Re: [PATCH 3/4] locking/mutex: Pass proper call-site ip

Peter Zijlstra
2022-03-01T19:49:21Z

On Tue, Mar 01, 2022 at 09:53:54AM -0500, Steven Rostedt wrote:
> On Tue, 1 Mar 2022 10:05:12 +0100
> Peter Zijlstra <peterz@infradead.org> wrote:
>
> > On Mon, Feb 28, 2022 at 05:04:11PM -0800, Namhyung Kim wrote:
> > > The __mutex_lock_slowpath() and friends are declared as noinline and
> > > _RET_IP_ returns its caller as mutex_lock which is not meaningful.
> > > Pass the ip from mutex_lock() to have actual caller info in the trace.
> >
> > Blergh, can't you do a very limited unwind when you do the tracing
> > instead? 3 or 4 levels should be plenty fast and sufficient.
>
> Is there a fast and sufficient way that works across architectures?

The normal stacktrace API? Or the fancy new arch_stack_walk() which is
already available on most architectures you actually care about and
risc-v :-)

Remember, this is the contention path, we're going to stall anyway,
doing a few levels of unwind shouldn't really hurt at that point.

Anyway; when I wrote that this morning, I was thinking:

unsigned long ips[4];
stack_trace_save(ips, 4, 0);

> Could objtool help here?

There's a contradition there... objtool is still x86_64 only :-/

IIRC there's been work on s390, arm64 and power objtool, but so far none
of them actually made it in.