Re: [PATCH] block: add missing block_bio_complete() tracepoint

[Namhyung Kim]

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@xxxxxxxxx>
> > Cc: Tejun Heo <tj@xxxxxxxxxx>
> > Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> > Cc: dm-devel@xxxxxxxxxx
>
> 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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