libceph: a per-osdc crush scratch buffer

With the addition of erasure coding support in the future, scratch
variable-length array in crush_do_rule_ary() is going to grow to at
least 200 bytes on average, on top of another 128 bytes consumed by
rawosd/osd arrays in the call chain.  Replace it with a buffer inside
struct osdmap and a mutex.  This shouldn't result in any contention,
because all osd requests were already serialized by request_mutex at
that point; the only unlocked caller was ceph_ioctl_get_dataloc().

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>

include/linux/ceph/osdmap.h

@@ -84,6 +84,9 @@ struct ceph_osdmap {
 	/* the CRUSH map specifies the mapping of placement groups to
 	 * the list of osds that store+replicate them. */
 	struct crush_map *crush;
+
+	struct mutex crush_scratch_mutex;
+	int crush_scratch_ary[CEPH_PG_MAX_SIZE * 3];
 };
 
 static inline void ceph_oid_set_name(struct ceph_object_id *oid,

net/ceph/osdmap.c

@@ -698,7 +698,9 @@ struct ceph_osdmap *osdmap_decode(void **p, void *end)
 	map = kzalloc(sizeof(*map), GFP_NOFS);
 	if (map == NULL)
 		return ERR_PTR(-ENOMEM);
+
 	map->pg_temp = RB_ROOT;
+	mutex_init(&map->crush_scratch_mutex);
 
 	ceph_decode_16_safe(p, end, version, bad);
 	if (version > 6) {
@@ -1142,14 +1144,20 @@ int ceph_oloc_oid_to_pg(struct ceph_osdmap *osdmap,
 }
 EXPORT_SYMBOL(ceph_oloc_oid_to_pg);
 
-static int crush_do_rule_ary(const struct crush_map *map, int ruleno, int x,
-			     int *result, int result_max,
-			     const __u32 *weight, int weight_max)
+static int do_crush(struct ceph_osdmap *map, int ruleno, int x,
+		    int *result, int result_max,
+		    const __u32 *weight, int weight_max)
 {
-	int scratch[result_max * 3];
+	int r;
 
-	return crush_do_rule(map, ruleno, x, result, result_max,
-			     weight, weight_max, scratch);
+	BUG_ON(result_max > CEPH_PG_MAX_SIZE);
+
+	mutex_lock(&map->crush_scratch_mutex);
+	r = crush_do_rule(map->crush, ruleno, x, result, result_max,
+			  weight, weight_max, map->crush_scratch_ary);
+	mutex_unlock(&map->crush_scratch_mutex);
+
+	return r;
 }
 
 /*
@@ -1205,9 +1213,8 @@ static int *calc_pg_raw(struct ceph_osdmap *osdmap, struct ceph_pg pgid,
 				      pool->pgp_num_mask) +
 			(unsigned)pgid.pool;
 	}
-	r = crush_do_rule_ary(osdmap->crush, ruleno, pps,
-			      osds, min_t(int, pool->size, *num),
-			      osdmap->osd_weight, osdmap->max_osd);
+	r = do_crush(osdmap, ruleno, pps, osds, min_t(int, pool->size, *num),
+		     osdmap->osd_weight, osdmap->max_osd);
 	if (r < 0) {
 		pr_err("error %d from crush rule: pool %lld ruleset %d type %d"
 		       " size %d\n", r, pgid.pool, pool->crush_ruleset,