Re: [PATCH 00/12 v5] Multiqueue for MMC/SD

On 10 November 2017 at 11:01, Linus Walleij [off-list ref] wrote:

This is the fifth iteration of this patch set.

I *HOPE* that we can scrap this patch set and merge Adrian's
patches instead, because they also bring CQE support which is
nice. I had some review comments on his series, mainly that
it needs to kill off the legacy block layer code path that
noone likes anyway.

So this is mainly an academic and inspirational exercise.
Whatever remains of this refactoring, if anything, I can
certainly do on top of Adrian's patches as well.

What changed since v4 is the error path, since Adrian pointed
out that the error handling seems to be fragile. It was indeed
fragile... To make sure things work properly I have run long
test rounds with fault injection, essentially:

Please correct me if I am wrong, the issues was observed already in
patch 11, before the actual switch to mq was done, right?

Enable FAULT_INJECTION, FAULT_INJECTION_DEBUG_FS,
       FAIL_MMC_REQUEST
cd /debug/mmc3/fail_mmc_request/
echo 1 > probability
echo -1 > times

Then running a dd to the card, also increased the error rate
to 10% and completed tests successfully, but at this error
rate the MMC stack sometimes exceeds the retry limit and the
dd command fails (as is appropriate).

That's great. I really appreciate that you have run these tests, that
gives me a good confidence from an overall point of view.

Removing a card during I/O does not work well however :/
So I guess I would need to work on that if this series should
continue. (Hopefully unlikely.)

Yeah, this has actually been rather cumbersome to deal with also in
the legacy request path. Let's dive into this in more detail as soon
as possible.

Linus Walleij (12):
  mmc: core: move the asynchronous post-processing
  mmc: core: add a workqueue for completing requests
  mmc: core: replace waitqueue with worker
  mmc: core: do away with is_done_rcv
  mmc: core: do away with is_new_req
  mmc: core: kill off the context info
  mmc: queue: simplify queue logic
  mmc: block: shuffle retry and error handling
  mmc: queue: stop flushing the pipeline with NULL
  mmc: queue/block: pass around struct mmc_queue_req*s
  mmc: block: issue requests in massive parallel
  mmc: switch MMC/SD to use blk-mq multiqueueing v5

 drivers/mmc/core/block.c    | 557 +++++++++++++++++++++++---------------------
 drivers/mmc/core/block.h    |   5 +-
 drivers/mmc/core/bus.c      |   1 -
 drivers/mmc/core/core.c     | 217 ++++++++++-------
 drivers/mmc/core/core.h     |  11 +-
 drivers/mmc/core/host.c     |   1 -
 drivers/mmc/core/mmc_test.c |  31 +--
 drivers/mmc/core/queue.c    | 252 ++++++++------------
 drivers/mmc/core/queue.h    |  16 +-
 include/linux/mmc/core.h    |   3 +-
 include/linux/mmc/host.h    |  31 +--
 11 files changed, 557 insertions(+), 568 deletions(-)

First, I haven't yet commented on the latest version of the mq patch
(patch12 in this series) and neither on Adrians (patch 3 in his
series), but before doing that, let me share my overall view of how we
could move forward, as to see if all of us can agree on that path.

So, what I really like in $subject series is the step by step method,
moving slowly forward enables an easy review, then the actual switch
to mq gets a diff of "3 files changed, 156 insertions(+), 181
deletions(-)". This shows to me, that it can be done! Great work!
Of course, I do realize that you may not have considered all
preparations needed for CQE, which Adrian may have thought of in his
mq patch from his series (patch 3), but still.

Moreover, for reasons brought up while reviewing Adrian's series,
regarding if mq is "ready", and because I see that the diff for patch
12 is small, I suggest that we just skip the step adding a Kconfig
option to allow an opt-in of the mq path. In other words, *the* patch
that makes the switch to mq, should also remove the entire left over
of rubbish code, from the legacy request path. That's also what you do
in patch12, nice!

Finally, I understand that you would be happy to scrap this series,
but instead let Adrian's series, when re-posted, to go first. Could
you perhaps re-consider that, because I wonder if it may not be
smother and less risky, to actually apply everything up to patch 11 in
this series?

I noticed that you reported issues with card removal during I/O (for
both yours and Adrian's mq patch), but does those problems exists at
patch 11 - or is those explicitly introduced with the mk patch (patch
12)?

Of course, I realize that if we apply everything up to patch11, that
would require a massive re-base of Adrian's mq/CQE series, but on the
other hand, then matter which mq patch we decide to go with, it should
be a rather small diff, thus easy to review and less risky.

Adrian, Linus - what do you think?

Kind regards
Uffe