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Message-ID: <73e8bdef-0f80-d255-e4f2-170813a12f5f@huawei.com>
Date: Wed, 18 May 2022 14:30:07 +0800
From: "yukuai (C)" <yukuai3@...wei.com>
To: Ming Lei <ming.lei@...hat.com>
CC: <tj@...nel.org>, <axboe@...nel.dk>, <cgroups@...r.kernel.org>,
<linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
<yi.zhang@...wei.com>
Subject: Re: [PATCH -next] blk-throttle: delay the setting of 'BIO_THROTTLED'
to when throttle is done
在 2022/05/18 12:07, Ming Lei 写道:
> On Tue, May 17, 2022 at 09:49:09PM +0800, Yu Kuai wrote:
>> commit 9f5ede3c01f9 ("block: throttle split bio in case of iops limit")
>> introduce a new problem, for example:
>>
>> [root@...alhost ~]# echo "8:0 1024" > /sys/fs/cgroup/blkio/blkio.throttle.write_bps_device
>> [root@...alhost ~]# echo $$ > /sys/fs/cgroup/blkio/cgroup.procs
>> [root@...alhost ~]# dd if=/dev/zero of=/dev/sda bs=10k count=1 oflag=direct &
>> [1] 620
>> [root@...alhost ~]# dd if=/dev/zero of=/dev/sda bs=10k count=1 oflag=direct &
>> [2] 626
>> [root@...alhost ~]# 1+0 records in
>> 1+0 records out
>> 10240 bytes (10 kB, 10 KiB) copied, 10.0038 s, 1.0 kB/s1+0 records in
>> 1+0 records out
>>
>> 10240 bytes (10 kB, 10 KiB) copied, 9.23076 s, 1.1 kB/s
>> -> the second bio is issued after 10s instead of 20s.
>>
>> This is because if some bios are already queued, current bio is queued
>> directly and the flag 'BIO_THROTTLED' is set. And later, when former
>> bios are dispatched, this bio will be dispatched without waiting at all,
>> this is due to tg_with_in_bps_limit() will return 0 if the flag is set.
>>
>> Instead of setting the flag when bio starts throttle, delay to when
>> throttle is done to fix the problem.
>>
>> Fixes: 9f5ede3c01f9 ("block: throttle split bio in case of iops limit")
>> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
>> ---
>> block/blk-throttle.c | 10 ++++++----
>> 1 file changed, 6 insertions(+), 4 deletions(-)
>>
>> diff --git a/block/blk-throttle.c b/block/blk-throttle.c
>> index 447e1b8722f7..f952f2d942ff 100644
>> --- a/block/blk-throttle.c
>> +++ b/block/blk-throttle.c
>> @@ -811,7 +811,7 @@ static bool tg_with_in_bps_limit(struct throtl_grp *tg, struct bio *bio,
>> unsigned int bio_size = throtl_bio_data_size(bio);
>>
>> /* no need to throttle if this bio's bytes have been accounted */
>> - if (bps_limit == U64_MAX || bio_flagged(bio, BIO_THROTTLED)) {
>> + if (bps_limit == U64_MAX) {
>
> This way may double account bio size for re-entered split bio.
Hi, Ming
Yes, you are right, I forgot that...
>
>
>> if (wait)
>> *wait = 0;
>> return true;
>> @@ -1226,8 +1226,10 @@ static void blk_throtl_dispatch_work_fn(struct work_struct *work)
>>
>> spin_lock_irq(&q->queue_lock);
>> for (rw = READ; rw <= WRITE; rw++)
>> - while ((bio = throtl_pop_queued(&td_sq->queued[rw], NULL)))
>> + while ((bio = throtl_pop_queued(&td_sq->queued[rw], NULL))) {
>> + bio_set_flag(bio, BIO_THROTTLED);
>> bio_list_add(&bio_list_on_stack, bio);
>> + }
>> spin_unlock_irq(&q->queue_lock);
>>
>> if (!bio_list_empty(&bio_list_on_stack)) {
>> @@ -2134,7 +2136,8 @@ bool __blk_throtl_bio(struct bio *bio)
>> }
>> break;
>> }
>> -
>> + /* this bio will be issued directly */
>> + bio_set_flag(bio, BIO_THROTTLED);
>> /* within limits, let's charge and dispatch directly */
>> throtl_charge_bio(tg, bio);
>
> Marking BIO_THROTTLED before throtle_charge_bio() causes the bio
> bytes not be charged.
Yes, thanks for spotting this.
>
> Another simple way is to compensate for previous extra bytes accounting,
> something like the following patch:
>
>
> diff --git a/block/blk-throttle.c b/block/blk-throttle.c
> index 139b2d7a99e2..44773d2ba257 100644
> --- a/block/blk-throttle.c
> +++ b/block/blk-throttle.c
> @@ -810,8 +810,7 @@ static bool tg_with_in_bps_limit(struct throtl_grp *tg, struct bio *bio,
> unsigned long jiffy_elapsed, jiffy_wait, jiffy_elapsed_rnd;
> unsigned int bio_size = throtl_bio_data_size(bio);
>
> - /* no need to throttle if this bio's bytes have been accounted */
> - if (bps_limit == U64_MAX || bio_flagged(bio, BIO_THROTTLED)) {
> + if (bps_limit == U64_MAX) {
> if (wait)
> *wait = 0;
> return true;
> @@ -921,10 +920,8 @@ static void throtl_charge_bio(struct throtl_grp *tg, struct bio *bio)
> unsigned int bio_size = throtl_bio_data_size(bio);
>
> /* Charge the bio to the group */
> - if (!bio_flagged(bio, BIO_THROTTLED)) {
> - tg->bytes_disp[rw] += bio_size;
> - tg->last_bytes_disp[rw] += bio_size;
> - }
> + tg->bytes_disp[rw] += bio_size;
> + tg->last_bytes_disp[rw] += bio_size;
>
> tg->io_disp[rw]++;
> tg->last_io_disp[rw]++;
> @@ -2125,6 +2122,20 @@ bool __blk_throtl_bio(struct bio *bio)
> if (sq->nr_queued[rw])
> break;
>
> + /*
> + * re-entered bio has accounted bytes already, so try to
> + * compensate previous over-accounting. However, if new
> + * slice is started, just forget it
> + */
> + if (bio_flagged(bio, BIO_THROTTLED)) {
> + unsigned int bio_size = throtl_bio_data_size(bio);
> +
> + if (tg->bytes_disp[rw] >= bio_size)
> + tg->bytes_disp[rw] -= bio_size;
> + if (tg->last_bytes_disp[rw] - bio_size)
> + tg->last_bytes_disp[rw] -= bio_size;
> + }
> +
If new slice is not started, then this should fix the problem.
From what I can see, other than tg_conf_updated, new silce can start if
there are still bio throttled:
tg_may_dispatch
if (!(tg->service_queue.nr_queued[rw]))
throtl_start_new_slice
Thus I think the change is ok. For the case in tg_conf_updated, I'll
remove the throtl_start_new_slice() to fix a hung problem. I'll add this
patch with this one in next version.
Thanks,
Kuai
> /* if above limits, break to queue */
> if (!tg_may_dispatch(tg, bio, NULL)) {
> tg->last_low_overflow_time[rw] = jiffies;
>
> Thanks,
> Ming
>
> .
>
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