blk-cgroup: introduce a new struct blkg_rwstat_sample

When sampling the blkcg counts we don't need atomics or per-cpu
variables.  Introduce a new structure just containing plain u64
counters.
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bfq-cgroup.c

@@ -935,7 +935,7 @@ static u64 bfqg_prfill_stat_recursive(struct seq_file *sf,
 static u64 bfqg_prfill_rwstat_recursive(struct seq_file *sf,
 					struct blkg_policy_data *pd, int off)
 {
-	struct blkg_rwstat sum;
+	struct blkg_rwstat_sample sum;
 
 	blkg_rwstat_recursive_sum(pd_to_blkg(pd), &blkcg_policy_bfq, off, &sum);
 	return __blkg_prfill_rwstat(sf, pd, &sum);
@@ -975,15 +975,13 @@ static int bfqg_print_stat_sectors(struct seq_file *sf, void *v)
 static u64 bfqg_prfill_sectors_recursive(struct seq_file *sf,
 					 struct blkg_policy_data *pd, int off)
 {
-	struct blkg_rwstat tmp;
-	u64 sum;
+	struct blkg_rwstat_sample tmp;
 
 	blkg_rwstat_recursive_sum(pd->blkg, NULL,
 			offsetof(struct blkcg_gq, stat_bytes), &tmp);
-	sum = atomic64_read(&tmp.aux_cnt[BLKG_RWSTAT_READ]) +
-		atomic64_read(&tmp.aux_cnt[BLKG_RWSTAT_WRITE]);
 
-	return __blkg_prfill_u64(sf, pd, sum >> 9);
+	return __blkg_prfill_u64(sf, pd,
+		(tmp.cnt[BLKG_RWSTAT_READ] + tmp.cnt[BLKG_RWSTAT_WRITE]) >> 9);
 }
 
 static int bfqg_print_stat_sectors_recursive(struct seq_file *sf, void *v)

block/blk-cgroup.c

@@ -544,7 +544,7 @@ EXPORT_SYMBOL_GPL(__blkg_prfill_u64);
  * Print @rwstat to @sf for the device assocaited with @pd.
  */
 u64 __blkg_prfill_rwstat(struct seq_file *sf, struct blkg_policy_data *pd,
-			 const struct blkg_rwstat *rwstat)
+			 const struct blkg_rwstat_sample *rwstat)
 {
 	static const char *rwstr[] = {
 		[BLKG_RWSTAT_READ]	= "Read",
@@ -562,12 +562,12 @@ u64 __blkg_prfill_rwstat(struct seq_file *sf, struct blkg_policy_data *pd,
 
 	for (i = 0; i < BLKG_RWSTAT_NR; i++)
 		seq_printf(sf, "%s %s %llu\n", dname, rwstr[i],
-			   (unsigned long long)atomic64_read(&rwstat->aux_cnt[i]));
+			   rwstat->cnt[i]);
 
-	v = atomic64_read(&rwstat->aux_cnt[BLKG_RWSTAT_READ]) +
-		atomic64_read(&rwstat->aux_cnt[BLKG_RWSTAT_WRITE]) +
-		atomic64_read(&rwstat->aux_cnt[BLKG_RWSTAT_DISCARD]);
-	seq_printf(sf, "%s Total %llu\n", dname, (unsigned long long)v);
+	v = rwstat->cnt[BLKG_RWSTAT_READ] +
+		rwstat->cnt[BLKG_RWSTAT_WRITE] +
+		rwstat->cnt[BLKG_RWSTAT_DISCARD];
+	seq_printf(sf, "%s Total %llu\n", dname, v);
 	return v;
 }
 EXPORT_SYMBOL_GPL(__blkg_prfill_rwstat);
@@ -597,7 +597,7 @@ EXPORT_SYMBOL_GPL(blkg_prfill_stat);
 u64 blkg_prfill_rwstat(struct seq_file *sf, struct blkg_policy_data *pd,
 		       int off)
 {
-	struct blkg_rwstat rwstat = { };
+	struct blkg_rwstat_sample rwstat = { };
 
 	blkg_rwstat_read((void *)pd + off, &rwstat);
 	return __blkg_prfill_rwstat(sf, pd, &rwstat);
@@ -607,7 +607,7 @@ EXPORT_SYMBOL_GPL(blkg_prfill_rwstat);
 static u64 blkg_prfill_rwstat_field(struct seq_file *sf,
 				    struct blkg_policy_data *pd, int off)
 {
-	struct blkg_rwstat rwstat = { };
+	struct blkg_rwstat_sample rwstat = { };
 
 	blkg_rwstat_read((void *)pd->blkg + off, &rwstat);
 	return __blkg_prfill_rwstat(sf, pd, &rwstat);
@@ -651,7 +651,7 @@ static u64 blkg_prfill_rwstat_field_recursive(struct seq_file *sf,
 					      struct blkg_policy_data *pd,
 					      int off)
 {
-	struct blkg_rwstat rwstat;
+	struct blkg_rwstat_sample rwstat;
 
 	blkg_rwstat_recursive_sum(pd->blkg, NULL, off, &rwstat);
 	return __blkg_prfill_rwstat(sf, pd, &rwstat);
@@ -734,7 +734,7 @@ EXPORT_SYMBOL_GPL(blkg_stat_recursive_sum);
  * @blkg: blkg of interest
  * @pol: blkcg_policy which contains the blkg_rwstat
  * @off: offset to the blkg_rwstat in blkg_policy_data or @blkg
- * @sum: blkg_rwstat structure containing the results
+ * @sum: blkg_rwstat_sample structure containing the results
  *
  * Collect the blkg_rwstat specified by @blkg, @pol and @off and all its
  * online descendants and their aux counts.  The caller must be holding the
@@ -744,7 +744,7 @@ EXPORT_SYMBOL_GPL(blkg_stat_recursive_sum);
  * is at @off bytes into @blkg's blkg_policy_data of the policy.
  */
 void blkg_rwstat_recursive_sum(struct blkcg_gq *blkg, struct blkcg_policy *pol,
-		int off, struct blkg_rwstat *sum)
+		int off, struct blkg_rwstat_sample *sum)
 {
 	struct blkcg_gq *pos_blkg;
 	struct cgroup_subsys_state *pos_css;
@@ -765,8 +765,7 @@ void blkg_rwstat_recursive_sum(struct blkcg_gq *blkg, struct blkcg_policy *pol,
 			rwstat = (void *)pos_blkg + off;
 
 		for (i = 0; i < BLKG_RWSTAT_NR; i++)
-			atomic64_set(&sum->aux_cnt[i],
-				blkg_rwstat_read_counter(rwstat, i));
+			sum->cnt[i] = blkg_rwstat_read_counter(rwstat, i);
 	}
 	rcu_read_unlock();
 }
@@ -934,7 +933,7 @@ static int blkcg_print_stat(struct seq_file *sf, void *v)
 	hlist_for_each_entry_rcu(blkg, &blkcg->blkg_list, blkcg_node) {
 		const char *dname;
 		char *buf;
-		struct blkg_rwstat rwstat;
+		struct blkg_rwstat_sample rwstat;
 		u64 rbytes, wbytes, rios, wios, dbytes, dios;
 		size_t size = seq_get_buf(sf, &buf), off = 0;
 		int i;
@@ -956,15 +955,15 @@ static int blkcg_print_stat(struct seq_file *sf, void *v)
 
 		blkg_rwstat_recursive_sum(blkg, NULL,
 				offsetof(struct blkcg_gq, stat_bytes), &rwstat);
-		rbytes = atomic64_read(&rwstat.aux_cnt[BLKG_RWSTAT_READ]);
-		wbytes = atomic64_read(&rwstat.aux_cnt[BLKG_RWSTAT_WRITE]);
-		dbytes = atomic64_read(&rwstat.aux_cnt[BLKG_RWSTAT_DISCARD]);
+		rbytes = rwstat.cnt[BLKG_RWSTAT_READ];
+		wbytes = rwstat.cnt[BLKG_RWSTAT_WRITE];
+		dbytes = rwstat.cnt[BLKG_RWSTAT_DISCARD];
 
 		blkg_rwstat_recursive_sum(blkg, NULL,
 					offsetof(struct blkcg_gq, stat_ios), &rwstat);
-		rios = atomic64_read(&rwstat.aux_cnt[BLKG_RWSTAT_READ]);
-		wios = atomic64_read(&rwstat.aux_cnt[BLKG_RWSTAT_WRITE]);
-		dios = atomic64_read(&rwstat.aux_cnt[BLKG_RWSTAT_DISCARD]);
+		rios = rwstat.cnt[BLKG_RWSTAT_READ];
+		wios = rwstat.cnt[BLKG_RWSTAT_WRITE];
+		dios = rwstat.cnt[BLKG_RWSTAT_DISCARD];
 
 		spin_unlock_irq(&blkg->q->queue_lock);
 

include/linux/blk-cgroup.h

@@ -63,8 +63,7 @@ struct blkcg {
 
 /*
  * blkg_[rw]stat->aux_cnt is excluded for local stats but included for
- * recursive.  Used to carry stats of dead children, and, for blkg_rwstat,
- * to carry result values from read and sum operations.
+ * recursive.  Used to carry stats of dead children.
  */
 struct blkg_stat {
 	struct percpu_counter		cpu_cnt;
@@ -76,6 +75,10 @@ struct blkg_rwstat {
 	atomic64_t			aux_cnt[BLKG_RWSTAT_NR];
 };
 
+struct blkg_rwstat_sample {
+	u64				cnt[BLKG_RWSTAT_NR];
+};
+
 /*
  * A blkcg_gq (blkg) is association between a block cgroup (blkcg) and a
  * request_queue (q).  This is used by blkcg policies which need to track
@@ -213,7 +216,7 @@ void blkcg_print_blkgs(struct seq_file *sf, struct blkcg *blkcg,
 		       bool show_total);
 u64 __blkg_prfill_u64(struct seq_file *sf, struct blkg_policy_data *pd, u64 v);
 u64 __blkg_prfill_rwstat(struct seq_file *sf, struct blkg_policy_data *pd,
-			 const struct blkg_rwstat *rwstat);
+			 const struct blkg_rwstat_sample *rwstat);
 u64 blkg_prfill_stat(struct seq_file *sf, struct blkg_policy_data *pd, int off);
 u64 blkg_prfill_rwstat(struct seq_file *sf, struct blkg_policy_data *pd,
 		       int off);
@@ -225,7 +228,7 @@ int blkg_print_stat_ios_recursive(struct seq_file *sf, void *v);
 u64 blkg_stat_recursive_sum(struct blkcg_gq *blkg,
 			    struct blkcg_policy *pol, int off);
 void blkg_rwstat_recursive_sum(struct blkcg_gq *blkg, struct blkcg_policy *pol,
-		int off, struct blkg_rwstat *sum);
+		int off, struct blkg_rwstat_sample *sum);
 
 struct blkg_conf_ctx {
 	struct gendisk			*disk;
@@ -701,13 +704,13 @@ static inline void blkg_rwstat_add(struct blkg_rwstat *rwstat,
  * Read the current snapshot of @rwstat and return it in the aux counts.
  */
 static inline void blkg_rwstat_read(struct blkg_rwstat *rwstat,
-		struct blkg_rwstat *result)
+		struct blkg_rwstat_sample *result)
 {
 	int i;
 
 	for (i = 0; i < BLKG_RWSTAT_NR; i++)
-		atomic64_set(&result->aux_cnt[i],
-			     percpu_counter_sum_positive(&rwstat->cpu_cnt[i]));
+		result->cnt[i] =
+			percpu_counter_sum_positive(&rwstat->cpu_cnt[i]);
 }
 
 /**
@@ -720,11 +723,10 @@ static inline void blkg_rwstat_read(struct blkg_rwstat *rwstat,
  */
 static inline uint64_t blkg_rwstat_total(struct blkg_rwstat *rwstat)
 {
-	struct blkg_rwstat tmp = { };
+	struct blkg_rwstat_sample tmp = { };
 
 	blkg_rwstat_read(rwstat, &tmp);
-	return atomic64_read(&tmp.aux_cnt[BLKG_RWSTAT_READ]) +
-		atomic64_read(&tmp.aux_cnt[BLKG_RWSTAT_WRITE]);
+	return tmp.cnt[BLKG_RWSTAT_READ] + tmp.cnt[BLKG_RWSTAT_WRITE];
 }
 
 /**