Date:	Fri, 12 Jun 2009 14:59:41 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Mike Frysinger <vapier.adi@...il.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] scripts/checksyscalls.sh: only whine perf_counter_open
	when supported


* Mike Frysinger <vapier.adi@...il.com> wrote:

> On Fri, Jun 12, 2009 at 08:31, Ingo Molnar wrote:
> > * Mike Frysinger <vapier.adi@...il.com> wrote:
> >> On Fri, Jun 12, 2009 at 08:17, Ingo Molnar wrote:
> >> > * Mike Frysinger <vapier.adi@...il.com> wrote:
> >> >> On Fri, Jun 12, 2009 at 08:05, Ingo Molnar wrote:
> >> >> > * Mike Frysinger <vapier@...too.org> wrote:
> >> >> >> If the port does not support HAVE_PERF_COUNTERS, then they can't
> >> >> >> support the perf_counter_open syscall either.  Rather than forcing
> >> >> >> everyone to add an ignore (or suffer the warning until they get
> >> >> >> around to implementing support), only whine about the syscall when
> >> >> >> applicable.
> >> >> >
> >> >> > No, this patch is wrong - it's really easy to add support: just hook
> >> >> > up the syscall. This should happen for every architecture really, so
> >> >> > the warning is correct and it should not be patched out.
> >> >> >
> >> >> > PMU support is not required to get perfcounters support: if an
> >> >> > architecture hooks up the syscall it will get generic software
> >> >> > counters and the tools will work as well.
> >> >> >
> >> >> > Profiling falls back to a hrtimer-based sampling method - this is a
> >> >> > much better fallback than oprofile's fall-back to the timer tick.
> >> >> > This hrtimer based sampling is dynticks/nohz-correct and can go
> >> >> > beyond HZ if the architecture supports hrtimers.
> >> >>
> >> >> if there is generic support available, why must every arch select
> >> >> HAVE_PERF_COUNTERS in their Kconfig ?
> >> >
> >> > Because we only want to enable it on architectures that have tested
> >> > it. It should only need a syscall addition, but nothing beats having
> >> > tested things, hence we have that additional Kconfig symbol.
> >>
> >> that is a pretty weak reason. [...]
> >
> > It isnt - this is proper isolation - dont offer something to the
> > user to enable that 1) cannot be used due to the lack of a syscall
> > 2) has not been tested by anyone on that architecture, ever.
> >
> > That way say build breakages or runtime failures due to perfcounters
> > only become possible on an architecture if the architecture
> > maintainer has hooked up the syscall and has provided
> > HAVE_PERF_COUNTERS explicitly.
> 
> except that the syscall presence is trivial to detect in the code by
> something like:
> #ifndef __NR_perf_counter_open
> # error sorry, your arch has not hooked up perf_counter_open syscall yet
> #endif
> 
> as for "no arch testing yet", there are plenty of drivers which lack
> arch depends in the Kconfig specifically so that it can be *easily*
> tested on random systems out there without requiring manual twiddling.

This is a new kernel subsystem, not just yet another driver.

Which bit of: "we dont want perfcounters to be enabled in the 
Kconfig on architectures that have no syscalls and no testing for 
it" is hard to understand? It is a valid technical concern.

I on the other hand fail to see what specific problem your patch is 
trying to solve.

Anyway - feel free to apply that hack to checksyscalls if you want 
to hide the lack of a feature that could be supported by the 
architecture - we certainly wont enable perfcounters on 
architectures that havent got it tested first.

	Ingo
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