[lttng-dev] LTTng: global_dirty_limit symbol lookup failed

[Gabbasov, Andrew]

> Hi Andrew,
> 
> Commit:
> 
> commit b87700e318c27267890cbd6fb5e50b687279131b
> Author: Andrew Gabbasov <andrew_gabbasov at mentor.com>
> Date:   Mon Dec 10 11:14:52 2012 -0500
> 
>     Add new kernel probes instrumentation
> 
>     Add kernel probes for btrfs, compaction, ext4, printk, random, rcu,
>     regmap, rpm, sunrpc, workqueue, writeback.
> 
>     Signed-off-by: Andrew Gabbasov <andrew_gabbasov at mentor.com>
>     Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
> 
> has 2 issues:
> 
> 1) it implements a wrapper around global_dirty_limit() in probes/. All
>    wrappers should be in /wrapper/, without exception, otherwise the
>    code will quickly become unmaintainable (we need to know what
>    wrappers we can eventually remove if we remove support for older
>    kernels in the future, and we need to be aware of those wrappers for
>    complete testing coverage). I have no problem with having a
>    probes/Makefile using a relative path (e.g. ../wrapper/somefile.o) as
>    input if needed.
> 
> 2) It fills my dmesg log output with:
>    "LTTng: global_dirty_limit symbol lookup failed."
>    messages. .config follows.
> 
> Can you look into those issues ?
> 
> Thanks,
> 
> Mathieu

Hi Mathieu,

Sorry for a late response.

I'm sending a separate e-mail with the patch to fix these issues. Here are some details:

1) I moved the wrapper to a separate file wrapper/writeback.h. Since it is supposed to be used
in writeback probe only, I think, using static variable will be harmless.

2) Since global_dirty_limit is not a function, but a global variable, its address is available
to kallsyms lookup only if CONFIG_KALLSYMS_ALL is set. In order not to introduce wrapper
related checks of this config into tracepoint definition include file, I decided not to exclude
separate tracepoints in case of this config absence, but to omit the whole probe from
building. So, writeback probe will be available only if CONFIG_KALLSYMS_ALL is set.

3) Additional issue that I noticed: in case of CONFIG_THUMB2_KERNEL setting for ARM,
kallsyms_lookup_funcptr modifies the address found for the symbol. In order to avoid
it for data addresses, I introduced a new kallsyms_lookup_dataptr function, that does not
do any address modifications. And this new function is used for global_dirty_limit wrapper.

Thanks.

Best regards,
Andrew