dm bufio: fix shrinker scans when (nr_to_scan < retain_target)

When system is under memory pressure it is observed that dm bufio shrinker often reclaims only one buffer per scan. This change fixes the following two issues in dm bufio shrinker that cause this behavior:

1. ((nr_to_scan - freed) <= retain_target) condition is used to terminate slab scan process. This assumes that nr_to_scan is equal to the LRU size, which might not be correct because do_shrink_slab() in vmscan.c calculates nr_to_scan using multiple inputs.
As a result when nr_to_scan is less than retain_target (64) the scan will terminate after the first iteration, effectively reclaiming one buffer per scan and making scans very inefficient. This hurts vmscan performance especially because mutex is acquired/released every time dm_bufio_shrink_scan() is called.
New implementation uses ((LRU size - freed) <= retain_target) condition for scan termination. LRU size can be safely determined inside __scan() because this function is called after dm_bufio_lock().

2. do_shrink_slab() uses value returned by dm_bufio_shrink_count() to determine number of freeable objects in the slab. However dm_bufio always retains retain_target buffers in its LRU and will terminate a scan when this mark is reached. Therefore returning the entire LRU size from dm_bufio_shrink_count() is misleading because that does not represent the number of freeable objects that slab will reclaim during a scan. Returning (LRU size - retain_target) better represents the number of freeable objects in the slab. This way do_shrink_slab() returns 0 when (LRU size < retain_target) and vmscan will not try to scan this shrinker avoiding scans that will not reclaim any memory.

Test: tested using Android device running <AOSP>/system/extras/alloc-stress that generates memory pressure and causes intensive shrinker scans

Signed-off-by Suren Baghdasaryan <[email protected]>

---
 drivers/md/dm-bufio.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

drivers/md/dm-bufio.c +6 -3

index b8ac591aaaa7..c546b567f3b5 100644
@@ -1611,7 +1611,8 @@ static unsigned long __scan(struct dm_bufio_client *c, unsigned long nr_to_scan,
        int l;
        struct dm_buffer *b, *tmp;
        unsigned long freed = 0;
-       unsigned long count = nr_to_scan;
+       unsigned long count = c->n_buffers[LIST_CLEAN] +
+                             c->n_buffers[LIST_DIRTY];
        unsigned long retain_target = get_retain_buffers(c);
 
        for (l = 0; l < LIST_SIZE; l++) {

@@ -1647,8 +1648,11 @@ static unsigned long
 dm_bufio_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
 {
        struct dm_bufio_client *c = container_of(shrink, struct dm_bufio_client, shrinker);
+       unsigned long count = READ_ONCE(c->n_buffers[LIST_CLEAN]) +
+                             READ_ONCE(c->n_buffers[LIST_DIRTY]);
+       unsigned long retain_target = get_retain_buffers(c);
 
-       return READ_ONCE(c->n_buffers[LIST_CLEAN]) + READ_ONCE(c->n_buffers[LIST_DIRTY]);
+       return (count < retain_target) ? 0 : (count - retain_target);
 }
 
 /*
--

2.15.0.531.g2ccb3012c9-goog

Dear kernel maintainers. I know it was close to holiday season when I send this patch last month, so delay was expected. Could you please take a look at it and provide your feedback?
Thanks!

Quoted 67 lines · Suren Baghdasaryan

When system is under memory pressure it is observed that dm bufio shrinker often reclaims only one buffer per scan. This change fixes the following two issues in dm bufio shrinker that cause this behavior:

1. ((nr_to_scan - freed) <= retain_target) condition is used to terminate slab scan process. This assumes that nr_to_scan is equal to the LRU size, which might not be correct because do_shrink_slab() in vmscan.c calculates nr_to_scan using multiple inputs.
As a result when nr_to_scan is less than retain_target (64) the scan will terminate after the first iteration, effectively reclaiming one buffer per scan and making scans very inefficient. This hurts vmscan performance especially because mutex is acquired/released every time dm_bufio_shrink_scan() is called.
New implementation uses ((LRU size - freed) <= retain_target) condition for scan termination. LRU size can be safely determined inside __scan() because this function is called after dm_bufio_lock().

2. do_shrink_slab() uses value returned by dm_bufio_shrink_count() to determine number of freeable objects in the slab. However dm_bufio always retains retain_target buffers in its LRU and will terminate a scan when this mark is reached. Therefore returning the entire LRU size from dm_bufio_shrink_count() is misleading because that does not represent the number of freeable objects that slab will reclaim during a scan. Returning (LRU size - retain_target) better represents the number of freeable objects in the slab. This way do_shrink_slab() returns 0 when (LRU size < retain_target) and vmscan will not try to scan this shrinker avoiding scans that will not reclaim any memory.

Test: tested using Android device running <AOSP>/system/extras/alloc-stress that generates memory pressure and causes intensive shrinker scans

Signed-off-by Suren Baghdasaryan <[email protected]>

---
 drivers/md/dm-bufio.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

drivers/md/dm-bufio.c +6 -3

index b8ac591aaaa7..c546b567f3b5 100644
@@ -1611,7 +1611,8 @@ static unsigned long __scan(struct dm_bufio_client *c, unsigned long nr_to_scan,
        int l;
        struct dm_buffer *b, *tmp;
        unsigned long freed = 0;
-       unsigned long count = nr_to_scan;
+       unsigned long count = c->n_buffers[LIST_CLEAN] +
+                             c->n_buffers[LIST_DIRTY];
        unsigned long retain_target = get_retain_buffers(c);
 
        for (l = 0; l < LIST_SIZE; l++) {

@@ -1647,8 +1648,11 @@ static unsigned long
 dm_bufio_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
 {
        struct dm_bufio_client *c = container_of(shrink, struct dm_bufio_client, shrinker);
+       unsigned long count = READ_ONCE(c->n_buffers[LIST_CLEAN]) +
+                             READ_ONCE(c->n_buffers[LIST_DIRTY]);
+       unsigned long retain_target = get_retain_buffers(c);
 
-       return READ_ONCE(c->n_buffers[LIST_CLEAN]) + READ_ONCE(c->n_buffers[LIST_DIRTY]);
+       return (count < retain_target) ? 0 : (count - retain_target);
 }
 
 /*
--

2.15.0.531.g2ccb3012c9-goog