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




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@xxxxxxxxxxxxxxxxxxx>
---
 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