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

[Kevin Wolf <kwolf@redhat.com>]

Am 25.02.2016 um 10:06 hat Pavel Dovgalyuk geschrieben:
> > From: Kevin Wolf [mailto:kwolf@redhat.com]
> > > > Coroutines aren't randomly assigned to threads, but threads actively
> > > > enter coroutines. To my knowledge this happens only when starting a
> > > > request (either vcpu or I/O thread; consistent per device) or by a
> > > > callback when some event happens (only I/O thread). I can't see any
> > > > non-determinism here.
> > >
> > > Behavior of coroutines looks strange for me.
> > > Consider the code below (co_readv function of the replay driver).
> > > In record mode it somehow changes the thread it assigned to.
> > > Code in point A is executed in CPU thread and code in point B - in some other thread.
> > > May this happen because this coroutine yields somewhere and its execution is restored
> > > by aio_poll, which is called from iothread?
> > > In this case event finishing callback cannot be executed deterministically
> > > (always in CPU thread or always in IO thread).
> > >
> > > static int coroutine_fn blkreplay_co_readv(BlockDriverState *bs,
> > >     int64_t sector_num, int nb_sectors, QEMUIOVector *qiov)
> > > {
> > >     BDRVBlkreplayState *s = bs->opaque;
> > >     uint32_t reqid = request_id++;
> > >     Request *req;
> > > // A
> > >     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);
> > >         if (reqid == current_request) {
> > >             current_finished = true;
> > >         } else {
> > >             req = block_request_insert(reqid, bs, qemu_coroutine_self());
> > >             qemu_coroutine_yield();
> > >             block_request_remove(req);
> > >         }
> > >     }
> > > // B
> > >     return 0;
> > > }
> > 
> > Yes, I guess this can happen. As I described above, the coroutine can be
> > entered from a vcpu thread initially. After yielding for the first time,
> > it is resumed from the I/O thread. So if there are paths where the
> > coroutine never yields, the coroutine completes in the original vcpu
> > thread. (It's not the common case that bdrv_co_readv() doesn't yield,
> > but it happens e.g. with unallocated sectors in qcow2.)
> > 
> > If this is a problem for you, you need to force the coroutine into the
> > I/O thread. You can do that by scheduling a BH, then yield, and then let
> > the BH reenter the coroutine.
> 
> Thanks, this approach seems to work. I got rid of replay_run_block_event,
> because BH basically does the same job.
> 
> There is one problem with flush event - callbacks for flush are called for
> all layers and I couldn't synchronize them correctly yet.
> I'll probably have to add new callback to block driver, which handles
> flush request for the whole stack of the drivers.

Flushes should be treated more or less the same a writes, I think.

Kevin