAX pages, reclaiming one THP
could fulfill it. But, if nr_reclaimed is not increased correctly,
direct reclaim may just waste time to reclaim more pages,
SWAP_CLUSTER_MAX * 512 pages in worst case.
And, it may cause pgsteal_{kswapd|direct} is greater than
pgscan_{kswapd|direct}, like the below:
pgsteal_kswapd 122933
pgsteal_direct 26600225
pgscan_kswapd 174153
pgscan_direct 14678312
nr_reclaimed and nr_scanned must be fixed in parallel otherwise it would
break some page reclaim logic, e.g.
vmpressure: this looks at the scanned/reclaimed ratio so it won't
change semantics as long as scanned & reclaimed are fixed in parallel.
compaction/reclaim: compaction wants a certain number of physical pages
freed up before going back to compacting.
kswapd priority raising: kswapd raises priority if we scan fewer pages
than the reclaim target (which itself is obviously expressed in order-0
pages). As a result, kswapd can falsely raise its aggressiveness even
when it's making great progress.
Other than nr_scanned and nr_reclaimed, some other counters, e.g.
pgactivate, nr_skipped, nr_ref_keep and nr_unmap_fail need to be fixed
too since they are user visible via cgroup, /proc/vmstat or trace
points, otherwise they would be underreported.
When isolating pages from LRUs, nr_taken has been accounted in base
page, but nr_scanned and nr_skipped are still accounted in THP. It
doesn't make too much sense too since this may cause trace point
underreport the numbers as well.
So accounting those counters in base page instead of accounting THP as
one page.
nr_dirty, nr_unqueued_dirty, nr_congested and nr_writeback are used by
file cache, so they are not impacted by THP swap.
This change may result in lower steal/scan ratio in some cases since
THP may get split during page reclaim, then a part of tail pages get
reclaimed instead of the whole 512 pages, but nr_scanned is accounted
by 512, particularly for direct reclaim. But, this should be not a
significant issue.
Cc: "Huang, Ying" <ying.huang@xxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: "Kirill A . Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
Cc: Hillf Danton <hdanton@xxxxxxxx>
Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
---
v6: Fixed the other double account issue introduced by v5 per Huang Ying
v5: Fixed sc->nr_scanned double accounting per Huang Ying
Added some comments to address the concern about premature OOM per Hillf Danton
v4: Fixed the comments from Johannes and Huang Ying
v3: Removed Shakeel's Reviewed-by since the patch has been changed significantly
Switched back to use compound_order per Matthew
Fixed more counters per Johannes
v2: Added Shakeel's Reviewed-by
Use hpage_nr_pages instead of compound_order per Huang Ying and William Kucharski
mm/vmscan.c | 47 +++++++++++++++++++++++++++++++++++------------
1 file changed, 35 insertions(+), 12 deletions(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index b65bc50..378edff 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1118,6 +1118,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
int may_enter_fs;
enum page_references references = PAGEREF_RECLAIM_CLEAN;
bool dirty, writeback;
+ unsigned int nr_pages;
cond_resched();
@@ -1129,7 +1130,10 @@ static unsigned long shrink_page_list(struct list_head *page_list,
VM_BUG_ON_PAGE(PageActive(page), page);
- sc->nr_scanned++;
+ nr_pages = 1 << compound_order(page);
+
+ /* Account the number of base pages even though THP */
+ sc->nr_scanned += nr_pages;
if (unlikely(!page_evictable(page)))
goto activate_locked;
@@ -1250,7 +1254,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
case PAGEREF_ACTIVATE:
goto activate_locked;
case PAGEREF_KEEP:
- stat->nr_ref_keep++;
+ stat->nr_ref_keep += nr_pages;
goto keep_locked;
case PAGEREF_RECLAIM:
case PAGEREF_RECLAIM_CLEAN:
@@ -1306,6 +1310,15 @@ static unsigned long shrink_page_list(struct list_head *page_list,
}
/*
+ * THP may get split above, need minus tail pages and update
+ * nr_pages to avoid accounting tail pages twice.
+ */
+ if ((nr_pages > 1) && !PageTransHuge(page)) {
+ sc->nr_scanned -= (nr_pages - 1);
+ nr_pages = 1;
+ }
After checking the code again, it appears there's another hole in the
code. In the following code snippet.
if (!add_to_swap(page)) {
if (!PageTransHuge(page))
goto activate_locked;
/* Fallback to swap normal pages */
if (split_huge_page_to_list(page,
page_list))
goto activate_locked;
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
count_vm_event(THP_SWPOUT_FALLBACK);
#endif
if (!add_to_swap(page))
goto activate_locked;
}
If the THP is split, but the first or the second add_to_swap() fails, we
still need to deal with sc->nr_scanned and nr_pages.
How about add a new label before "activate_locked" to deal with that?
It sounds not correct. If swapout fails it jumps to activate_locked too,
it has to be handled in if (!add_to_swap(page)). The below fix should be
good enough since only THP can reach here:
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 378edff..fff3937 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1294,8 +1294,15 @@ static unsigned long shrink_page_list(struct
list_head *page_list,
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
count_vm_event(THP_SWPOUT_FALLBACK);
#endif
- if (!add_to_swap(page))
+ if (!add_to_swap(page)) {
+ /*
+ * Minus tail pages and
reset
+ * nr_pages.
+ */
+ sc->nr_scanned -=
(nr_pages - 1);
+ nr_pages = 1;
goto activate_locked;
+ }
}
Best Regards,
Huang, Ying