On Thu, Dec 29, 2011 at 14:08, Lei Wen [off-list ref] wrote:

2. Seem dynamic ftrace also could involve some penalty for the running
system, although it patching the running kernel with nop stub...

For the second item, is there anyone done some research before that
could zero the cost for the running system when the tracing is not
enabled yet?

One thing that needs to be fixed (for ARM) is that for the new-style
mcounts, the nop that's currently being done is not really a nop -- it
removes the function call, but there is still an unnecessary push/pop
sequence.  This should be modified to have the push {lr} removed too.
(Two instructions replaced instead of one.)