Re: [dpdk-dev] [PATCH] mk: add support for gdb debug info generation

[Bruce Richardson <bruce.richardson@intel.com>]

On Tue, Mar 03, 2015 at 01:56:19PM +0100, Marc Sune wrote:
> 
> On 03/03/15 13:40, Panu Matilainen wrote:
> >On 03/03/2015 02:19 PM, Marc Sune wrote:
> >>
> >>On 03/03/15 10:33, Bruce Richardson wrote:
> >>>On Mon, Mar 02, 2015 at 06:32:13PM +0100, Marc Sune wrote:
> >>>>On 22/02/15 12:51, Marc Sune wrote:
> >>>>>I don't like the proposed patch, but I am recovering this old thread
> >>>>>because I agree on the problem statement.
> >>>>>
> >>>>>On 04/04/14 11:57, Ananyev, Konstantin wrote:
> >>>>>>Hi Cyril,
> >>>>>>We already do have 'EXTRA_CFLAGS' and 'EXTRA_LDFLAGS' that you
> >>>>>>can use
> >>>>>>to enable debug, or any other compiler/linker options you need.
> >>>>>>Wonder, why that is not enough?
> >>>>>EXTRA_FLAGS var affects all the DPDK libraries. I was wondering why
> >>>>>setting individually:
> >>>>>
> >>>>>diff --git a/config/common_linuxapp b/config/common_linuxapp
> >>>>>index 2f9643b..04adc0d 100644
> >>>>>--- a/config/common_linuxapp
> >>>>>+++ b/config/common_linuxapp
> >>>>>@@ -127,7 +127,7 @@ CONFIG_RTE_LIBRTE_KVARGS=y
> >>>>># Compile generic ethernet library
> >>>>>#
> >>>>>CONFIG_RTE_LIBRTE_ETHER=y
> >>>>>-CONFIG_RTE_LIBRTE_ETHDEV_DEBUG=n
> >>>>>+CONFIG_RTE_LIBRTE_ETHDEV_DEBUG=y
> >>>>>
> >>>>>
> >>>>>to put an example, does not set -g and -O0 in that particular module
> >>>>>only.
> >>>>>No one would ever use something compiled in DEBUG in production
> >>>>>anyway.
> >>>>>
> >>>>>I always end up modifying manually Makefiles in the lib library that
> >>>>>I am
> >>>>>interested in having insides, overriding CFLAGS=-O3, which is not
> >>>>>that
> >>>>>nice.
> >>>>I would like some feedback on this idea. If the community sees
> >>>>benefit, I
> >>>>will work on a patch for this.
> >>>>
> >>>>Marc
> >>>>
> >>>So, your proposal is to patch things so that any time one sets DEBUG=y
> >>>in the
> >>>build-time config for a library, we change the '-O3' to '-O0' and set
> >>>-g also.
> >>>Correct?
> >>
> >>I am not sure what you mean by 'patch things'. I would simply enable the
> >>build system to override the default compilation flags (now DPDK-wide,
> >>or specifically librte_ wide) when _DEBUG=y for a library, changing
> >>compilation flags from -O3 to -O0 -g and possibly also -fno-inline. I
> >>have to check if -O0 already implicitly means -fno-inline (even for
> >>__attribute__((always_inline)) ).
> >>
> >>I did a quick test. I chose KNI because it didn't have a DEBUG flag for
> >>the user-space library. For other libraries, the existing _DEBUG setting
> >>would be enough:
> >>
> >>marc@dpdk:~/dpdk$ git diff HEAD
> >>diff --git a/config/common_linuxapp b/config/common_linuxapp
> >>index 97f1c9e..8a3cef8 100644
> >>--- a/config/common_linuxapp
> >>+++ b/config/common_linuxapp
> >>@@ -405,6 +405,7 @@ CONFIG_RTE_LIBRTE_PIPELINE=y
> >>  #
> >>  CONFIG_RTE_LIBRTE_KNI=y
> >>  CONFIG_RTE_KNI_PREEMPT_DEFAULT=y
> >>+CONFIG_RTE_LIBRTE_KNI_DEBUG=y
> >>  CONFIG_RTE_KNI_KO_DEBUG=n
> >>  CONFIG_RTE_KNI_VHOST=n
> >>  CONFIG_RTE_KNI_VHOST_MAX_CACHE_SIZE=1024
> >>diff --git a/lib/librte_kni/Makefile b/lib/librte_kni/Makefile
> >>index 7107832..895f64e 100644
> >>--- a/lib/librte_kni/Makefile
> >>+++ b/lib/librte_kni/Makefile
> >>@@ -34,7 +34,7 @@ include $(RTE_SDK)/mk/rte.vars.mk
> >>  # library name
> >>  LIB = librte_kni.a
> >>
> >>-CFLAGS += $(WERROR_FLAGS) -I$(SRCDIR) -O3 -fno-strict-aliasing
> >>+CFLAGS += $(WERROR_FLAGS) -I$(SRCDIR) $(CONFIG_RTE_LIBRTE_KNI_CFLAGS)
> >>
> >>  EXPORT_MAP := rte_kni_version.map
> >>
> >>diff --git a/mk/rte.app.mk b/mk/rte.app.mk
> >>index 63a41e2..eee477d 100644
> >>--- a/mk/rte.app.mk
> >>+++ b/mk/rte.app.mk
> >>@@ -78,6 +78,13 @@ endif
> >>  ifeq ($(CONFIG_RTE_LIBRTE_KNI),y)
> >>  ifeq ($(CONFIG_RTE_EXEC_ENV_LINUXAPP),y)
> >>  LDLIBS += -lrte_kni
> >>+
> >>+ifeq ($(CONFIG_RTE_LIBRTE_KNI_DEBUG),y)
> >>+CONFIG_RTE_LIBRTE_KNI_CFLAGS = -O0 -g -fno-strict-aliasing -fno-inline
> >>+else
> >>+CONFIG_RTE_LIBRTE_KNI_CFLAGS = -O3 -fno-strict-aliasing
> >>+endif
> >>+
> >>  endif
> >>  endif
> >>
> >>
> >>Thoughts?
> >
> >My 5c is that if anything, DPDK needs *less* places that muck around with
> >compiler flags, not more. If you something like this for all the libraries
> >in DPDK the number doesn't just increase a bit, it explodes.
> 
> If you check the part below this one in my original email, that you stripped
> out (without notice), the suggestion was also to add a global _DEBUG
> parameter for the entire DPDK set of libraries, to change all the CFLAGS at
> once (not in the attached PATCH).
> 
> >
> >I dont see that much point in this thing, but I'd approach it by defining
> >the debug flags someplace central, say DEBUG_FLAGS, and append that to the
> >common cflags when *_DEBUG config is enabled. At least with gcc the last
> >option wins so if you just append -O0 when debugging then that's what
> >wins, the earlier -O3 does not matter.
> 
> The original problem is the one you expose; libraries hardcode the CFLAGS,
> ignoring user-flags. There is no way to change this unless you change the
> Makefiles directly.
> 
> But right now, each library does hardcode its *own* flags (check Makefiles
> for the libraries), so there is already not a unified approach here. I see
> for instance KNI having -fno-strict-aliasing while other libraries don't.
> 
> Having said that, there are moments, specially with -O3, in which to be able
> to reproduce a bug, you need to compile certain parts of code with -O3 and
> the rest with -O0 -g (the ones to be debugged). The approach proposed (both
> a global *and* a lib specific) allows that.
> 
> Marc
> 

I believe that the global option of overriding the CFLAGS is already sufficiently
covered - including being documented in programmers guide - by EXTRA_CFLAGS. The
ability to turn off optimization support for a single library is not covered
anywhere, and that suggestion seems reasonable to me. For each library, we can
just append '-O0 -g' to the CFLAGS in that libraries makefile if the debug option
is set. I don't see that as significantly complicating things [though I wouldn't
make any changes to the rte.app.mk to allow this, just have it per lib in the
lib's makefile]

/Bruce

> >    - Panu -
> >
> >    - Panu -
> >
>