Re: [PATCH 1/2] mm: page_alloc.c: Add tracepoints for slowpath

On Wed, 27 Jul 2016 10:20:28 -0700
Dave Hansen [off-list ref] wrote:

On 07/27/2016 08:23 AM, Steven Rostedt wrote:

quoted

quoted

quoted

+
+	trace_mm_slowpath_end(page);
+  

I'm thinking you only need one tracepoint, and use function_graph
tracer for the length of the function call.

 # cd /sys/kernel/debug/tracing
 # echo __alloc_pages_nodemask > set_ftrace_filter
 # echo function_graph > current_tracer
 # echo 1 > events/kmem/trace_mm_slowpath/enable  

I hesitate to endorse using the function_graph tracer for this kind of
stuff.  Tracepoints offer some level of stability in naming, and the
compiler won't ever make them go away.   While __alloc_pages_nodemask is
probably more stable than most things, there's no guarantee that it will
be there.

Well, then you are also advocating in a userspace ABI interface that
will have to be maintained forever. Just be warned.

BTW, what's the overhead of the function graph tracer if the filter is
set up to be really restrictive like above?  Is the overhead really just
limited to that one function?

Yes, if DYNAMIC_FTRACE is defined. Which it should be, because static
ftrace has a huge overhead without enabling the tracer.

It will enable only that function to be traced. I've recommend before
that if one wants to have a good idea of how long a function lasts,
they should filter to a single function. Anything else will include
overhead of the tracer itself.

-- Steve

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