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.
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" <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: "Kirill A . Shutemov" <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Shakeel Butt <[email protected]>
Signed-off-by: Yang Shi <[email protected]>
---
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 | 40 ++++++++++++++++++++++++++++------------
1 file changed, 28 insertions(+), 12 deletions(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index b65bc50..1044834 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1250,7 +1250,7 @@ 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 += (1 << compound_order(page));
goto keep_locked;
case PAGEREF_RECLAIM:
case PAGEREF_RECLAIM_CLEAN:
@@ -1294,6 +1294,17 @@ static unsigned long shrink_page_list(struct list_head *page_list,
goto activate_locked;
}
+ /*
+ * Account all tail pages when THP is added
+ * into swap cache successfully.
+ * The head page has been accounted at the
+ * first place.
+ */
+ if (PageTransHuge(page))
+ sc->nr_scanned +=
+ ((1 << compound_order(page)) -
+ 1);
+
may_enter_fs = 1;
/* Adding to swap updated mapping */
@@ -1315,7 +1326,8 @@ 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 +=
+ (1 << compound_order(page));
goto activate_locked;
}
}
@@ -1442,7 +1454,11 @@ 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 += (1 << compound_order(page));
/*
* Is there need to periodically free_page_list? It would
@@ -1464,7 +1480,6 @@ 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);
count_memcg_page_event(page, PGACTIVATE);
}
@@ -1475,6 +1490,8 @@ 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);
@@ -1642,14 +1659,12 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
unsigned long nr_zone_taken[MAX_NR_ZONES] = { 0 };
unsigned long nr_skipped[MAX_NR_ZONES] = { 0, };
unsigned long skipped = 0;
- unsigned long scan, total_scan, nr_pages;
+ unsigned long scan, nr_pages;
LIST_HEAD(pages_skipped);
isolate_mode_t mode = (sc->may_unmap ? 0 : ISOLATE_UNMAPPED);
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 && nr_taken < nr_to_scan && !list_empty(src)) {
struct page *page;
page = lru_to_page(src);
@@ -1659,7 +1674,8 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
if (page_zonenum(page) > sc->reclaim_idx) {
list_move(&page->lru, &pages_skipped);
- nr_skipped[page_zonenum(page)]++;
+ nr_skipped[page_zonenum(page)] +=
+ (1 << compound_order(page));
continue;
}
@@ -1669,7 +1685,7 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
* ineligible pages. This causes the VM to not reclaim any
* pages, triggering a premature OOM.
*/
- scan++;
+ scan += (1 << compound_order(page));
switch (__isolate_lru_page(page, mode)) {
case 0:
nr_pages = hpage_nr_pages(page);
@@ -1707,9 +1723,9 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
skipped += nr_skipped[zid];
}
}
- *nr_scanned = total_scan;
+ *nr_scanned = scan;
trace_mm_vmscan_lru_isolate(sc->reclaim_idx, sc->order, nr_to_scan,
- total_scan, skipped, nr_taken, mode, lru);
+ scan, skipped, nr_taken, mode, lru);
update_lru_sizes(lruvec, lru, nr_zone_taken);
return nr_taken;
}
--
1.8.3.1
On Tue, May 21, 2019 at 05:40:42PM +0800, Yang Shi wrote:
> 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.
>
> 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" <[email protected]>
> Cc: Johannes Weiner <[email protected]>
> Cc: Michal Hocko <[email protected]>
> Cc: Mel Gorman <[email protected]>
> Cc: "Kirill A . Shutemov" <[email protected]>
> Cc: Hugh Dickins <[email protected]>
> Cc: Shakeel Butt <[email protected]>
> Signed-off-by: Yang Shi <[email protected]>
> ---
> 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 | 40 ++++++++++++++++++++++++++++------------
> 1 file changed, 28 insertions(+), 12 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index b65bc50..1044834 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1250,7 +1250,7 @@ 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 += (1 << compound_order(page));
> goto keep_locked;
> case PAGEREF_RECLAIM:
> case PAGEREF_RECLAIM_CLEAN:
> @@ -1294,6 +1294,17 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> goto activate_locked;
> }
>
> + /*
> + * Account all tail pages when THP is added
> + * into swap cache successfully.
> + * The head page has been accounted at the
> + * first place.
> + */
> + if (PageTransHuge(page))
> + sc->nr_scanned +=
> + ((1 << compound_order(page)) -
> + 1);
> +
> may_enter_fs = 1;
Even if we don't split and reclaim the page, we should always account
the number of base pages in nr_scanned. Otherwise it's not clear what
nr_scanned means.
> /* Adding to swap updated mapping */
> @@ -1315,7 +1326,8 @@ 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 +=
> + (1 << compound_order(page));
> goto activate_locked;
> }
> }
> @@ -1442,7 +1454,11 @@ 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 += (1 << compound_order(page));
This expression is quite repetitive. Why not do
int nr_pages;
page = lru_to_page(page_list);
nr_pages = 1 << compound_order(page);
list_del(&page->lru);
if (!trylock_page(page))
...
at the head of the loop and add nr_pages to all these counters
instead?
> @@ -1642,14 +1659,12 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
> unsigned long nr_zone_taken[MAX_NR_ZONES] = { 0 };
> unsigned long nr_skipped[MAX_NR_ZONES] = { 0, };
> unsigned long skipped = 0;
> - unsigned long scan, total_scan, nr_pages;
> + unsigned long scan, nr_pages;
> LIST_HEAD(pages_skipped);
> isolate_mode_t mode = (sc->may_unmap ? 0 : ISOLATE_UNMAPPED);
>
> 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 && nr_taken < nr_to_scan && !list_empty(src)) {
> struct page *page;
Once you fixed the units, scan < nr_to_scan && nr_taken >= nr_to_scan
is an impossible condition. You should be able to write:
while (scan < nr_to_scan && !list_empty(src))
Also, you need to keep total_scan. The trace point wants to know how
many pages were actually looked at, including the ones from ineligible
zones that were skipped over.
>
> page = lru_to_page(src);
> @@ -1659,7 +1674,8 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>
> if (page_zonenum(page) > sc->reclaim_idx) {
> list_move(&page->lru, &pages_skipped);
> - nr_skipped[page_zonenum(page)]++;
> + nr_skipped[page_zonenum(page)] +=
> + (1 << compound_order(page));
> continue;
> }
>
> @@ -1669,7 +1685,7 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
> * ineligible pages. This causes the VM to not reclaim any
> * pages, triggering a premature OOM.
> */
> - scan++;
> + scan += (1 << compound_order(page));
> switch (__isolate_lru_page(page, mode)) {
> case 0:
> nr_pages = hpage_nr_pages(page);
Same here, you can calculate nr_pages at the top of the loop and use
it throughout.
> @@ -1707,9 +1723,9 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
> skipped += nr_skipped[zid];
> }
> }
> - *nr_scanned = total_scan;
> + *nr_scanned = scan;
> trace_mm_vmscan_lru_isolate(sc->reclaim_idx, sc->order, nr_to_scan,
> - total_scan, skipped, nr_taken, mode, lru);
> + scan, skipped, nr_taken, mode, lru);
> update_lru_sizes(lruvec, lru, nr_zone_taken);
> return nr_taken;
> }
> --
> 1.8.3.1
>
Yang Shi <[email protected]> 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.
>
> 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" <[email protected]>
> Cc: Johannes Weiner <[email protected]>
> Cc: Michal Hocko <[email protected]>
> Cc: Mel Gorman <[email protected]>
> Cc: "Kirill A . Shutemov" <[email protected]>
> Cc: Hugh Dickins <[email protected]>
> Cc: Shakeel Butt <[email protected]>
> Signed-off-by: Yang Shi <[email protected]>
> ---
> 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 | 40 ++++++++++++++++++++++++++++------------
> 1 file changed, 28 insertions(+), 12 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index b65bc50..1044834 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1250,7 +1250,7 @@ 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 += (1 << compound_order(page));
> goto keep_locked;
> case PAGEREF_RECLAIM:
> case PAGEREF_RECLAIM_CLEAN:
> @@ -1294,6 +1294,17 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> goto activate_locked;
> }
>
> + /*
> + * Account all tail pages when THP is added
> + * into swap cache successfully.
> + * The head page has been accounted at the
> + * first place.
> + */
> + if (PageTransHuge(page))
> + sc->nr_scanned +=
> + ((1 << compound_order(page)) -
> + 1);
> +
The "if" here could be changed to "else if" because if add_to_swap()
fails we don't need to call PageTransHuge() here. But this isn't a big
deal.
You have analyzed the code and found that nr_dirty, nr_unqueued_dirty,
nr_congested and nr_writeback are file cache related and not impacted by
THP swap out. How about add your findings in the patch description?
Best Regards,
Huang, Ying
On 5/22/19 12:00 AM, Johannes Weiner wrote:
> On Tue, May 21, 2019 at 05:40:42PM +0800, Yang Shi wrote:
>> 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.
>>
>> 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" <[email protected]>
>> Cc: Johannes Weiner <[email protected]>
>> Cc: Michal Hocko <[email protected]>
>> Cc: Mel Gorman <[email protected]>
>> Cc: "Kirill A . Shutemov" <[email protected]>
>> Cc: Hugh Dickins <[email protected]>
>> Cc: Shakeel Butt <[email protected]>
>> Signed-off-by: Yang Shi <[email protected]>
>> ---
>> 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 | 40 ++++++++++++++++++++++++++++------------
>> 1 file changed, 28 insertions(+), 12 deletions(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index b65bc50..1044834 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -1250,7 +1250,7 @@ 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 += (1 << compound_order(page));
>> goto keep_locked;
>> case PAGEREF_RECLAIM:
>> case PAGEREF_RECLAIM_CLEAN:
>> @@ -1294,6 +1294,17 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>> goto activate_locked;
>> }
>>
>> + /*
>> + * Account all tail pages when THP is added
>> + * into swap cache successfully.
>> + * The head page has been accounted at the
>> + * first place.
>> + */
>> + if (PageTransHuge(page))
>> + sc->nr_scanned +=
>> + ((1 << compound_order(page)) -
>> + 1);
>> +
>> may_enter_fs = 1;
> Even if we don't split and reclaim the page, we should always account
> the number of base pages in nr_scanned. Otherwise it's not clear what
> nr_scanned means.
Sure.
>
>> /* Adding to swap updated mapping */
>> @@ -1315,7 +1326,8 @@ 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 +=
>> + (1 << compound_order(page));
>> goto activate_locked;
>> }
>> }
>> @@ -1442,7 +1454,11 @@ 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 += (1 << compound_order(page));
> This expression is quite repetitive. Why not do
>
> int nr_pages;
>
> page = lru_to_page(page_list);
> nr_pages = 1 << compound_order(page);
> list_del(&page->lru);
>
> if (!trylock_page(page))
> ...
>
> at the head of the loop and add nr_pages to all these counters
> instead?
Because it is unknown whether the THP will be swapped out as a whole or
will be split at this point. nr_scanned is fine, but nr_reclaimed is not.
>
>> @@ -1642,14 +1659,12 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>> unsigned long nr_zone_taken[MAX_NR_ZONES] = { 0 };
>> unsigned long nr_skipped[MAX_NR_ZONES] = { 0, };
>> unsigned long skipped = 0;
>> - unsigned long scan, total_scan, nr_pages;
>> + unsigned long scan, nr_pages;
>> LIST_HEAD(pages_skipped);
>> isolate_mode_t mode = (sc->may_unmap ? 0 : ISOLATE_UNMAPPED);
>>
>> 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 && nr_taken < nr_to_scan && !list_empty(src)) {
>> struct page *page;
> Once you fixed the units, scan < nr_to_scan && nr_taken >= nr_to_scan
> is an impossible condition. You should be able to write:
>
> while (scan < nr_to_scan && !list_empty(src))
Yes.
>
> Also, you need to keep total_scan. The trace point wants to know how
> many pages were actually looked at, including the ones from ineligible
> zones that were skipped over.
Aha, yes. The total_scan includes both scanned and skipped. Will fix in v4.
>
>>
>> page = lru_to_page(src);
>> @@ -1659,7 +1674,8 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>>
>> if (page_zonenum(page) > sc->reclaim_idx) {
>> list_move(&page->lru, &pages_skipped);
>> - nr_skipped[page_zonenum(page)]++;
>> + nr_skipped[page_zonenum(page)] +=
>> + (1 << compound_order(page));
>> continue;
>> }
>>
>> @@ -1669,7 +1685,7 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>> * ineligible pages. This causes the VM to not reclaim any
>> * pages, triggering a premature OOM.
>> */
>> - scan++;
>> + scan += (1 << compound_order(page));
>> switch (__isolate_lru_page(page, mode)) {
>> case 0:
>> nr_pages = hpage_nr_pages(page);
> Same here, you can calculate nr_pages at the top of the loop and use
> it throughout.
Yes. Will fix in v4.
>
>> @@ -1707,9 +1723,9 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>> skipped += nr_skipped[zid];
>> }
>> }
>> - *nr_scanned = total_scan;
>> + *nr_scanned = scan;
>> trace_mm_vmscan_lru_isolate(sc->reclaim_idx, sc->order, nr_to_scan,
>> - total_scan, skipped, nr_taken, mode, lru);
>> + scan, skipped, nr_taken, mode, lru);
>> update_lru_sizes(lruvec, lru, nr_zone_taken);
>> return nr_taken;
>> }
>> --
>> 1.8.3.1
>>
On 5/22/19 9:23 AM, Huang, Ying wrote:
> Yang Shi <[email protected]> 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.
>>
>> 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" <[email protected]>
>> Cc: Johannes Weiner <[email protected]>
>> Cc: Michal Hocko <[email protected]>
>> Cc: Mel Gorman <[email protected]>
>> Cc: "Kirill A . Shutemov" <[email protected]>
>> Cc: Hugh Dickins <[email protected]>
>> Cc: Shakeel Butt <[email protected]>
>> Signed-off-by: Yang Shi <[email protected]>
>> ---
>> 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 | 40 ++++++++++++++++++++++++++++------------
>> 1 file changed, 28 insertions(+), 12 deletions(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index b65bc50..1044834 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -1250,7 +1250,7 @@ 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 += (1 << compound_order(page));
>> goto keep_locked;
>> case PAGEREF_RECLAIM:
>> case PAGEREF_RECLAIM_CLEAN:
>> @@ -1294,6 +1294,17 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>> goto activate_locked;
>> }
>>
>> + /*
>> + * Account all tail pages when THP is added
>> + * into swap cache successfully.
>> + * The head page has been accounted at the
>> + * first place.
>> + */
>> + if (PageTransHuge(page))
>> + sc->nr_scanned +=
>> + ((1 << compound_order(page)) -
>> + 1);
>> +
> The "if" here could be changed to "else if" because if add_to_swap()
> fails we don't need to call PageTransHuge() here. But this isn't a big
> deal.
This could be moved to the beginning according to Johannes.
>
> You have analyzed the code and found that nr_dirty, nr_unqueued_dirty,
> nr_congested and nr_writeback are file cache related and not impacted by
> THP swap out. How about add your findings in the patch description?
Yes, sure. Will add in v4.
>
> Best Regards,
> Huang, Ying
>