Re: [Linux PPC] Disable PREEMPT

["Michal Suchánek" <msuchanek@suse.de>]

On Mon, Sep 21, 2026 at 10:50:41PM +0530, Shrikanth Hegde wrote:
> Hi Michal,
> 
> On 9/21/26 10:16 PM, Michal Suchánek wrote:
> > On Mon, Sep 21, 2026 at 10:06:22AM +0200, Michal Suchánek wrote:
> > > On Thu, Sep 03, 2026 at 03:28:26PM +0200, Michal Suchánek wrote:
> > > > On Thu, Sep 03, 2026 at 06:06:16PM +0530, Harsh Prateek Bora wrote:
> > > > > + Amit, Gautam - since they recently came across a similar regression which
> > > > > is seen with non KVM workload as well.
> > > > > + Venkat - for CI improvement suggestion below.
> > > > > 
> > > > > Hi Michal,
> > > > > 
> > > > > On 03/09/26 5:55 pm, Michal Suchánek wrote:
> > > > 
> > > > > > 
> > > > > > There is another problem that Linux 7.2.2 often locks up on boot inside
> > > > > > the guest, and as a result a different kernel is needed in the guest to
> > > > > > reproduce the host crash. This is more recent, linux 6.12 does not have
> > > > > > this problem. Will try some different kernel combinations to see if I
> > > > > > can narrow this down. For this the repro is merely booting the kernel in
> > > > > > the VM, it typically fails early before userspace produces any messages.
> > > > 
> > > > This one regressed between 6.19.12
> > > > https://github.com/openSUSE/kernel-source/blob/9f6830f/config/ppc64le/default
> > > > and 7.0.12
> > > > https://github.com/openSUSE/kernel-source/blob/2ebf0bc/config/ppc64le/default
> > > 
> > > Which is when preempt was enabled.
> > > 
> > > Very likely preexisting problem that was exposed by enabling preemption
> > > again.
> > > 
> > > @@ -121,9 +122,7 @@ CONFIG_BPF_LSM=y
> > >   CONFIG_PREEMPT_BUILD=y
> > >   CONFIG_ARCH_HAS_PREEMPT_LAZY=y
> > > -# CONFIG_PREEMPT_NONE is not set
> > > -CONFIG_PREEMPT_VOLUNTARY=y
> > > -# CONFIG_PREEMPT is not set
> > > +CONFIG_PREEMPT=y
> > >   # CONFIG_PREEMPT_LAZY is not set
> > >   CONFIG_PREEMPT_COUNT=y
> > >   CONFIG_PREEMPTION=y
> > 
> > Looking more into this the problem is not reproducible with 6.19 with
> > full preempt, and can be reproduced with 7.0 with full preempt where
> > preemtion is mandatory. It only so happens that 1.19 was last kernel
> > where preemption was not mandatory and where this problem also happened
> > to not manifest with the Tumbleweed config.
> > 
> 
> I am bit confused. Is it different problem than the kprobe/bpf issue
> which was observed on host?

Yes, it appears to be different, a crash very early during boot with a
different stack trace.

> 
> Is this kvm/guest specific problem you are observing? Is it still seen
> even with that kprobe/bpf fix?

Yes, even with the fix this can be observed.

> 
> > It is not deterministic enough to bisect. It's probably some kind of
> > race condition or code generation problem that can surface and go away
> > or become too difficult to reproduce with seemingly unrelated changes.
> > 
> 
> Can you try latest kernel?

How much latest?

I tried with 7.2.6 with the kprobe fix as guest, and some earlier 7.2.x
as host already, and the problem is reproducible.

Thanks

Michal

> 
> There was change recently to move to use
> generic transfer to guest mode work. i.e VIRT_XFER_TO_GUEST_WORK.

> 
> > Crash happens very early but it's eemu so the memory can be saved
> > regardelss. The problem is that the crash tool refuses to load the core
> > bacause it detects the core as SMP and vmlinux as non-SMP or the other
> > way around.
> > 
> > Thanks
> > 
> > Michal
> >