Re: [Qemu-devel] [PATCH v13 19/25] replay: add BH oneshot event for block layer

["Pavel Dovgalyuk" <dovgaluk@ispras.ru>]

> From: Kevin Wolf [mailto:kwolf@redhat.com]
> Am 04.03.2019 um 13:17 hat Pavel Dovgalyuk geschrieben:
> > > From: Kevin Wolf [mailto:kwolf@redhat.com]
> > > Am 21.02.2019 um 12:05 hat Pavel Dovgalyuk geschrieben:
> > > > Replay is capable of recording normal BH events, but sometimes
> > > > there are single use callbacks scheduled with aio_bh_schedule_oneshot
> > > > function. This patch enables recording and replaying such callbacks.
> > > > Block layer uses these events for calling the completion function.
> > > > Replaying these calls makes the execution deterministic.
> > > >
> > > > Signed-off-by: Pavel Dovgalyuk <Pavel.Dovgaluk@ispras.ru>
> > > >
> > > > --
> > > >
> > > > v6:
> > > >  - moved stub function to the separate file for fixing linux-user build
> > > > v10:
> > > >  - replaced all block layer aio_bh_schedule_oneshot calls
> > > This still doesn't catch all instances, e.g. everything that goes
> > > through aio_co_schedule() is missing.
> >
> > It seems, that everything else is synchronized with blkreplay driver
> > which is mandatory when using block devices in rr mode.
> 
> Ah, yes, this is a good point. blkreplay goes through
> replay_block_event(), which is where things get synchronised, right?

Right.

> Does this mean that most of the places where you replaced a BH with your
> new function don't actually need it either because they are called
> through blkreplay and will go through replay_block_event() before
> reaching the guest?

I'm not sure, maybe some of them are going through the blkreplay driver.

> > > > @@ -1349,8 +1351,8 @@ static BlockAIOCB *blk_aio_prwv(BlockBackend *blk, int64_t offset,
> int
> > > bytes,
> > > >
> > > >      acb->has_returned = true;
> > > >      if (acb->rwco.ret != NOT_DONE) {
> > > > -        aio_bh_schedule_oneshot(blk_get_aio_context(blk),
> > > > -                                blk_aio_complete_bh, acb);
> > > > +        replay_bh_schedule_oneshot_event(blk_get_aio_context(blk),
> > > > +                                         blk_aio_complete_bh, acb);
> > > >      }
> > >
> > > This, and a few other places that you convert, are in fast paths and add
> > > some calls that are unnecessary for non-replay cases.
> >
> > I don't think that this can make a noticeable slowdown, but we can run
> > the tests if you want.
> > We have the test suite which performs disk-intensive computation.
> > It was created to measure the effect of running BH callbacks through
> > the virtual timer infrastructure.
> 
> I think this requires quite fast storage to possibly make a difference.

True.

> Or if you don't have that, maybe a ramdisk or even a null-co:// backend
> could do the trick. Maybe null-co:// is actually the best option.

We've got tests with file copying and compression on qcow2 disks.
How the null backend can be applied there?

> Anyway, if it's not too much work for you, running some tests would be
> good.
> 
> > > I wonder if we could make replay optional in ./configure and then make
> > > replay_bh_schedule_oneshot_event() a static inline function that can get
> > > optimised away at compile time if the feature is disabled.
> >
> > It is coupled with icount. However, some icount calls are also lie on
> > the fast paths and are completely useless when icount is not enabled.
> 
> Well, the common fast path is KVM, which doesn't have icount at all, so
> that might make it less critical. :-)

I see.

> I get your point, though maybe that just means that both should be
> possible to be disabled at configure time.

Right, it may be reasonable for some TCG use cases.
This could be a separate patch set, because everything changes.

Pavel Dovgalyuk