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

Namhyung Kim
2022-03-04T07:29:02Z

On Tue, Mar 1, 2022 at 11:47 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> 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);

When I collected stack traces in a BPF, it already consumed 3 or 4
entries in the BPF so I had to increase the size to 8 and skip 4.
But it didn't add noticeable overheads in my test.

Thanks,
Namhyung