linux-kernel - Re: [patch] block: add blktrace C events for bio-based drivers

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Date:   Wed, 18 Jan 2017 11:01:46 -0500
From:   Jeff Moyer <jmoyer@...hat.com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     linux-block@...r.kernel.org, agk@...hat.com, snitzer@...hat.com,
        dm-devel@...hat.com, shli@...nel.org, linux-kernel@...r.kernel.org,
        linux-raid@...r.kernel.org, hch@....de
Subject: Re: [patch] block: add blktrace C events for bio-based drivers

Hi, Jens,

Jens Axboe <axboe@...nel.dk> writes:

> I like the change, hate the naming. I'd prefer one of two things:
>
> - Add bio_endio_complete() instead. That name sucks too, the
>   important part is flipping the __name() to have a trace
>   version instead.

ITYM a notrace version.  By default, we want tracing for bio_endio.  The
only callers that need the inverse of that are in the request-based
path, and there are only 2 of them.

> - Mark the bio as trace completed, and keep the naming. Since
>   it's only off the completion path, that can be just marking
>   the bi_flags non-atomically.

One issue with this is in generic_make_request_checks, where we can call
bio_endio without having called trace_block_bio_queue (so you could get
a C event with no corresponding Q).  To address that, we could make the
flag indicate that trace_block_bio_queue was performed, and clear it in
bio_complete, like so:

	if (test_and_clear_bit(BIO_QUEUE_TRACED, &bio->bi_flags))
		trace_block_bio_complete(...);

That would solve the problem of duplicate completions, but requires
setting the flag in the submission path and clearing it in the
completion path.  I think the former can be done with just a
bio_set_flag (i.e. non-atomic), right?  Of course, where to stick that
bio_set_flag call is another bike-shedding discussion waiting to happen
(i.e. does it go in the tracepoint itself?).

Alternatively, we could set the trace_completed flag in the paths where
we end I/O without having done the trace_block_bio_queue, but that seems
way uglier to me.

Can you think of any other options?  If we're choosing from the above,
my preference is for adding the bio_endio_notrace(), since it's so much
simpler.

Cheers,
Jeff