Re: [PATCH 6/7] arch: __get_wchan || STACKTRACE_SUPPORT

Josh Poimboeuf
2021-10-14T18:49:08Z

On Thu, Oct 14, 2021 at 07:03:07PM +0100, Mark Rutland wrote:
> On Fri, Oct 08, 2021 at 03:45:59PM +0200, Peter Zijlstra wrote:
> > On Fri, Oct 08, 2021 at 01:40:52PM +0100, Mark Rutland wrote:
> > > [Adding Josh, since there might be a concern here from a livepatch pov]
> > >
> >
> > > > +static unsigned long __get_wchan(struct task_struct *p)
> > > > +{
> > > > + unsigned long entry = 0;
> > > > +
> > > > + stack_trace_save_tsk(p, &entry, 1, 0);
> > >
> > > This assumes stack_trace_save_tsk() will skip sched functions, but I
> > > don't think that's ever been a requirement? It's certinaly not
> > > documented anywhere that I could find, and arm64 doesn't do so today,
> > > and this patch causes wchan to just log `__switch_to` for everything.
> >
> > Confused, arm64 has arch_stack_walk() and should thus use
> > kernel/stacktrace.c's stack_trace_consume_entry_nosched.
>
> Looking at this arm64's *current* get_wchan() unwinds once before
> checking in_sched_functions(), so it skips __switch_to(). As of this
> patch, we check in_sched_functions() first, which stops the unwind
> immediately as __switch_to() isn't marked as __sched.
>
> I think x86 gets away with this because switch_to() is asm, and that
> tail-calls __switch_to() when returning.
>
> Does switch_to() and below need to be marked __sched?

Yes, I would think so, for arches where they otherwise show up on the
stacktrace.

--
Josh