[PATCH 4/8] mm: Only drain per-cpu pagevecs once per pagevec usage

Mel Gorman
2017-10-18T08:00:57Z

When a pagevec is initialised on the stack, it is generally used multiple
times over a range of pages, looking up entries and then releasing them.
On each pagevec_release, the per-cpu deferred LRU pagevecs are drained
on the grounds the page being released may be on those queues and the
pages may be cache hot. In many cases only the first drain is necessary
as it's unlikely that the range of pages being walked is racing against
LRU addition. Even if there is such a race, the impact is marginal where
as constantly redraining the lru pagevecs costs.

This patch ensures that pagevec is only drained once in a given lifecycle
without increasing the cache footprint of the pagevec structure. Only
sparsetruncate tiny is shown here as large files have many exceptional
entries and calls pagecache_release less frequently.

sparsetruncate (tiny)
4.14.0-rc4 4.14.0-rc4
batchshadow-v1r1 onedrain-v1r1
Min Time 141.00 ( 0.00%) 141.00 ( 0.00%)
1st-qrtle Time 142.00 ( 0.00%) 142.00 ( 0.00%)
2nd-qrtle Time 142.00 ( 0.00%) 142.00 ( 0.00%)
3rd-qrtle Time 143.00 ( 0.00%) 143.00 ( 0.00%)
Max-90% Time 144.00 ( 0.00%) 144.00 ( 0.00%)
Max-95% Time 146.00 ( 0.00%) 145.00 ( 0.68%)
Max-99% Time 198.00 ( 0.00%) 194.00 ( 2.02%)
Max Time 254.00 ( 0.00%) 208.00 ( 18.11%)
Amean Time 145.12 ( 0.00%) 144.30 ( 0.56%)
Stddev Time 12.74 ( 0.00%) 9.62 ( 24.49%)
Coeff Time 8.78 ( 0.00%) 6.67 ( 24.06%)
Best99%Amean Time 144.29 ( 0.00%) 143.82 ( 0.32%)
Best95%Amean Time 142.68 ( 0.00%) 142.31 ( 0.26%)
Best90%Amean Time 142.52 ( 0.00%) 142.19 ( 0.24%)
Best75%Amean Time 142.26 ( 0.00%) 141.98 ( 0.20%)
Best50%Amean Time 141.90 ( 0.00%) 141.71 ( 0.13%)
Best25%Amean Time 141.80 ( 0.00%) 141.43 ( 0.26%)

The impact on bonnie is marginal and within the noise because a significant
percentage of the file being truncated has been reclaimed and consists of
shadow entries which reduce the hotness of the pagevec_release path.

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
---
include/linux/pagevec.h | 4 +++-
mm/swap.c | 5 ++++-
2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/include/linux/pagevec.h b/include/linux/pagevec.h
index 4dcd5506f1ed..4231979be982 100644
--- a/include/linux/pagevec.h
+++ b/include/linux/pagevec.h
@@ -16,7 +16,8 @@ struct address_space;

struct pagevec {
unsigned long nr;
- unsigned long cold;
+ bool cold;
+ bool drained;
struct page *pages[PAGEVEC_SIZE];
};

@@ -45,6 +46,7 @@ static inline void pagevec_init(struct pagevec *pvec, int cold)
{
pvec->nr = 0;
pvec->cold = cold;
+ pvec->drained = false;
}

static inline void pagevec_reinit(struct pagevec *pvec)
diff --git a/mm/swap.c b/mm/swap.c
index a77d68f2c1b6..31bd9d8a5db7 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -833,7 +833,10 @@ EXPORT_SYMBOL(release_pages);
*/
void __pagevec_release(struct pagevec *pvec)
{
- lru_add_drain();
+ if (!pvec->drained) {
+ lru_add_drain();
+ pvec->drained = true;
+ }
release_pages(pvec->pages, pagevec_count(pvec), pvec->cold);
pagevec_reinit(pvec);
}
--
2.14.0