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

[Waiman Long <longman@xxxxxxxxxx>]

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