Re: [patch 15/14] x86/dumpstack/64: Speedup in_exception_stack()

[Peter Zijlstra]

On Wed, Apr 03, 2019 at 10:08:28AM +0200, Peter Zijlstra wrote:
> On Tue, Apr 02, 2019 at 10:51:49AM -0500, Josh Poimboeuf wrote:
> > On Tue, Apr 02, 2019 at 05:48:56PM +0200, Thomas Gleixner wrote:
> > > > With the above "(stk <= begin || stk >= end)" check, removing the loop
> > > > becomes not all that important since exception stack dumps are quite
> > > > rare and not performance sensitive.  With all the macros this code
> > > > becomes a little more obtuse, so I'm not sure whether removal of the
> > > > loop is a net positive.
> > > 
> > > What about perf? It's NMI context and probably starts from there. Peter?
> > 
> > I believe perf unwinds starting from the regs from the context which was
> > interrupted by the NMI.
> 
> Aah, indeed. So then we only see exception stacks when the NMI lands in
> an exception, which is, as you say, quite rare.

Aah, ftrace OTOH might still trigger this lots. When you do function
tracer with stacktrace enabled it'll do unwinds _everywhere_.