[PATCH v10 06/35] mm: new shrinker API

Glauber Costa
2013-06-03T19:29:35Z

From: Dave Chinner <dchinner-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>

The current shrinker callout API uses an a single shrinker call for
multiple functions. To determine the function, a special magical
value is passed in a parameter to change the behaviour. This
complicates the implementation and return value specification for
the different behaviours.

Separate the two different behaviours into separate operations, one
to return a count of freeable objects in the cache, and another to
scan a certain number of objects in the cache for freeing. In
defining these new operations, ensure the return values and
resultant behaviours are clearly defined and documented.

Modify shrink_slab() to use the new API and implement the callouts
for all the existing shrinkers.

Signed-off-by: Dave Chinner <dchinner-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Signed-off-by: Glauber Costa <glommer-bzQdu9zFT3WakBO8gow8eQ@public.gmane.org>
Acked-by: Mel Gorman <mgorman-l3A5Bk7waGM@public.gmane.org>
---
include/linux/shrinker.h | 36 ++++++++++++++++++++++++----------
mm/vmscan.c | 50 +++++++++++++++++++++++++++++++-----------------
2 files changed, 58 insertions(+), 28 deletions(-)

diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
index ac6b8ee..c277b4e 100644
--- a/include/linux/shrinker.h
+++ b/include/linux/shrinker.h
@@ -4,31 +4,47 @@
/*
* This struct is used to pass information from page reclaim to the shrinkers.
* We consolidate the values for easier extention later.
+ *
+ * The 'gfpmask' refers to the allocation we are currently trying to
+ * fulfil.
+ *
+ * Note that 'shrink' will be passed nr_to_scan == 0 when the VM is
+ * querying the cache size, so a fastpath for that case is appropriate.
*/
struct shrink_control {
gfp_t gfp_mask;

/* How many slab objects shrinker() should scan and try to reclaim */
- unsigned long nr_to_scan;
+ long nr_to_scan;
};

/*
* A callback you can register to apply pressure to ageable caches.
*
- * 'sc' is passed shrink_control which includes a count 'nr_to_scan'
- * and a 'gfpmask'. It should look through the least-recently-used
- * 'nr_to_scan' entries and attempt to free them up. It should return
- * the number of objects which remain in the cache. If it returns -1, it means
- * it cannot do any scanning at this time (eg. there is a risk of deadlock).
+ * @shrink() should look through the least-recently-used 'nr_to_scan' entries
+ * and attempt to free them up. It should return the number of objects which
+ * remain in the cache. If it returns -1, it means it cannot do any scanning at
+ * this time (eg. there is a risk of deadlock).
*
- * The 'gfpmask' refers to the allocation we are currently trying to
- * fulfil.
+ * @count_objects should return the number of freeable items in the cache. If
+ * there are no objects to free or the number of freeable items cannot be
+ * determined, it should return 0. No deadlock checks should be done during the
+ * count callback - the shrinker relies on aggregating scan counts that couldn't
+ * be executed due to potential deadlocks to be run at a later call when the
+ * deadlock condition is no longer pending.
*
- * Note that 'shrink' will be passed nr_to_scan == 0 when the VM is
- * querying the cache size, so a fastpath for that case is appropriate.
+ * @scan_objects will only be called if @count_objects returned a positive
+ * value for the number of freeable objects. The callout should scan the cache
+ * and attempt to free items from the cache. It should then return the number of
+ * objects freed during the scan, or -1 if progress cannot be made due to
+ * potential deadlocks. If -1 is returned, then no further attempts to call the
+ * @scan_objects will be made from the current reclaim context.
*/
struct shrinker {
int (*shrink)(struct shrinker *, struct shrink_control *sc);
+ long (*count_objects)(struct shrinker *, struct shrink_control *sc);
+ long (*scan_objects)(struct shrinker *, struct shrink_control *sc);
+
int seeks; /* seeks to recreate an obj */
long batch; /* reclaim batch size, 0 = default */

diff --git a/mm/vmscan.c b/mm/vmscan.c
index b1b38ad..6ac3ec2 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -205,19 +205,19 @@ static inline int do_shrinker_shrink(struct shrinker *shrinker,
*
* Returns the number of slab objects which we shrunk.
*/
-unsigned long shrink_slab(struct shrink_control *shrink,
+unsigned long shrink_slab(struct shrink_control *shrinkctl,
unsigned long nr_pages_scanned,
unsigned long lru_pages)
{
struct shrinker *shrinker;
- unsigned long ret = 0;
+ unsigned long freed = 0;

if (nr_pages_scanned == 0)
nr_pages_scanned = SWAP_CLUSTER_MAX;

if (!down_read_trylock(&shrinker_rwsem)) {
/* Assume we'll be able to shrink next time */
- ret = 1;
+ freed = 1;
goto out;
}

@@ -225,13 +225,16 @@ unsigned long shrink_slab(struct shrink_control *shrink,
unsigned long long delta;
long total_scan;
long max_pass;
- int shrink_ret = 0;
long nr;
long new_nr;
long batch_size = shrinker->batch ? shrinker->batch
: SHRINK_BATCH;

- max_pass = do_shrinker_shrink(shrinker, shrink, 0);
+ if (shrinker->scan_objects) {
+ max_pass = shrinker->count_objects(shrinker, shrinkctl);
+ WARN_ON(max_pass < 0);
+ } else
+ max_pass = do_shrinker_shrink(shrinker, shrinkctl, 0);
if (max_pass <= 0)
continue;

@@ -248,8 +251,8 @@ unsigned long shrink_slab(struct shrink_control *shrink,
do_div(delta, lru_pages + 1);
total_scan += delta;
if (total_scan < 0) {
- printk(KERN_ERR "shrink_slab: %pF negative objects to "
- "delete nr=%ld\n",
+ printk(KERN_ERR
+ "shrink_slab: %pF negative objects to delete nr=%ld\n",
shrinker->shrink, total_scan);
total_scan = max_pass;
}
@@ -277,20 +280,31 @@ unsigned long shrink_slab(struct shrink_control *shrink,
if (total_scan > max_pass * 2)
total_scan = max_pass * 2;

- trace_mm_shrink_slab_start(shrinker, shrink, nr,
+ trace_mm_shrink_slab_start(shrinker, shrinkctl, nr,
nr_pages_scanned, lru_pages,
max_pass, delta, total_scan);

while (total_scan >= batch_size) {
- int nr_before;
+ long ret;
+
+ if (shrinker->scan_objects) {
+ shrinkctl->nr_to_scan = batch_size;
+ ret = shrinker->scan_objects(shrinker, shrinkctl);
+
+ if (ret == -1)
+ break;
+ freed += ret;
+ } else {
+ int nr_before;
+ nr_before = do_shrinker_shrink(shrinker, shrinkctl, 0);
+ ret = do_shrinker_shrink(shrinker, shrinkctl,
+ batch_size);
+ if (ret == -1)
+ break;
+ if (ret < nr_before)
+ freed += nr_before - ret;
+ }

- nr_before = do_shrinker_shrink(shrinker, shrink, 0);
- shrink_ret = do_shrinker_shrink(shrinker, shrink,
- batch_size);
- if (shrink_ret == -1)
- break;
- if (shrink_ret < nr_before)
- ret += nr_before - shrink_ret;
count_vm_events(SLABS_SCANNED, batch_size);
total_scan -= batch_size;

@@ -308,12 +322,12 @@ unsigned long shrink_slab(struct shrink_control *shrink,
else
new_nr = atomic_long_read(&shrinker->nr_in_batch);

- trace_mm_shrink_slab_end(shrinker, shrink_ret, nr, new_nr);
+ trace_mm_shrink_slab_end(shrinker, freed, nr, new_nr);
}
up_read(&shrinker_rwsem);
out:
cond_resched();
- return ret;
+ return freed;
}

static inline int is_page_cache_freeable(struct page *page)
--
1.8.1.4