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From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	LKML <linux-kernel@vger.kernel.org>,
	x86@kernel.org, Andy Lutomirski <luto@kernel.org>
Subject: Re: [patch 15/14] x86/dumpstack/64: Speedup in_exception_stack()
Date: Wed, 3 Apr 2019 10:11:57 -0500	[thread overview]
Message-ID: <20190403151157.dcjmunsl7mna4ore@treble> (raw)
In-Reply-To: <20190403081041.GB14281@hirez.programming.kicks-ass.net>

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

  reply	other threads:[~2019-04-03 15:12 UTC|newest]

Thread overview: 43+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-03-31 21:40 [patch 00/14] x86/exceptions: Add guard patches to IST stacks Thomas Gleixner
2019-03-31 21:40 ` [patch 01/14] x86/irq/64: Limit IST stack overflow check to #DB stack Thomas Gleixner
2019-04-01 18:03   ` Borislav Petkov
2019-04-02 16:34   ` Sean Christopherson
2019-03-31 21:40 ` [patch 02/14] x86/idt: Remove unused macro SISTG Thomas Gleixner
2019-04-01  4:04   ` Andy Lutomirski
2019-03-31 21:40 ` [patch 03/14] x86/exceptions: Remove unused stack defines on 32bit Thomas Gleixner
2019-03-31 21:40 ` [patch 04/14] x86/exceptions: Make IST index zero based Thomas Gleixner
2019-04-01  7:30   ` Peter Zijlstra
2019-04-01  7:33     ` Thomas Gleixner
2019-04-02 16:49   ` Sean Christopherson
2019-04-03 16:35   ` Borislav Petkov
2019-03-31 21:40 ` [patch 05/14] x86/cpu_entry_area: Cleanup setup functions Thomas Gleixner
2019-03-31 21:40 ` [patch 06/14] x86/exceptions: Add structs for exception stacks Thomas Gleixner
2019-03-31 21:40 ` [patch 07/14] x86/cpu_entry_area: Prepare for IST guard pages Thomas Gleixner
2019-03-31 21:40 ` [patch 08/14] x86/cpu_entry_area: Provide exception stack accessor Thomas Gleixner
2019-03-31 21:40 ` [patch 09/14] x86/traps: Use cpu_entry_area instead of orig_ist Thomas Gleixner
2019-03-31 21:40 ` [patch 10/14] x86/irq/64: Use cpu entry area " Thomas Gleixner
2019-03-31 21:40 ` [patch 11/14] x86/dumpstack/64: Use cpu_entry_area " Thomas Gleixner
2019-03-31 21:40 ` [patch 12/14] x86/cpu: Prepare TSS.IST setup for guard pages Thomas Gleixner
2019-04-02 16:57   ` Sean Christopherson
2019-03-31 21:40 ` [patch 13/14] x86/cpu: Remove orig_ist array Thomas Gleixner
2019-03-31 21:40 ` [patch 14/14] x86/exceptions: Enable IST guard pages Thomas Gleixner
2019-04-02 10:19   ` [patch 15/14] x86/dumpstack/64: Speedup in_exception_stack() Thomas Gleixner
2019-04-02 15:43     ` Josh Poimboeuf
2019-04-02 15:48       ` Thomas Gleixner
2019-04-02 15:51         ` Josh Poimboeuf
2019-04-02 15:53           ` Josh Poimboeuf
2019-04-03  8:08           ` Peter Zijlstra
2019-04-03  8:10             ` Peter Zijlstra
2019-04-03 15:11               ` Josh Poimboeuf [this message]
2019-04-02 16:11         ` Andy Lutomirski
2019-04-02 18:27           ` Thomas Gleixner
2019-04-02 19:29             ` Thomas Gleixner
2019-04-03  0:36               ` Andy Lutomirski
2019-04-03 16:26                 ` Thomas Gleixner
2019-04-03 19:42                   ` Thomas Gleixner
2019-04-04  0:03                     ` Andy Lutomirski
2019-04-02 19:02         ` Rasmus Villemoes
2019-04-02 19:21           ` Thomas Gleixner
2019-04-03  8:02       ` Peter Zijlstra
2019-04-01  4:03 ` [patch 00/14] x86/exceptions: Add guard patches to IST stacks Andy Lutomirski
2019-04-03 16:30   ` Thomas Gleixner

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