Re: [PATCH v3] blk-cgroup: Replace u64 sync with spinlock for iostat update

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On 7/17/24 12:55, tj@xxxxxxxxxx wrote:
Hello,

Does something like the following work for you?

Thanks.

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 37e6cc91d576..ec1d191f5c83 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -329,7 +329,6 @@ static struct blkcg_gq *blkg_alloc(struct blkcg *blkcg, struct gendisk *disk,
  	INIT_WORK(&blkg->async_bio_work, blkg_async_bio_workfn);
  #endif
- u64_stats_init(&blkg->iostat.sync);
  	for_each_possible_cpu(cpu) {
  		u64_stats_init(&per_cpu_ptr(blkg->iostat_cpu, cpu)->sync);
  		per_cpu_ptr(blkg->iostat_cpu, cpu)->blkg = blkg;
@@ -632,24 +631,26 @@ static void blkg_iostat_set(struct blkg_iostat *dst, struct blkg_iostat *src)
  static void __blkg_clear_stat(struct blkg_iostat_set *bis)
  {
  	struct blkg_iostat cur = {0};
-	unsigned long flags;
- flags = u64_stats_update_begin_irqsave(&bis->sync);
  	blkg_iostat_set(&bis->cur, &cur);
  	blkg_iostat_set(&bis->last, &cur);
-	u64_stats_update_end_irqrestore(&bis->sync, flags);
  }
static void blkg_clear_stat(struct blkcg_gq *blkg)
  {
+	unsigned long flags;
  	int cpu;
+ raw_spin_lock_irqsave(&blkg_stat_lock, flags);
+
  	for_each_possible_cpu(cpu) {
  		struct blkg_iostat_set *s = per_cpu_ptr(blkg->iostat_cpu, cpu);
__blkg_clear_stat(s);
  	}
  	__blkg_clear_stat(&blkg->iostat);
+
+	raw_spin_unlock_irqrestore(&blkg_stat_lock, flags);
  }
static int blkcg_reset_stats(struct cgroup_subsys_state *css,
@@ -998,12 +999,10 @@ static void blkcg_iostat_update(struct blkcg_gq *blkg, struct blkg_iostat *cur,
  	unsigned long flags;
/* propagate percpu delta to global */
-	flags = u64_stats_update_begin_irqsave(&blkg->iostat.sync);
  	blkg_iostat_set(&delta, cur);
  	blkg_iostat_sub(&delta, last);
  	blkg_iostat_add(&blkg->iostat.cur, &delta);
  	blkg_iostat_add(last, &delta);
-	u64_stats_update_end_irqrestore(&blkg->iostat.sync, flags);
  }
static void __blkcg_rstat_flush(struct blkcg *blkcg, int cpu)
@@ -1134,9 +1133,9 @@ static void blkcg_fill_root_iostats(void)
  				cpu_dkstats->sectors[STAT_DISCARD] << 9;
  		}
- flags = u64_stats_update_begin_irqsave(&blkg->iostat.sync);
+		raw_spin_lock_irqsave(&blkg_stat_lock, flags);
  		blkg_iostat_set(&blkg->iostat.cur, &tmp);
-		u64_stats_update_end_irqrestore(&blkg->iostat.sync, flags);
+		raw_spin_unlock_irqrestore(&blkg_stat_lock, flags);
  	}
  }
@@ -1145,7 +1144,6 @@ static void blkcg_print_one_stat(struct blkcg_gq *blkg, struct seq_file *s)
  	struct blkg_iostat_set *bis = &blkg->iostat;
  	u64 rbytes, wbytes, rios, wios, dbytes, dios;
  	const char *dname;
-	unsigned seq;
  	int i;
if (!blkg->online)
@@ -1157,16 +1155,14 @@ static void blkcg_print_one_stat(struct blkcg_gq *blkg, struct seq_file *s)
seq_printf(s, "%s ", dname); - do {
-		seq = u64_stats_fetch_begin(&bis->sync);
-
-		rbytes = bis->cur.bytes[BLKG_IOSTAT_READ];
-		wbytes = bis->cur.bytes[BLKG_IOSTAT_WRITE];
-		dbytes = bis->cur.bytes[BLKG_IOSTAT_DISCARD];
-		rios = bis->cur.ios[BLKG_IOSTAT_READ];
-		wios = bis->cur.ios[BLKG_IOSTAT_WRITE];
-		dios = bis->cur.ios[BLKG_IOSTAT_DISCARD];
-	} while (u64_stats_fetch_retry(&bis->sync, seq));
+	raw_spin_lock_irq(&blkg_stat_lock);
+	rbytes = bis->cur.bytes[BLKG_IOSTAT_READ];
+	wbytes = bis->cur.bytes[BLKG_IOSTAT_WRITE];
+	dbytes = bis->cur.bytes[BLKG_IOSTAT_DISCARD];
+	rios = bis->cur.ios[BLKG_IOSTAT_READ];
+	wios = bis->cur.ios[BLKG_IOSTAT_WRITE];
+	dios = bis->cur.ios[BLKG_IOSTAT_DISCARD];
+	raw_spin_unlock_irq(&blkg_stat_lock, flags);
if (rbytes || wbytes || rios || wios) {
  		seq_printf(s, "rbytes=%llu wbytes=%llu rios=%llu wios=%llu dbytes=%llu dios=%llu",

bis->sync is still being used in blk_cgroup_bio_start(). Replacing it with a global lock may kill performance. We may have to use a per-cpu lock if we want to go this route of eliminating bis->sync.

Cheers,
Longman





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