[PATCH] ARM: imx_v6_v7_defconfig: Explicitly restore CONFIG_DEBUG_FS
Paul E. McKenney
paulmck at linux.vnet.ibm.com
Mon May 29 08:11:00 PDT 2017
On Mon, May 29, 2017 at 02:58:36PM +0300, Leonard Crestez wrote:
> On Fri, 2017-05-26 at 08:42 -0700, Paul E. McKenney wrote:
> > On Fri, May 26, 2017 at 02:26:06PM +0300, Leonard Crestez wrote:
> > >
> > > This option was removed by "make savedefconfig" in
> > > commit c5054a98bce4 ("ARM: imx_v6_v7_defconfig: Select SMSC_PHY")
> > >
> > > This happened because CONFIG_DEBUG_FS was implicitly selected by
> > > CONFIG_TREE_RCU_TRACE which defaulted to true because CONFIG_RCU_TRACE
> > > was enabled by default by commit 961518259b3b ("rcu: Enable RCU
> > > tracepoints by default to aid in debugging")
> > >
> > > Recently however CONFIG_RCU_TRACE was completely removed by
> > > commit 6e74c237c410 ("rcu: Remove debugfs tracing")
>
> > CONFIG_RCU_TRACE is still very much alive in its new home at
> > kernel/rcu/Kconfig.debug:
>
> Sorry, what was removed is the dependency on DEBUG_FS. In particular
> this snippet:
>
> diff --git a/init/Kconfig b/init/Kconfig
> index 2aa14ff..3025383 100644
> --
> - a/init/Kconfig
> +++ b/init/Kconfig
> @@ -659,14 +659,6 @@ config
> RCU_FAST_NO_HZ
>
> Say N if you are unsure.
>
> -config
> TREE_RCU_TRACE
> - def_bool RCU_TRACE && ( TREE_RCU || PREEMPT_RCU )
> - select DEBUG_FS
Ah, you are right. RCU no longer needs DEBUG_FS, so it no longer
selects it.
> - help
> - This option provides
> tracing for the TREE_RCU and
> - PREEMPT_RCU implementations,
> permitting Makefile to
> - trivially select
> kernel/rcutree_trace.c.
> -
> config RCU_BOOST
> bool "Enable RCU
> priority boosting"
> depends on RT_MUTEXES && PREEMPT_RCU &&
> RCU_EXPERT
>
> > config RCU_TRACE
> > bool "Enable tracing for RCU"
> > depends on DEBUG_KERNEL
> > default y if TREE_RCU
> > select TRACE_CLOCK
> > help
> > This option enables additional tracepoints for ftrace-style
> > event tracing.
> >
> > Say Y here if you want to enable RCU tracing
> > Say N if you are unsure.
> >
> > That said, I need to make it default to "y" if PREEMPT_RCU as well as
> > the current TREE_RCU. Would that help?
>
> I don't think you can help unless you want to make RCU_TRACE depend on
> DEBUG_FS for no reason. The proper fix is to have DEBUG_FS inside the
> imx_v6_v7_defconfig.
>
> My problem is just an unfortunate accident with default dependencies
> and overzealous savedefconfig.
Right you are!
> As far as I understand after recent changes RCU_TRACE is now only
> useful with tracepoints. Shouldn't it depend on TRACEPOINTS in some
> way? It's still possible to compile with RCU_TRACE on and TRACEPOINTS
> off, is this intentional? It seems that when this happens you're just
> relying on tracepoint macros to expand to nothing.
>
> Fr example maybe RCU_TRACE should be "default y if (TREE_RCU &&
> TRACEPOINTS); depends on TRACEPOINTS"?
>
> In theory you could also "select TRACEPOINTS" but then you would end up
> enabling TRACEPOINTS by default in all configurations that use TREE_RCU
> but don't explicitly disable RCU_TRACE.
>
> I'd say that if $SUBSYSTEM_DEBUG depends on $MAJOR_DEBUG_FEATURE it
> should try to avoid selecting $MAJOR_DEBUG_FEATURE by default if it's
> not otherwise enabled. As far as I can tell this is how RCU ended up
> indirectly pulling in DEBUG_FS and later unexpectedly removed it.
>
> Or I could be completely wrong, kconfig can be very confusing.
Interesting questions! I will give them some thought.
Perhaps TRACEPOINTS is accidentally being enabled for me just as
DEBUG_FS was being accidentally enabled for you. ;-)
Thanx, Paul
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