Re: v4.16+ seeing many unaligned access in dequeue_task_fair() on IA64

[Peter Zijlstra]

On Mon, Apr 02, 2018 at 04:24:49PM -0700, Luck, Tony wrote:
> v4.16 boots cleanly. But with the first bunch of merges
> (Linus HEAD = 46e0d28bdb8e6d00e27a0fe9e1d15df6098f0ffb)
> I see a bunch of:
> 
> ia64_handle_unaligned: 4863 callbacks suppressed
> kernel unaligned access to 0xe00000031660fd74, ip=0xa0000001000f23e0
> kernel unaligned access to 0xe00000033bdffbcc, ip=0xa0000001000f2370
> kernel unaligned access to 0xe00000031660fd74, ip=0xa0000001000f23e0
> kernel unaligned access to 0xe00000033bdffbcc, ip=0xa0000001000f2370
> kernel unaligned access to 0xe00000031660fd74, ip=0xa0000001000f23e0
> 
> The addresses are all 4-byte, but not 8-byte aligned.
> 
> Any guesses before I start to bisect?

That doesn't sound good. The only guess I have at this moment is you
accidentially enabled RANDSTRUCT_PLUGIN and that messes things up.

struct task_struct whould be at least L1_CACHE_BYTES aligned, and C
otherwise makes it fairly hard to cause unaligned accesses. Packed
structures and/or casting are required, and I don't think we added
anything dodgy like that here.