linux-kernel - Re: [PATCH] block: add missing block_bio

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Date:	Mon, 30 Jan 2012 10:44:19 +0900
From:	Namhyung Kim <namhyung@...il.com>
To:	Tejun Heo <tj@...nel.org>
CC:	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
	Steven Rostedt <rostedt@...dmis.org>, dm-devel@...hat.com
Subject: Re: [PATCH] block: add missing block_bio_complete() tracepoint

Hello,

2012-01-30 4:24 AM, Tejun Heo wrote:
> On Sun, Jan 29, 2012 at 06:41:33 PM +0900, Namhyung Kim wrote:
>> The block_bio_complete() TP has been missed so long, so that bio-based
>> drivers haven't been able to trace its IO behavior. Add it.
>>
>> In some rare cases, such as loop_switch, @bio->bi_bdev can be NULL.
>> Thus convert it to TRACE_EVENT_CONDITION() as Steven suggested.
>>
>>  From now on, request-based drivers will also get BLK_TA_COMPLETEs for
>> all bio's in requests. This needs to be handled in userland properly.
>>
>> Also remove external use of the TP in DM and unexport it.
>>
>> Signed-off-by: Namhyung Kim <namhyung@...il.com>
>> Cc: Tejun Heo <tj@...nel.org>
>> Cc: Steven Rostedt <rostedt@...dmis.org>
>> Cc: dm-devel@...hat.com
>
> I like the smiplicity change but do we know how we can filter this out
> from userland?  Also, what's the reason not to do it from blktrace.c?
> What would be the downside of doing that?
>
> Thanks.
>

The userland tool cannot distinguish bounced bio from original one at 
completion TP, but it can expect there will be a duplicated 
BLK_TA_COMPLETE as it sees BLK_TA_BOUNCE for the bio before.

Filtering it out from kernel side seems to hide a real information that 
(paranoid?) user might want to get, and it looks like providing "polcy 
not mechanism" IMHO. That's why I changed my mind finally.

I cannot think of the downside, anyway it's not a big deal, if you think 
it's wrong choice, I'm OK to change it again.


Thanks,
Namhyung
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