Re: Reduction of guru use in systemtap war stories

[William Cohen <wcohen@redhat.com>]

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Frank Ch. Eigler wrote:
> wcohen wrote:
> 
>> I am looking through the war stories and figuring out how to clean
>> things up.  [...]
> 
> Thanks.
> 
>> WSthreadCPUshare uses guru mode to determine the instruction pointer
>> value for the sample and then determine whether this value is in
>> kernel or user-space. For some 32-bit kernels this approach is not
>> correct. A better approach would be make the kernel function
>> user_mode() available in the tapsets. [...]
> 
> Indeed, please make it so!

Attached is a simple little patch for the context.stp tapset and documentation 
to "make it so." Assuming that it looks okay I will commit it.

> 
>> Some of the other war stories (WSPfiles, WSPsig, and WSPlimit) the
>> idiom to map a pid to a task struct point comes up and would be a
>> candidate to add to the tapsets. It looks like some of the other guru
>> functions in those scripts are already available and can be recast as
>> task.stp tapset functions. [...]
> 
> See bug #6525 for a probably more robust way.  The one used in some of
> that embedded-c code, find_task_by_pid(), is not safe to call from all
> contexts.

Thanks for the pointer to 6525.

>> Several of the war stories (such as WSPfiles) include header files
>> and make use of defines to extract bits/flags out of values. Seems
>> like there should be a cleaner way to make those bit values
>> available to scripts.
> 
> One possibility is to lobby the kernel to run gcc with -g3 or somesuch
> to get it to dump macro definitions into the dwarf data; then we could
> pull it out of there with $MACRO.  Sounds unlikely.

At the moment asking for -g3 sounds unlikely. The debuginfo is already large. 
Adding in all the macros is going to make the debug information even larger.

> Other ideas?

-Will


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diff --git a/stapfuncs.5.in b/stapfuncs.5.in
index 6d4546a..d8a1a55 100644
--- a/stapfuncs.5.in
+++ b/stapfuncs.5.in
@@ -252,6 +252,9 @@ Return the probe point's module name, if known.
 target:long ()
 Return the pid of the target process.
 .TP
+user_mode:long ()
+Return 1 if the probe point occurred in user-mode.
+.TP
 is_return:long ()
 Return 1 if the probe point is a return probe.  Deprecated.
 
diff --git a/tapset/context.stp b/tapset/context.stp
index 45e3be3..d2195f8 100644
--- a/tapset/context.stp
+++ b/tapset/context.stp
@@ -140,6 +140,11 @@ function registers_valid:long () %{ /* pure */
 	THIS->__retvalue = (CONTEXT->regs != NULL);
 %}
 
+function user_mode:long () %{ /* pure */ /* currently a user-mode address? */
+  if (CONTEXT->regs)
+    THIS->__retvalue = (uint64_t) user_mode (CONTEXT->regs);
+%}
+
 function is_return:long () %{ /* pure */
 	if (CONTEXT->pi)
 		THIS->__retvalue = 1;