Messages in this thread | | | Date | Wed, 3 Apr 2019 10:11:57 -0500 | From | Josh Poimboeuf <> | Subject | Re: [patch 15/14] x86/dumpstack/64: Speedup in_exception_stack() |
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On Wed, Apr 03, 2019 at 10:10:41AM +0200, Peter Zijlstra wrote: > 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_.
Even then, ftrace stacktrace will be really slow regardless, and this loop removal would be a tiny performance improvement for a tiny fraction of those stack traces. Unless the improvement is measurable I would personally rather err on the side of code readability.
-- Josh
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