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

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

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

>
>     - Panu -
>
>     - Panu -
>