linux-kernel - Re: [PATCH] block: per-cpu counters for in-flight IO accounting

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Date:	Wed, 04 Jun 2014 20:16:32 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Shaohua Li <shli@...nel.org>
CC:	Matias Bjørling <m@...rling.me>,
	"Sam Bradshaw (sbradshaw)" <sbradshaw@...ron.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] block: per-cpu counters for in-flight IO accounting

On 2014-06-04 20:09, Shaohua Li wrote:
> On Wed, Jun 04, 2014 at 02:08:46PM -0600, Jens Axboe wrote:
>> On 06/04/2014 05:29 AM, Matias Bjørling wrote:
>>> It's in
>>>
>>> blk_io_account_start
>>>    part_round_stats
>>>      part_round_state_single
>>>        part_in_flight
>>>
>>> I like the granularity idea.
>>
>> And similarly from blk_io_account_done() - which makes it even worse,
>> since it at both ends of the IO chain.
>
> But part_round_state_single is supposed to only call part_in_flight every
> jiffery. Maybe we need something below:
> 1. set part->stamp immediately
> 2. fixed granularity
> Untested though.
>
>
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 40d6548..5f0acaa 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -1270,17 +1270,19 @@ static void part_round_stats_single(int cpu, struct hd_struct *part,
>   				    unsigned long now)
>   {
>   	int inflight;
> +	unsigned long old_stamp;
>
> -	if (now == part->stamp)
> +	if (time_before(now, part->stamp + msecs_to_jiffies(10)))
>   		return;
> +	old_stamp = part->stamp;
> +	part->stamp = now;
>
>   	inflight = part_in_flight(part);
>   	if (inflight) {
>   		__part_stat_add(cpu, part, time_in_queue,
> -				inflight * (now - part->stamp));
> -		__part_stat_add(cpu, part, io_ticks, (now - part->stamp));
> +				inflight * (now - old_stamp));
> +		__part_stat_add(cpu, part, io_ticks, (now - old_stamp));
>   	}
> -	part->stamp = now;
>   }
>
>   /**

It'd be a good improvement, and one we should be able to do without 
screwing anything up. It'd be identical to anyone running at HZ==100 
right now.

So the above we can easily do, and arguably should just do. We wont see 
real scaling in the IO stats path before we fixup the hd_struct 
referencing as well, however.

-- 
Jens Axboe

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