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

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Message-ID: <Zpf3ks2drDZ7ULTa@slm.duckdns.org>
Date: Wed, 17 Jul 2024 06:55:46 -1000
From: "tj@...nel.org" <tj@...nel.org>
To: Boy Wu (吳勃誼) <Boy.Wu@...iatek.com>
Cc: "boris@....io" <boris@....io>,
	"linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
	"linux-mediatek@...ts.infradead.org" <linux-mediatek@...ts.infradead.org>,
	"axboe@...nel.dk" <axboe@...nel.dk>,
	Iverlin Wang (王苳霖) <Iverlin.Wang@...iatek.com>,
	"josef@...icpanda.com" <josef@...icpanda.com>,
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
	"matthias.bgg@...il.com" <matthias.bgg@...il.com>,
	"angelogioacchino.delregno@...labora.com" <angelogioacchino.delregno@...labora.com>
Subject: Re: [PATCH v3] blk-cgroup: Replace u64 sync with spinlock for iostat
 update

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",