Re: [PATCH v2 6/7] x86: Allow disabling HW_BREAKPOINTS, PERF_EVENTS, INSTRUCTION_DECODER, IRQ_WORK

[Josh Triplett <josh@joshtriplett.org>]

On Mon, Mar 10, 2014 at 05:14:11PM +0100, Frederic Weisbecker wrote:
> On Sun, Mar 09, 2014 at 08:12:53PM -0700, Josh Triplett wrote:
> > From: Andi Kleen <ak@linux.intel.com>
> > 
> > Make HW_BREAKPOINTS a config option. HW_BREAKPOINTS depends on perf.
> > This allows disabling PERF_EVENTS for systems that don't need it (e.g.
> > anything not used for development)  This then allows disabling IRQ_WORK
> > and INSTRUCTION_DECODER.
> > 
> > This change reduces the size of allnoconfig by 161k, a full 7.6%
> > reduction:
> >    text	   data	    bss	    dec	    hex	filename
> >  788816	 131952	1214432	2135200	 2094a0	vmlinux-allnoconfig-with-perf
> >  666361	  93020	1214368	1973749	 1e1df5	vmlinux-allnoconfig-no-perf
> > 
> > bloat-o-meter summary:
> > add/remove: 1/1040 grow/shrink: 1/25 up/down: 82/-152214 (-152132)
> 
> My review hasn't changed much since last time (https://lkml.org/lkml/2013/10/4/483),
> so I'm mostly copy pasting it here so that we can discuss if you want:
> 
> I'm personally not against the idea of allowing hw breakpoint support
> to be disabled but hpa did not like much the idea. I must say he had
> valid concerns though, see the thread of this patchset: https://lkml.org/lkml/2011/7/14/163
> 
> IMHO, a better solution would be to make the hw breakpoint subsystem work
> without using perf.

It looks like hpa's concern was primarily that it should be possible to
have CONFIG_HW_BREAKPOINTS=y without CONFIG_PERF_EVENTS.  I agree, but
since right now you can't have *either* of them disabled, allowing both
to be disabled seems like an improvement.

> > +config HW_BREAKPOINTS
> > +	bool "Enable hardware breakpoints support" if EXPERT
> > +	default y
> > +	select HAVE_HW_BREAKPOINT
> > +	select HAVE_MIXED_BREAKPOINTS_REGS
> > +	---help---
> > +	  Enable support for x86 hardware breakpoints for debuggers
> > +	  and perf.  This will implicitly enable perf-events.
> > +
> 
> HW_BREAKPOINTS must depend on HAVE_HW_BREAKPOINT, not the other way round.
> HAVE_* Kconfig symbols must only express constant backend support, not a variable user choice...
> 
> ie, that's what I did in this patchset
> https://lkml.org/lkml/2011/7/14/163

Fair enough.

- Josh Triplett

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