On Tue, 11 Feb 2025 20:03:38 +0800 Menglong Dong <menglong8.dong@xxxxxxxxx> wrote: > > Another beneficiary can be ftrace. For now, all the kernel functions that > are enabled by dynamic ftrace will be added to a filter hash. And hash > lookup will happen when then traced functions are called, which has an > impact on the performance, see > __ftrace_ops_list_func() -> ftrace_ops_test(). With the per-function metadata > support, we can store the information that if the ftrace ops is enabled on the > kernel function to the metadata. Note, ftrace only uses ftrace_ops_list if there's more than one callback attached to the same function. Otherwise it calls directly to a single trampoline, and is rather efficient. No meta data needed. > > Arm64 and other archs add meta data before the functions too. Can we have > > an effort to perhaps share these methods? > > I have not done research on arm64 yet. AFAIK, arm64 insn is 16-bytes aligned, > so the way we process can be a little different here, as making kernel function > non 16-bytes aligned can have a huge influence. Arm64 already uses the meta data before every function. That's where it stores a pointer to the ftrace_ops. So in ftrace, when there's a single callback attached to a function in arm64, it jumps to a ftrace trampoline, that will reference the function's meta data to find the ftrace_ops to use for that callback. If more than one callback is attached to the same function, then it acts just like x86 and does the loop. -- Steve
