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Re: [Qemu-devel] [PATCH v2 0/9] block: Delay poll when ending drained se

From: Kevin Wolf
Subject: Re: [Qemu-devel] [PATCH v2 0/9] block: Delay poll when ending drained sections
Date: Tue, 16 Jul 2019 16:40:16 +0200
User-agent: Mutt/1.11.3 (2019-02-01)

Am 19.06.2019 um 17:25 hat Max Reitz geschrieben:
> Hi,
> This is v2 to “block: Keep track of parent quiescing”.
> Please read this cover letter, because I’m very unsure about the design
> in this series and I’d appreciate some comments.
> As Kevin wrote in his reply to that series, the actual problem is that
> bdrv_drain_invoke() polls on every node whenever ending a drain.  This
> may cause graph changes, which is actually forbidden.
> To solve that problem, this series makes the drain code construct a list
> of undrain operations that have been initiated, and then polls all of
> them on the root level once graph changes are acceptable.
> Note that I don’t like this list concept very much, so I’m open to
> alternatives.

So drain_end is different from drain_begin in that it wants to wait only
for all bdrv_drain_invoke() calls to complete, but not for other
requests that are in flight. Makes sense.

Though instead of managing a whole list, wouldn't a counter suffice?

> Furthermore, all BdrvChildRoles with BDS parents have a broken
> .drained_end() implementation.  The documentation clearly states that
> this function is not allowed to poll, but it does.  So this series
> changes it to a variant (using the new code) that does not poll.
> There is a catch, which may actually be a problem, I don’t know: The new
> variant of that .drained_end() does not poll at all, never.  As
> described above, now every bdrv_drain_invoke() returns an object that
> describes when it will be done and which can thus be polled for.  These
> objects are just discarded when using BdrvChildRole.drained_end().  That
> does not feel quite right.  It would probably be more correct to let
> BdrvChildRole.drained_end() return these objects so the top level
> bdrv_drained_end() can poll for their completion.
> I decided not to do this, for two reasons:
> (1) Doing so would spill the “list of objects to poll for” design to
>     places outside of block/io.c.  I don’t like the design very much as
>     it is, but I can live with it as long as it’s constrained to the
>     core drain code in block/io.c.
>     This is made worse by the fact that currently, those objects are of
>     type BdrvCoDrainData.  But it shouldn’t be a problem to add a new
>     type that is externally visible (we only need the AioContext and
>     whether bdrv_drain_invoke_entry() is done).
> (2) It seems to work as it is.
> The alternative would be to add the same GSList ** parameter to
> BdrvChildRole.drained_end() that I added in the core drain code in patch
> 2, and then let the .drained_end() implementation fill that with objects
> to poll for.  (Which would be accomplished by adding a frontend to
> bdrv_do_drained_end() that lets bdrv_child_cb_drained_poll() pass the
> parameter through.)
> Opinions?

I think I would add an int* to BdrvChildRole.drained_end() so that we
can just increase the counter whereever we need to.

> And then we have bdrv_replace_child_noperm(), which actually wants a
> polling BdrvChildRole.drained_end().  So this series adds
> BdrvChildRole.drained_end_unquiesce(), which takes that role (if there
> is any polling to do).
> Note that if I implemented the alternative described above
> (.drained_end() gets said GSList ** parameter), a
> .drained_end_unquiesce() wouldn’t be necessary.
> bdrv_parent_drained_end_single() could just poll the list returned by
> .drained_end() by itself.

The split between .drained_end/.drained_end_unquiesce feels wrong. It
shouldn't be the job of the BdrvChildRole to worry about this. Polling
should be handled inside bdrv_parent_drained_end_single(), like we do in
bdrv_parent_drained_begin_single(), so that the BdrvChildRole never has
to poll.

> Finally, patches 1, 8, and 9 are unmodified from v1.
> [...]
>  include/block/block.h      |  22 +++++-
>  include/block/block_int.h  |  23 ++++++
>  block.c                    |  24 +++---
>  block/io.c                 | 155 ++++++++++++++++++++++++++++++-------
>  python/qemu/__init__.py    |   5 +-
>  tests/qemu-iotests/040     |  40 +++++++++-
>  tests/qemu-iotests/040.out |   4 +-
>  tests/qemu-iotests/255     |   2 +-
>  8 files changed, 231 insertions(+), 44 deletions(-)

I feel this series should add something to tests/test-bdrv-drain.c, too.
qemu-iotests can only test high-level block job commands that happen to
trigger the bug today, but that code may change in the future. Unit
tests allow us to test the problematic cases more directly.


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