Re: [PATCH] arm64: add HAVE_LATENCYTOP_SUPPORT config

[Will Deacon]

On Tue, Nov 10, 2015 at 12:01:45PM +0100, Heiko Carstens wrote:
> On Tue, Nov 10, 2015 at 10:05:48AM +0000, Will Deacon wrote:
> > Hi Heiko,
> > 
> > On Tue, Nov 10, 2015 at 08:41:24AM +0100, Heiko Carstens wrote:
> > > On Fri, Nov 06, 2015 at 04:21:10PM +0000, Will Deacon wrote:
> > > > On Sat, Nov 07, 2015 at 12:11:16AM +0800, yalin wang wrote:
> > > > > i just enable it on ARM64,
> > > > > and it can work,
> > > > > i donât see some special requirement to enable this config .
> > > > 
> > > > Right, so why does HAVE_LATENCYTOP_SUPPORT exist?
> [...]
> > > And looking through the kernel there is at least avr32 which would break
> > > at build time if the config option would be removed completely.
> > > 
> > > So.. renaming it to STACKTRACE_TSK_SUPPORT would be a good idea.
> > 
> > ftrace has a similar issue and solves it by having architectures define
> > a `config STACKTRACE_SUPPORT' symbol. Over in kernel/trace/Kconfig,
> > there's a `select STACKTRACE if STACKTRACE_SUPPORT', which means
> > that kernel/stacktrace.c gets built and a dummy (weak symbol) version of
> > save_stack_trace_tsk appears.
> 
> Ah, I wasn't aware of the weak symbol.
> 
> > I don't think adding another STACKTRACE-related Kconfig option is
> > necessarily the best thing to do. Maybe we should instead have LATENCYTOP
> > depend on STACKTRACE_SUPPORT (already the case) and select STACKTRACE?
> 
> LATENCYTOP also already selects STACKTRACE. So it looks like
> HAVE_LATENCYTOP_SUPPORT could simply be removed.

Ha, so it does! Patch below. The only non-trivial part was arch/arm/,
which has a dependency on !SMP which I believe is no longer required
as of d5996b2ff0e2 ("ARM: fix /proc/$PID/stack on SMP").

Will

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