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Message-ID: <BL0PR04MB65146F70831B61CB6B9AFE7CE7759@BL0PR04MB6514.namprd04.prod.outlook.com>
Date: Wed, 7 Apr 2021 10:27:57 +0000
From: Damien Le Moal <Damien.LeMoal@....com>
To: Changheun Lee <nanich.lee@...sung.com>,
"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
CC: Johannes Thumshirn <Johannes.Thumshirn@....com>,
"asml.silence@...il.com" <asml.silence@...il.com>,
"axboe@...nel.dk" <axboe@...nel.dk>,
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"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"ming.lei@...hat.com" <ming.lei@...hat.com>,
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Subject: Re: [RESEND,v5,1/2] bio: limit bio max size
On 2021/04/07 18:46, Changheun Lee wrote:
>> On Wed, Apr 07, 2021 at 03:55:07PM +0900, Changheun Lee wrote:
>>>> On Wed, Apr 07, 2021 at 02:06:33PM +0900, Changheun Lee wrote:
>>>>>> On Wed, Apr 07, 2021 at 09:16:12AM +0900, Changheun Lee wrote:
>>>>>>>> On Tue, Apr 06, 2021 at 10:31:28AM +0900, Changheun Lee wrote:
>>>>>>>>>> bio size can grow up to 4GB when muli-page bvec is enabled.
>>>>>>>>>> but sometimes it would lead to inefficient behaviors.
>>>>>>>>>> in case of large chunk direct I/O, - 32MB chunk read in user space -
>>>>>>>>>> all pages for 32MB would be merged to a bio structure if the pages
>>>>>>>>>> physical addresses are contiguous. it makes some delay to submit
>>>>>>>>>> until merge complete. bio max size should be limited to a proper size.
>>>>>>>>>>
>>>>>>>>>> When 32MB chunk read with direct I/O option is coming from userspace,
>>>>>>>>>> kernel behavior is below now in do_direct_IO() loop. it's timeline.
>>>>>>>>>>
>>>>>>>>>> | bio merge for 32MB. total 8,192 pages are merged.
>>>>>>>>>> | total elapsed time is over 2ms.
>>>>>>>>>> |------------------ ... ----------------------->|
>>>>>>>>>> | 8,192 pages merged a bio.
>>>>>>>>>> | at this time, first bio submit is done.
>>>>>>>>>> | 1 bio is split to 32 read request and issue.
>>>>>>>>>> |--------------->
>>>>>>>>>> |--------------->
>>>>>>>>>> |--------------->
>>>>>>>>>> ......
>>>>>>>>>> |--------------->
>>>>>>>>>> |--------------->|
>>>>>>>>>> total 19ms elapsed to complete 32MB read done from device. |
>>>>>>>>>>
>>>>>>>>>> If bio max size is limited with 1MB, behavior is changed below.
>>>>>>>>>>
>>>>>>>>>> | bio merge for 1MB. 256 pages are merged for each bio.
>>>>>>>>>> | total 32 bio will be made.
>>>>>>>>>> | total elapsed time is over 2ms. it's same.
>>>>>>>>>> | but, first bio submit timing is fast. about 100us.
>>>>>>>>>> |--->|--->|--->|---> ... -->|--->|--->|--->|--->|
>>>>>>>>>> | 256 pages merged a bio.
>>>>>>>>>> | at this time, first bio submit is done.
>>>>>>>>>> | and 1 read request is issued for 1 bio.
>>>>>>>>>> |--------------->
>>>>>>>>>> |--------------->
>>>>>>>>>> |--------------->
>>>>>>>>>> ......
>>>>>>>>>> |--------------->
>>>>>>>>>> |--------------->|
>>>>>>>>>> total 17ms elapsed to complete 32MB read done from device. |
>>>>>>>>>>
>>>>>>>>>> As a result, read request issue timing is faster if bio max size is limited.
>>>>>>>>>> Current kernel behavior with multipage bvec, super large bio can be created.
>>>>>>>>>> And it lead to delay first I/O request issue.
>>>>>>>>>>
>>>>>>>>>> Signed-off-by: Changheun Lee <nanich.lee@...sung.com>
>>>>>>>>>> ---
>>>>>>>>>> block/bio.c | 13 ++++++++++++-
>>>>>>>>>> include/linux/bio.h | 2 +-
>>>>>>>>>> include/linux/blkdev.h | 3 +++
>>>>>>>>>> 3 files changed, 16 insertions(+), 2 deletions(-)
>>>>>>>>>>
>>>>>>>>>> diff --git a/block/bio.c b/block/bio.c
>>>>>>>>>> index 1f2cc1fbe283..c528e1f944c7 100644
>>>>>>>>>> --- a/block/bio.c
>>>>>>>>>> +++ b/block/bio.c
>>>>>>>>>> @@ -287,6 +287,17 @@ void bio_init(struct bio *bio, struct bio_vec *table,
>>>>>>>>>> }
>>>>>>>>>> EXPORT_SYMBOL(bio_init);
>>>>>>>>>>
>>>>>>>>>> +unsigned int bio_max_size(struct bio *bio)
>>>>>>>>>> +{
>>>>>>>>>> + struct request_queue *q = bio->bi_disk->queue;
>>>>>>>>>> +
>>>>>>>>>> + if (blk_queue_limit_bio_size(q))
>>>>>>>>>> + return blk_queue_get_max_sectors(q, bio_op(bio))
>>>>>>>>>> + << SECTOR_SHIFT;
>>>>>>>>>> +
>>>>>>>>>> + return UINT_MAX;
>>>>>>>>>> +}
>>>>>>>>>> +
>>>>>>>>>> /**
>>>>>>>>>> * bio_reset - reinitialize a bio
>>>>>>>>>> * @bio: bio to reset
>>>>>>>>>> @@ -877,7 +888,7 @@ bool __bio_try_merge_page(struct bio *bio, struct page *page,
>>>>>>>>>> struct bio_vec *bv = &bio->bi_io_vec[bio->bi_vcnt - 1];
>>>>>>>>>>
>>>>>>>>>> if (page_is_mergeable(bv, page, len, off, same_page)) {
>>>>>>>>>> - if (bio->bi_iter.bi_size > UINT_MAX - len) {
>>>>>>>>>> + if (bio->bi_iter.bi_size > bio_max_size(bio) - len) {
>>>>>>>>>> *same_page = false;
>>>>>>>>>> return false;
>>>>>>>>>> }
>>>>>>>>>> diff --git a/include/linux/bio.h b/include/linux/bio.h
>>>>>>>>>> index 1edda614f7ce..13b6f6562a5b 100644
>>>>>>>>>> --- a/include/linux/bio.h
>>>>>>>>>> +++ b/include/linux/bio.h
>>>>>>>>>> @@ -113,7 +113,7 @@ static inline bool bio_full(struct bio *bio, unsigned len)
>>>>>>>>>> if (bio->bi_vcnt >= bio->bi_max_vecs)
>>>>>>>>>> return true;
>>>>>>>>>>
>>>>>>>>>> - if (bio->bi_iter.bi_size > UINT_MAX - len)
>>>>>>>>>> + if (bio->bi_iter.bi_size > bio_max_size(bio) - len)
>>>>>>>>>> return true;
>>>>>>>>>>
>>>>>>>>>> return false;
>>>>>>>>>> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
>>>>>>>>>> index f94ee3089e01..3aeab9e7e97b 100644
>>>>>>>>>> --- a/include/linux/blkdev.h
>>>>>>>>>> +++ b/include/linux/blkdev.h
>>>>>>>>>> @@ -621,6 +621,7 @@ struct request_queue {
>>>>>>>>>> #define QUEUE_FLAG_RQ_ALLOC_TIME 27 /* record rq->alloc_time_ns */
>>>>>>>>>> #define QUEUE_FLAG_HCTX_ACTIVE 28 /* at least one blk-mq hctx is active */
>>>>>>>>>> #define QUEUE_FLAG_NOWAIT 29 /* device supports NOWAIT */
>>>>>>>>>> +#define QUEUE_FLAG_LIMIT_BIO_SIZE 30 /* limit bio size */
>>>>>>>>>>
>>>>>>>>>> #define QUEUE_FLAG_MQ_DEFAULT ((1 << QUEUE_FLAG_IO_STAT) | \
>>>>>>>>>> (1 << QUEUE_FLAG_SAME_COMP) | \
>>>>>>>>>> @@ -667,6 +668,8 @@ bool blk_queue_flag_test_and_set(unsigned int flag, struct request_queue *q);
>>>>>>>>>> #define blk_queue_fua(q) test_bit(QUEUE_FLAG_FUA, &(q)->queue_flags)
>>>>>>>>>> #define blk_queue_registered(q) test_bit(QUEUE_FLAG_REGISTERED, &(q)->queue_flags)
>>>>>>>>>> #define blk_queue_nowait(q) test_bit(QUEUE_FLAG_NOWAIT, &(q)->queue_flags)
>>>>>>>>>> +#define blk_queue_limit_bio_size(q) \
>>>>>>>>>> + test_bit(QUEUE_FLAG_LIMIT_BIO_SIZE, &(q)->queue_flags)
>>>>>>>>>>
>>>>>>>>>> extern void blk_set_pm_only(struct request_queue *q);
>>>>>>>>>> extern void blk_clear_pm_only(struct request_queue *q);
>>>>>>>>>> --
>>>>>>>>>> 2.28.0
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> Please feedback to me if more modification is needed to apply. :)
>>>>>>>>
>>>>>>>> You are adding code that tests for a value to be set, yet you never set
>>>>>>>> it in this code so why is it needed at all?
>>>>>>>
>>>>>>> This patch is a solution for some inefficient case of multipage bvec like
>>>>>>> as current DIO scenario. So it's not set as a default.
>>>>>>> It will be set when bio size limitation is needed in runtime.
>>>>>>
>>>>>> Set where?
>>>>>
>>>>> In my environment, set it on init.rc file like as below.
>>>>> "echo 1 > /sys/block/sda/queue/limit_bio_size"
>>>>
>>>> I do not see any sysfs file in this patch, and why would you ever want
>>>> to be forced to manually do this? The hardware should know the limits
>>>> itself, and should automatically tune things like this, do not force a
>>>> user to do it as that's just not going to go well at all.
>>>
>>> Patch for sysfs is sent "[RESEND,v5,2/2] bio: add limit_bio_size sysfs".
>>> Actually I just suggested constant - 1MB - value to limit bio size at first.
>>> But I got a feedback that patch will be better if it's optional, and
>>> getting meaningful value from device queue on patchwork.
>>> There are some differences for each system environment I think.
>>>
>>> But there are inefficient logic obviously by applying of multipage bvec.
>>> So it will be shown in several system environment.
>>> Currently providing this patch as a option would be better to select
>>> according to each system environment, and policy I think.
>>>
>>> Please, revisit applying this patch.
>>>
>>>>
>>>> So if this patch series is forcing a new option to be configured by
>>>> sysfs only, that's not acceptable, sorry.
>>>
>>> If it is not acceptable ever with current, may I progress review again
>>> with default enabled?
>>
>> I am sorry, I can not parse this, can you rephrase this?
>>
>> thanks,
>>
>> greg k-h
>>
>
> I'll prepare new patch as you recommand. It will be added setting of
> limit_bio_size automatically when queue max sectors is determined.
Please do that in the driver for the HW that benefits from it. Do not do this
for all block devices.
>
>
> Thanks,
>
> Changheun Lee
>
--
Damien Le Moal
Western Digital Research
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