blk-cgroup: introduce a new struct blkg_rwstat_sample [Linux 5.3]

This Linux kernel change "blk-cgroup: introduce a new struct blkg_rwstat_sample" is included in the Linux 5.3 release. This change is authored by Christoph Hellwig <hch [at] lst.de> on Thu Jun 6 12:26:21 2019 +0200. The commit for this change in Linux stable tree is 7af6fd9 (patch).

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>

There are 71 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 block/bfq-cgroup.c         | 10 ++++------
 block/blk-cgroup.c         | 39 +++++++++++++++++++--------------------
 include/linux/blk-cgroup.h | 22 ++++++++++++----------
 3 files changed, 35 insertions(+), 36 deletions(-)

diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
index 66abc82..624374a9 100644
--- a/block/bfq-cgroup.c
+++ b/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)
diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index db039a8..664c098 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -544,7 +544,7 @@ u64 __blkg_prfill_u64(struct seq_file *sf, struct blkg_policy_data *pd, u64 v)
  * 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 @@ 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)
 {
-   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 @@ u64 blkg_prfill_rwstat(struct seq_file *sf, struct blkg_policy_data *pd,
 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 @@ u64 blkg_stat_recursive_sum(struct blkcg_gq *blkg,
  * @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 @@ u64 blkg_stat_recursive_sum(struct blkcg_gq *blkg,
  * 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);

diff --git a/include/linux/blk-cgroup.h b/include/linux/blk-cgroup.h
index 3ee8581..e4a8176 100644
--- a/include/linux/blk-cgroup.h
+++ b/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 @@ u64 blkg_prfill_rwstat(struct seq_file *sf, struct blkg_policy_data *pd,
 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];
 }

 /**

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