linux-kernel - Re: [PATCH -next v4 4/4] blk-throttle: fix io hung due to config updates

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Date:   Tue, 24 May 2022 11:59:36 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Yu Kuai <yukuai3@...wei.com>
Cc:     tj@...nel.org, axboe@...nel.dk, ming.lei@...hat.com,
        cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, yi.zhang@...wei.com
Subject: Re: [PATCH -next v4 4/4] blk-throttle: fix io hung due to config
 updates

On Mon, May 23, 2022 at 04:26:33PM +0800, Yu Kuai <yukuai3@...wei.com> wrote:
> Fix the problem by respecting the time that throttled bio aready waited.
> In order to do that, add new fields to record how many bytes/io already
> waited, and use it to calculate wait time for throttled bio under new
> configuration.

This new approach is correctly conserving the bandwidth upon changes.
(Looking and BPS paths.)

> 
> Some simple test:
> 1)
> cd /sys/fs/cgroup/blkio/
> echo $$ > cgroup.procs
> echo "8:0 2048" > blkio.throttle.write_bps_device
> {
>         sleep 3
>         echo "8:0 1024" > blkio.throttle.write_bps_device
> } &
> sleep 1
> dd if=/dev/zero of=/dev/sda bs=8k count=1 oflag=direct
> 
> 2)
> cd /sys/fs/cgroup/blkio/
> echo $$ > cgroup.procs
> echo "8:0 1024" > blkio.throttle.write_bps_device
> {
>         sleep 5
>         echo "8:0 2048" > blkio.throttle.write_bps_device
> } &
> sleep 1
> dd if=/dev/zero of=/dev/sda bs=8k count=1 oflag=direct
> 

It's interesting that you're getting these numbers (w/patch)

> test results: io finish time
> 	before this patch	with this patch
> 1)	10s			6s
> 2)	8s			6s

wait := (disp + bio - Δt*l_old) / l_new

1)
wait = (0k + 8k - 3s*2k/s) / 1k/s = 2s -> i.e. 5s absolute

2)
wait = (0k + 8k - 5s*1k/s) / 2k/s = 2.5s -> i.e. 6.5s absolute

Are you numbers noisy+rounded or do I still mis anything?

(Also isn't it worth having this more permanent in tools/testing/selftest?)

> +static void tg_update_skipped(struct throtl_grp *tg)
> +{
> +	if (tg->service_queue.nr_queued[READ])
> +		__tg_update_skipped(tg, READ);
> +	if (tg->service_queue.nr_queued[WRITE])
> +		__tg_update_skipped(tg, WRITE);

On one hand, the callers of tg_update_skipped() know whether R/W limit
is changed, so only the respective variant could be called.
On the other hand, this conditions look implied by tg->flags &
THROTL_TG_PENDING.
(Just noting, it's likely still not possibly to pass the skipped value
only via stack.)


> @@ -115,6 +115,10 @@ struct throtl_grp {
>  	uint64_t bytes_disp[2];
>  	/* Number of bio's dispatched in current slice */
>  	unsigned int io_disp[2];
> +	/* Number of bytes will be skipped in current slice */
> +	uint64_t bytes_skipped[2];
> +	/* Number of bio's will be skipped in current slice */
> +	unsigned int io_skipped[2];

Please add a comment these fields exists to facilitate config updates
(the bytes to be skipped is sort of obvious from the name :-).

Thanks,
Michal