Re: [Qemu-devel] [PATCH 3/3] replay: introduce block devices record/replay

[Kevin Wolf]

Am 16.02.2016 um 07:25 hat Pavel Dovgalyuk geschrieben:
> > From: Kevin Wolf [mailto:address@hidden
> > Am 15.02.2016 um 15:24 hat Pavel Dovgalyuk geschrieben:
> > > > From: Kevin Wolf [mailto:address@hidden
> > >
> > > > > There could be asynchronous events that occur in non-cpu threads.
> > > > > For now these events are shutdown request and block task execution.
> > > > > They may "hide" following clock (or another one) events. That is why
> > > > > we process them until synchronous event (like clock, instructions
> > > > > execution, or checkpoint) is met.
> > > > >
> > > > >
> > > > > > Anyway, what does "can't proceed" mean? The coroutine yields because
> > > > > > it's waiting for I/O, but it is never reentered? Or is it hanging 
> > > > > > while
> > > > > > trying to acquire a lock?
> > > > >
> > > > > I've solved this problem by slightly modifying the queue.
> > > > > I haven't yet made BlockDriverState assignment to the request ids.
> > > > > Therefore aio_poll was temporarily replaced with usleep.
> > > > > Now execution starts and hangs at some random moment of OS loading.
> > > > >
> > > > > Here is the current version of blkreplay functions:
> > > > >
> > > > > static int coroutine_fn blkreplay_co_readv(BlockDriverState *bs,
> > > > >     int64_t sector_num, int nb_sectors, QEMUIOVector *qiov)
> > > > > {
> > > > >     uint32_t reqid = request_id++;
> > > > >     Request *req;
> > > > >     req = block_request_insert(reqid, bs, qemu_coroutine_self());
> > > > >     bdrv_co_readv(bs->file->bs, sector_num, nb_sectors, qiov);
> > > > >
> > > > >     if (replay_mode == REPLAY_MODE_RECORD) {
> > > > >         replay_save_block_event(reqid);
> > > > >     } else {
> > > > >         assert(replay_mode == REPLAY_MODE_PLAY);
> > > > >         qemu_coroutine_yield();
> > > > >     }
> > > > >     block_request_remove(req);
> > > > >
> > > > >     return 0;
> > > > > }
> > > > >
> > > > > void replay_run_block_event(uint32_t id)
> > > > > {
> > > > >     Request *req;
> > > > >     if (replay_mode == REPLAY_MODE_PLAY) {
> > > > >         while (!(req = block_request_find(id))) {
> > > > >             //aio_poll(bdrv_get_aio_context(req->bs), true);
> > > > >             usleep(1);
> > > > >         }
> > > >
> > > > How is this loop supposed to make any progress?
> > >
> > > This loop does not supposed to make any progress. It waits until 
> > > block_request_insert
> > > call is added to the queue.
> > 
> > Yes. And who is supposed to add something to the queue? You are looping
> > and doing nothing, so no other code gets to run in this thread. It's
> > only aio_poll() that would run event handlers that could eventually add
> > something to the list.
> 
> blkreplay_co_readv adds request to the queue.

I don't see blkreplay_co_readv() called in this loop without aio_poll().

> > > > And I still don't understand why aio_poll() doesn't work and where it
> > > > hangs.
> > >
> > > aio_poll hangs if "req = block_request_insert(reqid, bs, 
> > > qemu_coroutine_self());" line
> > > is executed after bdrv_co_readv. When bdrv_co_readv yields, 
> > > replay_run_block_event has no
> > > information about pending request and cannot jump to its coroutine.
> > > Maybe I should implement aio_poll execution there to make progress in 
> > > that case?
> > 
> > Up in the thread, I posted code that was a bit more complex than what
> > you have, and which considered both cases (completion before
> > replay/completion after replay). If you implement it like this, it might
> > just work. And yes, it involved an aio_poll() loop in the replay
> > function.
> 
> You are right, but I found kind of fundamental problem here.
> There are two possible approaches for replaying coroutine events:
> 
> 1. Waiting with aio_poll in replay_run_block_event.
>    In this case replay cannot be made, because of recursive mutex lock:
>    aio_poll -> qemu_clock_get_ns -> <lock replay mutex> -> 
>      replay_run_block_event -> aio_poll -> qemu_clock_get_ns -> <lock replay 
> mutex> ->
>      <assert failed>
>    I.e. we have recursive aio_poll function calls that lead to recursive 
> replay calls
>    and to recursive mutex lock.

That's what you get for replaying stuff in qemu_clock_get_ns() when it
should really be replayed in the hardware emulation.

This is a problem you would also have had with your original patch, as
there are some places in the block layer that use aio_poll() internally.
Using the blkreplay driver only made you find the bug earlier.

But... can you detail how we get from aio_poll() to qemu_clock_get_ns()?
I don't see it directly calling that function, so it might just be a
callback to some guest device that is running now.

> 2. Adding block events to the queue until checkpoint is met.
>    This will not work with synchronous requests, because they will wait 
> infinitely
>    and checkpoint will never happen. Correct me, if I'm not right for this 
> case.
>    If blkreplay_co_readv will yield, can vcpu thread unlock the BQL and wait 
> for 
>    the checkpoint in iothread?

I don't know enough checkpoints and about CPU emulation to give a
definite answer. This kind of blocking the I/O thread feels wrong,
though.

Kevin