Re: [PATCH RFC 1/5] block: return ns precision from disk_start_io_acct

[Ming Lei <ming.lei@redhat.com>]

On Wed, Jul 08, 2020 at 09:58:15AM +0200, Guoqing Jiang wrote:
> Currently the duration accounting of bio based driver is converted from
> jiffies to ns, means it could be less accurate as request based driver.
> 
> So let disk_start_io_acct return from ns precision, instead of convert
> jiffies to ns in disk_end_io_acct.
> 
> Cc: Philipp Reisner <philipp.reisner@linbit.com>
> Cc: Lars Ellenberg <lars.ellenberg@linbit.com>
> Cc: drbd-dev@lists.linbit.com
> Signed-off-by: Guoqing Jiang <guoqing.jiang@cloud.ionos.com>
> ---
>  block/blk-core.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index d9d632639bd1..0e806a8c62fb 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -1466,6 +1466,7 @@ unsigned long disk_start_io_acct(struct gendisk *disk, unsigned int sectors,
>  	struct hd_struct *part = &disk->part0;
>  	const int sgrp = op_stat_group(op);
>  	unsigned long now = READ_ONCE(jiffies);
> +	unsigned long start_ns = ktime_get_ns();
>  
>  	part_stat_lock();
>  	update_io_ticks(part, now, false);
> @@ -1474,7 +1475,7 @@ unsigned long disk_start_io_acct(struct gendisk *disk, unsigned int sectors,
>  	part_stat_local_inc(part, in_flight[op_is_write(op)]);
>  	part_stat_unlock();
>  
> -	return now;
> +	return start_ns;
>  }
>  EXPORT_SYMBOL(disk_start_io_acct);
>  
> @@ -1484,11 +1485,11 @@ void disk_end_io_acct(struct gendisk *disk, unsigned int op,
>  	struct hd_struct *part = &disk->part0;
>  	const int sgrp = op_stat_group(op);
>  	unsigned long now = READ_ONCE(jiffies);
> -	unsigned long duration = now - start_time;
> +	unsigned long duration = ktime_get_ns() - start_time;
>  
>  	part_stat_lock();
>  	update_io_ticks(part, now, true);
> -	part_stat_add(part, nsecs[sgrp], jiffies_to_nsecs(duration));
> +	part_stat_add(part, nsecs[sgrp], duration);
>  	part_stat_local_dec(part, in_flight[op_is_write(op)]);
>  	part_stat_unlock();

Hi Guoqing,

Cost of ktime_get_ns() can be observed as not cheap in high IOPS device,
so not sure the conversion is good. Also could you share what benefit we can
get with this change?

Thanks,
Ming