[v7 PATCH 1/2] mm: vmscan: remove double slab pressure by inc'ing sc->nr_scanned

[v7 PATCH 1/2] mm: vmscan: remove double slab pressure by inc'ing sc->nr_scanned

[Yang Shi]

The commit 9092c71bb724 ("mm: use sc->priority for slab shrink targets")
has broken up the relationship between sc->nr_scanned and slab pressure.
The sc->nr_scanned can't double slab pressure anymore.  So, it sounds no
sense to still keep sc->nr_scanned inc'ed.  Actually, it would prevent
from adding pressure on slab shrink since excessive sc->nr_scanned would
prevent from scan->priority raise.

The bonnie test doesn't show this would change the behavior of
slab shrinkers.

				w/		w/o
			  /sec    %CP      /sec      %CP
Sequential delete: 	3960.6    94.6    3997.6     96.2
Random delete: 		2518      63.8    2561.6     64.6

The slight increase of "/sec" without the patch would be caused by the
slight increase of CPU usage.

Cc: Josef Bacik <josef@toxicpanda.com>
Cc: Michal Hocko <mhocko@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
---
v4: Added Johannes's ack

 mm/vmscan.c | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 7acd0af..b65bc50 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1137,11 +1137,6 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 		if (!sc->may_unmap && page_mapped(page))
 			goto keep_locked;
 
-		/* Double the slab pressure for mapped and swapcache pages */
-		if ((page_mapped(page) || PageSwapCache(page)) &&
-		    !(PageAnon(page) && !PageSwapBacked(page)))
-			sc->nr_scanned++;
-
 		may_enter_fs = (sc->gfp_mask & __GFP_FS) ||
 			(PageSwapCache(page) && (sc->gfp_mask & __GFP_IO));
 
-- 
1.8.3.1

[v7 PATCH 2/2] mm: vmscan: correct some vmscan counters for THP swapout

[Yang Shi]

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@intel.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Hillf Danton <hdanton@sina.com>
Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
---
v7: Fixed more incorrect account for split page per Huang Ying
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 | 63 +++++++++++++++++++++++++++++++++++++++++++++++--------------
 1 file changed, 49 insertions(+), 14 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index b65bc50..58d8a8e 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:
@@ -1282,7 +1286,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 				}
 				if (!add_to_swap(page)) {
 					if (!PageTransHuge(page))
-						goto activate_locked;
+						goto activate_locked_split;
 					/* Fallback to swap normal pages */
 					if (split_huge_page_to_list(page,
 								    page_list))
@@ -1291,7 +1295,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 					count_vm_event(THP_SWPOUT_FALLBACK);
 #endif
 					if (!add_to_swap(page))
-						goto activate_locked;
+						goto activate_locked_split;
 				}
 
 				may_enter_fs = 1;
@@ -1306,6 +1310,18 @@ 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.
+		 *
+		 * The tail pages that are added into swap cache successfully
+		 * reach here.
+		 */
+		if ((nr_pages > 1) && !PageTransHuge(page)) {
+			sc->nr_scanned -= (nr_pages - 1);
+			nr_pages = 1;
+		}
+
+		/*
 		 * The page is mapped into the page tables of one or more
 		 * processes. Try to unmap it here.
 		 */
@@ -1315,7 +1331,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 			if (unlikely(PageTransHuge(page)))
 				flags |= TTU_SPLIT_HUGE_PMD;
 			if (!try_to_unmap(page, flags)) {
-				stat->nr_unmap_fail++;
+				stat->nr_unmap_fail += nr_pages;
 				goto activate_locked;
 			}
 		}
@@ -1442,7 +1458,11 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 
 		unlock_page(page);
 free_it:
-		nr_reclaimed++;
+		/*
+		 * THP may get swapped out in a whole, need account
+		 * all base pages.
+		 */
+		nr_reclaimed += nr_pages;
 
 		/*
 		 * Is there need to periodically free_page_list? It would
@@ -1455,6 +1475,15 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 			list_add(&page->lru, &free_pages);
 		continue;
 
+activate_locked_split:
+		/*
+		 * The tail pages that are failed to add into swap cache
+		 * reach here.  Fixup nr_scanned and nr_pages.
+		 */
+		if (nr_pages > 1) {
+			sc->nr_scanned -= (nr_pages - 1);
+			nr_pages = 1;
+		}
 activate_locked:
 		/* Not a candidate for swapping, so reclaim swap space. */
 		if (PageSwapCache(page) && (mem_cgroup_swap_full(page) ||
@@ -1464,8 +1493,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 		if (!PageMlocked(page)) {
 			int type = page_is_file_cache(page);
 			SetPageActive(page);
-			pgactivate++;
-			stat->nr_activate[type] += hpage_nr_pages(page);
+			stat->nr_activate[type] += nr_pages;
 			count_memcg_page_event(page, PGACTIVATE);
 		}
 keep_locked:
@@ -1475,6 +1503,8 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 		VM_BUG_ON_PAGE(PageLRU(page) || PageUnevictable(page), page);
 	}
 
+	pgactivate = stat->nr_activate[0] + stat->nr_activate[1];
+
 	mem_cgroup_uncharge_list(&free_pages);
 	try_to_unmap_flush();
 	free_unref_page_list(&free_pages);
@@ -1646,10 +1676,9 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
 	LIST_HEAD(pages_skipped);
 	isolate_mode_t mode = (sc->may_unmap ? 0 : ISOLATE_UNMAPPED);
 
+	total_scan = 0;
 	scan = 0;
-	for (total_scan = 0;
-	     scan < nr_to_scan && nr_taken < nr_to_scan && !list_empty(src);
-	     total_scan++) {
+	while (scan < nr_to_scan && !list_empty(src)) {
 		struct page *page;
 
 		page = lru_to_page(src);
@@ -1657,9 +1686,12 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
 
 		VM_BUG_ON_PAGE(!PageLRU(page), page);
 
+		nr_pages = 1 << compound_order(page);
+		total_scan += nr_pages;
+
 		if (page_zonenum(page) > sc->reclaim_idx) {
 			list_move(&page->lru, &pages_skipped);
-			nr_skipped[page_zonenum(page)]++;
+			nr_skipped[page_zonenum(page)] += nr_pages;
 			continue;
 		}
 
@@ -1668,11 +1700,14 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
 		 * return with no isolated pages if the LRU mostly contains
 		 * ineligible pages.  This causes the VM to not reclaim any
 		 * pages, triggering a premature OOM.
+		 *
+		 * Account all tail pages of THP.  This would not cause
+		 * premature OOM since __isolate_lru_page() returns -EBUSY
+		 * only when the page is being freed somewhere else.
 		 */
-		scan++;
+		scan += nr_pages;
 		switch (__isolate_lru_page(page, mode)) {
 		case 0:
-			nr_pages = hpage_nr_pages(page);
 			nr_taken += nr_pages;
 			nr_zone_taken[page_zonenum(page)] += nr_pages;
 			list_move(&page->lru, dst);
-- 
1.8.3.1

Re: [v7 PATCH 2/2] mm: vmscan: correct some vmscan counters for THP swapout

[Huang, Ying]

Yang Shi <yang.shi@linux.alibaba.com> 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@intel.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Mel Gorman <mgorman@techsingularity.net>
> Cc: "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Shakeel Butt <shakeelb@google.com>
> Cc: Hillf Danton <hdanton@sina.com>
> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>

Looks good to me!  Thanks for your effort!

Reviewed-by: "Huang, Ying" <ying.huang@intel.com>

Best Regards,
Huang, Ying

Re: [v7 PATCH 1/2] mm: vmscan: remove double slab pressure by inc'ing sc->nr_scanned

[Oscar Salvador]

On Tue, 2019-05-28 at 14:44 +0800, Yang Shi wrote:
> The commit 9092c71bb724 ("mm: use sc->priority for slab shrink
> targets")
> has broken up the relationship between sc->nr_scanned and slab
> pressure.
> The sc->nr_scanned can't double slab pressure anymore.  So, it sounds
> no
> sense to still keep sc->nr_scanned inc'ed.  Actually, it would
> prevent
> from adding pressure on slab shrink since excessive sc->nr_scanned
> would
> prevent from scan->priority raise.

Hi Yang,

I might be misunderstanding this, but did you mean "prevent from scan-
priority decreasing"?
I guess we are talking about balance_pgdat(), and in case
kswapd_shrink_node() returns true (it means we have scanned more than
we had to reclaim), raise_priority becomes false, and this does not let
sc->priority to be decreased, which has the impact that less pages will
 be reclaimed the next round.

Sorry for bugging here, I just wanted to see if I got this right.


-- 
Oscar Salvador
SUSE L3

Re: [v7 PATCH 1/2] mm: vmscan: remove double slab pressure by inc'ing sc->nr_scanned

[Yang Shi]

On 6/10/19 2:36 PM, Oscar Salvador wrote:
> On Tue, 2019-05-28 at 14:44 +0800, Yang Shi wrote:
>> The commit 9092c71bb724 ("mm: use sc->priority for slab shrink
>> targets")
>> has broken up the relationship between sc->nr_scanned and slab
>> pressure.
>> The sc->nr_scanned can't double slab pressure anymore.  So, it sounds
>> no
>> sense to still keep sc->nr_scanned inc'ed.  Actually, it would
>> prevent
>> from adding pressure on slab shrink since excessive sc->nr_scanned
>> would
>> prevent from scan->priority raise.
> Hi Yang,
>
> I might be misunderstanding this, but did you mean "prevent from scan-
> priority decreasing"?
> I guess we are talking about balance_pgdat(), and in case
> kswapd_shrink_node() returns true (it means we have scanned more than
> we had to reclaim), raise_priority becomes false, and this does not let
> sc->priority to be decreased, which has the impact that less pages will
>   be reclaimed the next round.

Yes, exactly.

>
> Sorry for bugging here, I just wanted to see if I got this right.
>
>

Re: [v7 PATCH 1/2] mm: vmscan: remove double slab pressure by inc'ing sc->nr_scanned

[Yang Shi]

On 6/11/19 10:12 AM, Yang Shi wrote:
>
>
> On 6/10/19 2:36 PM, Oscar Salvador wrote:
>> On Tue, 2019-05-28 at 14:44 +0800, Yang Shi wrote:
>>> The commit 9092c71bb724 ("mm: use sc->priority for slab shrink
>>> targets")
>>> has broken up the relationship between sc->nr_scanned and slab
>>> pressure.
>>> The sc->nr_scanned can't double slab pressure anymore.  So, it sounds
>>> no
>>> sense to still keep sc->nr_scanned inc'ed.  Actually, it would
>>> prevent
>>> from adding pressure on slab shrink since excessive sc->nr_scanned
>>> would
>>> prevent from scan->priority raise.
>> Hi Yang,
>>
>> I might be misunderstanding this, but did you mean "prevent from scan-
>> priority decreasing"?
>> I guess we are talking about balance_pgdat(), and in case
>> kswapd_shrink_node() returns true (it means we have scanned more than
>> we had to reclaim), raise_priority becomes false, and this does not let
>> sc->priority to be decreased, which has the impact that less pages will
>>   be reclaimed the next round.
>
> Yes, exactly.

BTW, for the scan priority, the smaller number the higher priority. So, 
either "raise" or "decrease" sounds correct. "raise" means the real 
priority, "decrease" means the number itself.

>
>>
>> Sorry for bugging here, I just wanted to see if I got this right.
>>
>>
>