Re: post-copy is broken?

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Li, Liang Z (liang.z.li@intel.com) wrote:
> > > > > > > > Interesting; it's failing reliably for me - but only with a
> > > > > > > > reasonably freshly booted machine (so that the pages get THPd).
> > > > > > >
> > > > > > > The same here. Freshly booted machine with 64GiB ram. I've
> > > > > > > checked
> > > > > > > /proc/vmstat: huge pages were allocated
> > > > > >
> > > > > > Thanks for testing.
> > > > > >
> > > > > > Damn; this is confusing now.  I've got a RHEL7 box with
> > > > > > 4.6.0-rc3 on where it works, and a fedora24 VM where it fails
> > > > > > (the f24 VM is where I did the bisect so it works fine with the
> > > > > > older kernel on the f24
> > > > userspace in that VM).
> > > > > >
> > > > > > So lets see:
> > > > > >    works: Kirill's (64GB machine)
> > > > > >           Dave's RHEL7 host (24GB RAM, dual xeon, RHEL7
> > > > > > userspace and kernel
> > > > > > config)
> > > > > >    fails: Dave's f24 VM (4GB RAM, 4 vcpus VM on my laptop24
> > > > > > userspace and kernel config)
> > > > > >
> > > > > > So it's any of userspace, kernel config, machine hardware or hmm.
> > > > > >
> > > > > > My f24 box has transparent_hugepage_madvise, where my rhel7 has
> > > > > > transparent_hugepage_always (but still works if I flip it to
> > > > > > madvise at run time).  I'll try and get the configs closer together.
> > > > > >
> > > > > > Liang Li: Can you run my test on your setup which fails the
> > > > > > migrate and tell me what your userspace is?
> > > > > >
> > > > > > (If you've not built my test yet, you might find you need to add a :
> > > > > >    tests/postcopy-test$(EXESUF): tests/postcopy-test.o
> > > > > >
> > > > > >   to the tests/Makefile)
> > > > > >
> > > > >
> > > > > Hi Dave,
> > > > >
> > > > >   How to build and run you test? I didn't do that before.
> > > >
> > > > Apply the code in:
> > > > http://lists.gnu.org/archive/html/qemu-devel/2016-04/msg02138.html
> > > >
> > > > fix the:
> > > > +            if ( ((b + 1) % 255) == last_byte && !hit_edge) {
> > > > to:
> > > > +            if ( ((b + 1) % 256) == last_byte && !hit_edge) {
> > > >
> > > > to tests/Makefile
> > > >    tests/postcopy-test$(EXESUF): tests/postcopy-test.o
> > > >
> > > > and do a:
> > > >     make check
> > > >
> > > > in qemu.
> > > > Then you can rerun the test with:
> > > >     QTEST_QEMU_BINARY=path/to/qemu-system-
> > x86_64 ./tests/postcopy-
> > > > test
> > > >
> > > > if it works, reboot and check it still works from a fresh boot.
> > > >
> > > > Can you describe the system which your full test failed on? What
> > > > distro on the host? What type of host was it tested on?
> > > >
> > > > Dave
> > > >
> > >
> > >
> > > Thanks, Dave
> > >
> > > The host is CenOS7, its original kernel is 3.10.0-327.el7.x86_64
> > > (CentOS 7.1?), The hardware platform is HSW-EP with 64GB RAM.
> > 
> > OK, so your test fails on real hardware; my guess is that my test will work on
> > there.
> > Can you try your test with THP disabled on the host:
> > 
> > echo never > /sys/kernel/mm/transparent_hugepage/enabled
> > 
> 
> If the THP is disabled, no fails.
> And your test was always passed, even when  real post-copy was failed. 
> 
> In my env, the output of 
> 'cat /sys/kernel/mm/transparent_hugepage/enabled'  is:
> 
>  [always] ...

OK, I can't get my test to fail on real hardware - only in a VM; but my
suspicion is we're looking at the same bug; both of them it goes away
if we disable THP, both of them work on 4.4.x and fail on 4.5.x.
I'd love to be able to find a nice easy test to be able to give to Andrea
and Kirill

I've also just confirmed that running (in a VM) a fedora-24 4.5.0 kernel
with a fedora-23 userspace (qemu built under f23) still fails with my test.
So the problem there is definitely triggered by the newer kernel not
the newer userspace.

Dave

> 
> Liang
> 
> > Dave
> > 
> > >
> > >
> > > > >
> > > > > Thanks!
> > > > > Liang
> > > > >
> > > > > >
> > > > > > Dave
> > > > > > >
> > > > > > > --
> > > > > > >  Kirill A. Shutemov
> > > > > > --
> > > > > > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> > > > --
> > > > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> > --
> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK

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