CONFIG_DEBUG_INFO_SPLIT impacts on faddr2line

[Fengguang Wu]

CC Andi and more DEBUG_INFO_SPLIT people.

On Sun, Nov 12, 2017 at 11:31:56AM -0800, Linus Torvalds wrote:
> On Wed, Nov 8, 2017 at 9:12 AM, Fengguang Wu <fengguang.wu@xxxxxxxxx> wrote:
> > OK. Here is the original faddr2line output:
> >
> > $ ~/linux/scripts/faddr2line vmlinux vlan_device_event+0x7f5/0xa40
> > vlan_device_event+0x7f5/0xa40:
> > vlan_device_event at net/8021q/vlan.h:60
> >
> > And below is call trace embedded with full faddr2line output.
> >
> > I notice that this trace shows no additional inline files at all.
> > Is it because I did some kconfig option wrong, so that inline info is
> > lost? Eg.
> >
> > CONFIG_OPTIMIZE_INLINING=y (it looks better set to N)
> > CONFIG_DEBUG_INFO_REDUCED=y
> > CONFIG_DEBUG_INFO_SPLIT=y
>
> Ok, this annoyed me, so I went back and looked.
>
> It's the "CONFIG_DEBUG_INFO_SPLIT" thing that makes faddr2line unable
> to see the inlining information,
>
> Using OPTIMIZE_INLINING is fine.

Good to know that!

> I'm not sure that addr2line could be made to understand the .dwo files
> that DEBUG_INFO_SPLIT causes (particularly since we munge the vmlinux
> file itself, who knows how that could confuse things).
>
> So can I ask that you make the 0day build scripts always use
>
>  CONFIG_DEBUG_INFO=y
>  CONFIG_DEBUG_INFO_REDUCED=y
>  # CONFIG_DEBUG_INFO_SPLIT is not set
>
> because with that "DEBUG_INFO_REDUCED=y", the use of DEBUG_INFO_SPLIT
> shouldn't be _that_ big of a deal.
>
> Yes, splitting the debug info does help reduce disk usage for the
> build, and presumably speed it up a bit too due to less IO and reduced
> copying of the debug info data, but right now it really makes the
> debug info much less useful.

Yes DEBUG_INFO_SPLIT helps reduce build cost. Equally importantly,
it helps cut down the *.ko sizes, which saves boot test cost, too.
Since in our test scheme, the below modules.cgz will be loaded as part
of initrd on boot testing. Which will cost memory, and to the lesser
degree, IO and uncompressing time.

Here is the diff of the modules.cgz size:

Big files under /pkg/linux/x86_64-rhel-7.2+CONFIG_DEBUG_INFO_REDUCED/gcc-6/v4.14-rc7/,
comparing to +CONFIG_DEBUG_INFO_SPLIT:

=>    54M  135M  modules.cgz
    7.3M  7.3M  vmlinuz-4.14.0-rc7
    1.2M  1.2M  linux-headers.cgz
    7.6M  7.7M  linux-selftests.cgz
     31M   31M  linux-perf.cgz

Nevertheless, that's machine cost. If DEBUG_INFO_SPLIT hurts our
ability to analyze bugs, I think the forthright way would be to
disable it in our tests.

> Just to see the difference:
>
> - with DEBUG_INFO_SPLIT=y
>
>    [torvalds@i7 linux]$ ./scripts/faddr2line vmlinux __schedule+0x314
>    __schedule+0x314/0x840:
>    __schedule at kernel/sched/stats.h:12
>
> - with DEBUG_INFO_SPLIT is not set
>
>    [torvalds@i7 linux]$ ./scripts/faddr2line vmlinux __schedule+0x314
>    __schedule+0x314/0x840:
>    rq_sched_info_arrive at kernel/sched/stats.h:12
>     (inlined by) sched_info_arrive at kernel/sched/stats.h:99
>     (inlined by) __sched_info_switch at kernel/sched/stats.h:151
>     (inlined by) sched_info_switch at kernel/sched/stats.h:158
>     (inlined by) prepare_task_switch at kernel/sched/core.c:2582
>     (inlined by) context_switch at kernel/sched/core.c:2755
>     (inlined by) __schedule at kernel/sched/core.c:3366
>
> and while (once again) this is a pretty extreme case, we do use a lot
> of inlines, and gcc will add its own inlining. Getting this whole
> information - particularly for the faulting IP - would really help in
> some situations.
>
> I love what the 0day robot is doing, this would be another big step forward.

Thank you for the helpful information and appreciations!
I'll make the change to disable DEBUG_INFO_SPLIT.

> Oh - and talking about "big step forward" - does the 0day robot do any
> suspend/resume testing at all?

Yes, we do. CC Rui and Aaron on power testing.

> Even on non-laptop hardware, it should be possible to do something like
>
>    echo platform > /sys/power/pm_test
>    echo freeze > /sys/power/state
>
> or similar (assuming CONFIG_PM_DEBUG is enabled).
>
> Maybe you already do something like this?

Rui/Aaron have better knowledge on the current status. It does look an
error-prone area that's worth more testing efforts.

> Anyway, regardless this was a good release for the 0day robot. Thanks.

My (and our) pleasure. I'd like to thank you and all the people who
take time to analyze/fix the bugs. It's great to see the long standing
bugs being fixed in mainline -- they have been a big source of noises
that hurt our auto bisect&reporting capabilities.

Regards,
Fengguang