Hi, Jens,
Jens Axboe [off-list ref] 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.
quoted hunk
- 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