GCC 里经常碰到的option: -fomit-frame-pointer，意思是：
Don’t keep the frame pointer in a register for functions that don’t need one. This avoids the instructions to save, set up and restore frame pointers; it also makes an extra register available in many functions. It also makes debugging impossible on some machines.
其实就是在x86编译的时候忽略EBP， 在arm编译时忽略r11。这两个东东其实就是栈基地址。
kernel默认都是忽略fp，可以通过kernel debug选项开启。但是sched.c默认是用-fno-omit-frame-pointer 编译的，主要是由于/proc//wchan需要做栈回溯。
讨论见：
内核分析见：http://weichong78.blogspot.com/2006/10/wchan-waiting-channel.html
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