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Subject: Re: [PATCH v2] block: fix trace completion for chained bio
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From:   Edward Hsieh <edwardh@synology.com>
To:     NeilBrown <neilb@suse.de>, axboe@kernel.dk, neilb@suse.com
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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