From: Edward Hsieh <edwardh@synology.com>
To: NeilBrown <neilb@suse.de>, axboe@kernel.dk, neilb@suse.com
Cc: linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
s3t@synology.com, bingjingc@synology.com, cccheng@synology.com
Subject: Re: [PATCH v2] block: fix trace completion for chained bio
Date: Tue, 25 May 2021 17:37:29 +0800 [thread overview]
Message-ID: <3600b6c0-f83d-f375-bebc-cd5ac811c3d5@synology.com> (raw)
In-Reply-To: <10f3e198-f34c-47e9-608a-e5f84e3379a1@synology.com>
On 5/10/2021 10:06 AM, Edward Hsieh wrote:
>
> On 4/23/2021 4:04 PM, Edward Hsieh wrote:
>> On 3/23/2021 5:22 AM, NeilBrown wrote:
>>> On Wed, Mar 03 2021, edwardh wrote:
>>>
>>>> From: Edward Hsieh <edwardh@synology.com>
>>>>
>>>> For chained bio, trace_block_bio_complete in bio_endio is currently
>>>> called
>>>> only by the parent bio once upon all chained bio completed.
>>>> However, the sector and size for the parent bio are modified in
>>>> bio_split.
>>>> Therefore, the size and sector of the complete events might not
>>>> match the
>>>> queue events in blktrace.
>>>>
>>>> The original fix of bio completion trace <fbbaf700e7b1> ("block: trace
>>>> completion of all bios.") wants multiple complete events to correspond
>>>> to one queue event but missed this.
>>>>
>>>> md/raid5 read with bio cross chunks can reproduce this issue.
>>>>
>>>> To fix, move trace completion into the loop for every chained bio to
>>>> call.
>>>
>>> Thanks. I think this is correct as far as tracing goes.
>>> However the code still looks a bit odd.
>>>
>>> The comment for the handling of bio_chain_endio suggests that the *only*
>>> purpose for that is to avoid deep recursion. That suggests it should be
>>> at the end of the function.
>>> As it is blk_throtl_bio_endio() and bio_unint() are only called on the
>>> last bio in a chain.
>>> That seems wrong.
>>>
>>> I'd be more comfortable if the patch moved the bio_chain_endio()
>>> handling to the end, after all of that.
>>> So the function would end.
>>>
>>> if (bio->bi_end_io == bio_chain_endio) {
>>> bio = __bio_chain_endio(bio);
>>> goto again;
>>> } else if (bio->bi_end_io)
>>> bio->bi_end_io(bio);
>>>
>>> Jens: can you see any reason why that functions must only be called on
>>> the last bio in the chain?
>>>
>>> Thanks,
>>> NeilBrown
>>>
>>
>> Hi Neil and Jens,
>>
>> From the commit message, bio_uninit is put here for bio allocated in
>> special ways (e.g., on stack), that will not be release by bio_free. For
>> chained bio, __bio_chain_endio invokes bio_put and release the
>> resources, so it seems that we don't need to call bio_uninit for chained
>> bio.
>>
>> The blk_throtl_bio_endio is used to update the latency for the throttle
>> group. I think the latency should only be updated after the whole bio is
>> finished?
>>
>> To make sense for the "tail call optimization" in the comment, I'll
>> suggest to wrap the whole statement with an else. What do you think?
>>
>> if (bio->bi_end_io == bio_chain_endio) {
>> bio = __bio_chain_endio(bio);
>> goto again;
>> } else {
>> blk_throtl_bio_endio(bio);
>> /* release cgroup info */
>> bio_uninit(bio);
>> if (bio->bi_end_io)
>> bio->bi_end_io(bio);
>> }
>>
>> Thanks,
>> Edward Hsieh
>
> Hi Neil and Jens,
>
> Any feedback on this one?
>
> Thank you,
> Edward Hsieh >
Hi Neil and Jens,
Any comments?
Thank you,
Edward Hsieh
prev parent reply other threads:[~2021-05-25 9:38 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-03 3:22 [PATCH v2] block: fix trace completion for chained bio edwardh
2021-03-16 10:30 ` Edward Hsieh
2021-03-22 21:22 ` NeilBrown
2021-04-23 8:04 ` Edward Hsieh
2021-05-10 2:06 ` Edward Hsieh
2021-05-25 9:37 ` Edward Hsieh [this message]
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