scripts: ftrace - move the nop-processing in ftrace_init to compile time

When ftrace is enabled, ftrace_init will consume a period of
time, usually around 15~20ms. Approximately 60% of the time is
consumed by nop-processing. Moving the nop-processing to the
compile time can speed up the kernel boot process.

performance test:
        env:    Intel(R) Xeon(R) CPU E5-2682 v4 @ 2.50GHz
        method: before and after patching, compare the
                total time of ftrace_init(), and verify
                the functionality of ftrace.

        avg_time of ftrace_init:
                with patch: 7.114ms
                without patch: 15.763ms

Signed-off-by: Yinan Liu <yinan@linux.alibaba.com>

kernel/trace/ftrace.c

@@ -6261,6 +6261,10 @@ static int ftrace_process_locs(struct module *mod,
 	 * until we are finished with it, and there's no
 	 * reason to cause large interrupt latencies while we do it.
 	 */
+#if defined CONFIG_X86 || defined CONFIG_X86_64 || defined CONFIG_ARM || defined CONFIG_ARM64
+	ret = 0;
+	goto out;
+#endif
 	if (!mod)
 		local_irq_save(flags);
 	ftrace_update_code(mod, start_pg);

scripts/recordmcount.h

@@ -406,6 +406,8 @@ static uint_t *sift_rel_mcount(uint_t *mlocp,
 			       uint_t const recval,
 			       unsigned const reltype)
 {
+	Elf_Shdr *const shdr0 = (Elf_Shdr *)(_w(ehdr->e_shoff) + (void *)ehdr);
+	Elf_Shdr const *const shdr = &shdr0[w(relhdr->sh_info)];
 	uint_t *const mloc0 = mlocp;
 	Elf_Rel *mrelp = *mrelpp;
 	Elf_Sym const *sym0;
@@ -419,6 +421,7 @@ static uint_t *sift_rel_mcount(uint_t *mlocp,
 	get_sym_str_and_relp(relhdr, ehdr, &sym0, &str0, &relp);
 
 	for (t = nrel; t; --t) {
+		int ret = -1;
 		if (!mcountsym)
 			mcountsym = get_mcountsym(sym0, relp, str0);
 
@@ -436,6 +439,17 @@ static uint_t *sift_rel_mcount(uint_t *mlocp,
 				*mlocp++ = addend;
 
 			mrelp = (Elf_Rel *)(rel_entsize + (void *)mrelp);
+			/* convert mcount into nop */
+			if (make_nop)
+				ret = make_nop((void *)ehdr,
+						_w(shdr->sh_offset) + _w(relp->r_offset));
+			if (!ret) {
+				Elf_Rel rel;
+				rel = *(Elf_Rel *)relp;
+				Elf_r_info(&rel, Elf_r_sym(relp), rel_type_nop);
+				ulseek(fd_map, (void *)relp - (void *)ehdr, SEEK_SET);
+				uwrite(fd_map, &rel, sizeof(rel));
+			}
 		}
 		relp = (Elf_Rel const *)(rel_entsize + (void *)relp);
 	}