Re: [Linux PPC] Disable PREEMPT

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:

quoted

On Mon, Sep 21, 2026 at 10:06:22AM +0200, Michal Suchánek wrote:

quoted

On Thu, Sep 03, 2026 at 03:28:26PM +0200, Michal Suchánek wrote:

quoted

On Thu, Sep 03, 2026 at 06:06:16PM +0530, Harsh Prateek Bora wrote:

quoted

+ 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:

quoted

quoted

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.

quoted

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.

quoted

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