perlhacktips: Add callgrind profiling documentation

One of the benefits of callgrind is you can profile perls that
haven't been built with debugging symbols, which can be real useful
on live-ish datasets

pod/perlhacktips.pod

@@ -1451,6 +1451,51 @@ on the use of F<gcov> and basic block profiling with gcc, see the
 latest GNU CC manual.  As of gcc 4.8, this is at
 L<http://gcc.gnu.org/onlinedocs/gcc/Gcov-Intro.html#Gcov-Intro>
 
+=head2 callgrind profiling
+
+callgrind is a valgrind tool for profiling source code. Paired
+with kcachegrind (a Qt based UI), it gives you an overview of
+where code is taking up time, as well as the ability
+to examine callers, call trees, and more. One of its benefits
+is you can use it on perl and XS modules that have not been
+compiled with debugging symbols.
+
+If perl is compiled with debugging symbols (C<-g>), you can view
+the annotated source and click around, much like Devel::NYTProf's
+HTML output.
+
+For basic usage:
+
+    valgrind --tool=callgrind ./perl ...
+
+By default it will write output to F<callgrind.out.PID>, but you
+can change that with C<--callgrind-out-file=...>
+
+To view the data, do:
+
+    kcachegrind callgrind.out.PID
+
+If you'd prefer to view the data in a terminal, you can use
+F<callgrind_annotate>. In it's basic form:
+
+    callgrind_annotate callgrind.out.PID | less
+
+Some useful options are:
+
+=over 4
+
+=item * --threshold
+
+Percentage of counts (of primary sort event) we are interested in.
+The default is 99%, 100% might show things that seem to be missing.
+
+=item * --auto
+
+Annotate all source files containing functions that helped reach
+the event count threshold.
+
+=back
+
 =head1 MISCELLANEOUS TRICKS
 
 =head2 PERL_DESTRUCT_LEVEL