linux-kernel - Re: [patch 15/14] x86/dumpstack/64: Speedup in_exception

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Date:   Wed, 3 Apr 2019 10:08:28 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [patch 15/14] x86/dumpstack/64: Speedup in_exception_stack()

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.