Re: [PATCH] tracing/events: Add module tracepoints

[Li Zefan <lizf@cn.fujitsu.com>]

Frederic Weisbecker wrote:
> On Tue, Jul 21, 2009 at 04:56:33PM +0800, Li Zefan wrote:
>> Add trace points to trace module_load, module_free, module_get,
>> module_put and module_request, and use trace_event facility
>> to get the trace output.
>>
>> Here's the sample output:
>>
>>      TASK-PID    CPU#    TIMESTAMP  FUNCTION
>>         | |       |          |         |
>>     <...>-42    [000]     1.758380: module_request: fb0 wait=1 call_site=fb_open
>>     ...
>>     <...>-60    [000]     3.269403: module_load: scsi_wait_scan 
>>     <...>-60    [000]     3.269432: module_put: scsi_wait_scan call_site=sys_init_module refcnt=0
>>     <...>-61    [001]     3.273168: module_free: scsi_wait_scan
>>     ...
>>     <...>-1021  [000]    13.836081: module_load: sunrpc 
>>     <...>-1021  [000]    13.840589: module_put: sunrpc call_site=sys_init_module refcnt=-1
>>     <...>-1027  [000]    13.848098: module_get: sunrpc call_site=try_module_get refcnt=0
>>     <...>-1027  [000]    13.848308: module_get: sunrpc call_site=get_filesystem refcnt=1
>>     <...>-1027  [000]    13.848692: module_put: sunrpc call_site=put_filesystem refcnt=0
>>     ...
>>  modprobe-2587  [001]  1088.437213: module_load: trace_events_sample F
>>  modprobe-2587  [001]  1088.437786: module_put: trace_events_sample call_site=sys_init_module refcnt=0
>>
>>
>> Note:
>>
>> - the taints flag can be 'F', 'C' and/or 'P' if mod->taints != 0
>>
>> - the module refcnt is percpu, so it can be negative in a specific cpu
>>
>> Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
> 
> 
> Nice.
> 
> Just two worries about it.
> 
> The ring buffer are flushed on module unloading right?
> That won't make it easy to perform module event tracing.
> 

Yes, but only when the module has "_ftrace_events" section.
They are ext4 and gfs2 for the current kernel.

> Also the events selftests do a lot of random things to trigger
> each kind of events, I guess some new others will be needed to
> tests these, unless they will seem to fail on every selftests.
> Although I can't imagine a module loading/unloading for
> every ftrace event selftest... I guess these will require
> a specific treatement and also will need to be selftested once
> the filesystem is set to be able to load modules.
> 

It's nice to have more selftests but I don't think it is a
necessity.

Actually the events selftests just confirm the events won't
crash the system, and no trace entries are generated by them
in the selftests.