Re: [Qemu-devel] [PATCH] Add delay option to blkdebug

[Alex Bligh]

Kevin,

--On 1 July 2013 12:23:53 +0200 Kevin Wolf <address@hidden> wrote:

> "inject-error" doesn't really describe this well. Shouldn't we rather
> introduce a new section "[delay]" or something like that?

That's how I started off. Then I realised you might want to make
the delay dependent on the sector or state, or the operation
concerned. Changing the name of the section to '[inject]' would
probably be best.

> I remember that I once tried something similar, but never submitted it.
> I think the reason was that using timers inside requests doesn't really
> work. It works as long as everything is indeed asynchronous, but
> bdrv_drain_all() or a loop waiting for a single request that tries to
> make progress with qemu_aio_wait() will simply hang because timers
> aren't processed in these nested event loops.

Yes I discovered that the hard way. In particular, timers are not
run at all in (e.g.) qemu-img. I think they are not run at all outside
qemu itself.

This is why I don't use timers.

What it currently does is schedule a bh which looks at the time
(as time still moves forwards) - using idle scheduling, and if it's
not time to run the bh, the bh reschedules itself (using idle scheduling
again). Because of the use of idle scheduling it isn't busy-waiting,
but it is hardly ideal.

I /think/ this is safe because if bh's aren't running, lots of
drivers (e.g. blkverify) would not work. And as far as I can tell,
idle bh's are scheduled whenever non-idle ones are. However, I am
newbie as far as the async code and the block code are concerned.

--
Alex Bligh