linux-kernel - Re: [PATCH] ARM: imx_v6_v7_defconfig: Explicitly restore CONFIG_DEBUG

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Message-Id: <20170529151100.GJ3956@linux.vnet.ibm.com>
Date:   Mon, 29 May 2017 08:11:00 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Leonard Crestez <leonard.crestez@....com>
Cc:     Shawn Guo <shawnguo@...nel.org>,
        Fabio Estevam <fabio.estevam@....com>,
        Dong Aisheng <aisheng.dong@....com>,
        linux-arm-kernel@...ts.infradead.org,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Octavian Purdila <octavian.purdila@....com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ARM: imx_v6_v7_defconfig: Explicitly restore
 CONFIG_DEBUG_FS

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