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

[Marc Sune <marc.sune@bisdn.de>]

On 03/03/15 14:31, Ananyev, Konstantin wrote:
>
>> -----Original Message-----
>> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Bruce Richardson
>> Sent: Tuesday, March 03, 2015 1:03 PM
>> To: Marc Sune
>> Cc: dev@dpdk.org
>> Subject: Re: [dpdk-dev] [PATCH] mk: add support for gdb debug info generation
>>
>> 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]
>>
> Well, but then people would say 'why only -g -O0' for _DEBUG config option?
> And would ask for ability to add/change other compile options to add/change for particular library.
> If we need that, probably we can introduce RTE_LIBRTE_<LIBNAME>_EXTRA_CFLAGS config macros for each library?
> So people can set them in their config files to whatever they like?

It is not a bad idea either. However, there are already those _DEBUG 
config elements (the KNI is a special case).

Maybe both can be combined (i.e. RTE_LIBRTE_<LIBNAME>_EXTRA_CFLAGS 
should always override _DEBUG). It would probably get a bit 
over-engineered then, though.

marc
>
> Konstantin