linux-kernel - Re: [PATCH 4/4] block: account io: convert part->in

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Message-ID: <20150716144043.GU15934@mtj.duckdns.org>
Date:	Thu, 16 Jul 2015 10:40:43 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Ming Lei <tom.leiming@...il.com>
Cc:	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
	Mike Snitzer <snitzer@...hat.com>
Subject: Re: [PATCH 4/4] block: account io: convert part->in_fligh[] into
 percpu variable

Hello,

On Thu, Jul 16, 2015 at 11:16:47AM +0800, Ming Lei wrote:
...
> --- a/drivers/md/dm.c
> +++ b/drivers/md/dm.c
> @@ -651,9 +651,9 @@ static void start_io_acct(struct dm_io *io)
>  
>  	cpu = part_stat_lock();
>  	part_round_stats(cpu, &dm_disk(md)->part0);
> +	part_stat_set(cpu, &dm_disk(md)->part0, in_flight[rw],
> +			atomic_inc_return(&md->pending[rw]));
>  	part_stat_unlock();
> -	atomic_set(&dm_disk(md)->part0.in_flight[rw],
> -		atomic_inc_return(&md->pending[rw]));

Why is this correct?  Isn't the code trying to transfer its stat to
block stat verbatim?  Why does part_stat_set() have @cpu param at all?
Shouldn't it clear the whole thing and set one of the cpu counters to
the target value?

> @@ -679,7 +679,9 @@ static void end_io_acct(struct dm_io *io)
>  	 * a flush.
>  	 */
>  	pending = atomic_dec_return(&md->pending[rw]);
> -	atomic_set(&dm_disk(md)->part0.in_flight[rw], pending);
> +	cpu = part_stat_lock();
> +	part_stat_set(cpu, &dm_disk(md)->part0, in_flight[rw], pending);
> +	part_stat_unlock();

Ditto.

Thanks.

-- 
tejun
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