Re: [patch] nd_blk,nd_pmem,nd_btt: add endio blktrace events

[Jeff Moyer <jmoyer@redhat.com>]

Christoph Hellwig <hch@infradead.org> writes:

> On Wed, Nov 09, 2016 at 02:43:58PM -0500, Jeff Moyer wrote:
>> But on the issue side, we have different trace actions: Q vs. I.  On the
>> completion side, we just have C.  You'd end up getting two C events for
>> each Q, and that may confuse existing utilities (such as blkparse, btt,
>> iowatcher, fio, etc), not to mention any scripts built around the
>> tracepoints, and any users looking at the raw blkparse output.
>> 
>> So, are you suggesting we add another action on the endio side?  If so,
>> that's a different patch set.  ;-)  If you're suggesting this multiple C
>> event thing, I'm not on board with that.
>
> Ok, good point.  It's a little bit annoying how asymetic the tracepoints
> are, but fixing it now might cause more harm than it helps.
>
> That being said, it might still be a good idea to have bio_endio call
> the tracepoint, we'll just need a __bio_endio to bypass the tracepoints
> for calls from the request layer.  That way all bio-based drivers will
> automatically do the right thing.

OK, I'll look into that.  I'm also still trying to decide whether a
separate endio event would be useful.  Any opinions on that are welcome.
It could show up in blkparse as 'E'.  For btt, I guess we could add a
Q2E column.  I'm not sure C2E would ever be interesting, but maybe?

-Jeff