linux-kernel - [PATCH v2 2/2] scripts: ftrace - move the nop-processing in ftrace

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Date:   Tue, 16 Nov 2021 10:49:42 +0800
From:   Yinan Liu <yinan@...ux.alibaba.com>
To:     rostedt@...dmis.org
Cc:     mark-pk.tsai@...iatek.com, peterz@...radead.org, mingo@...hat.com,
        linux-kernel@...r.kernel.org
Subject: [PATCH v2 2/2] scripts: ftrace - move the nop-processing in ftrace_init to compile time

In some business scenarios, GCC versions are so old that
optimizations in ftrace cannot be completed, such as
-mrecord-mcount and -mnop-mcount. The recordmCount in the
kernel is actually used. In this case, ftrace_init will
consume a period of time, usually around 9~12ms. Do nop
substitution in recordmcount.c to speed up ftrace_init.

Signed-off-by: Yinan Liu <yinan@...ux.alibaba.com>
---
 kernel/trace/ftrace.c  |  7 +++++++
 scripts/recordmcount.h | 14 ++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index c776a2956237..ccc690e81556 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -6478,6 +6478,13 @@ 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.
 	 */
+#ifdef CONFIG_FTRACE_MCOUNT_USE_RECORDMCOUNT
+#ifdef CONFIG_HAVE_C_RECORDMCOUNT
+	ret = 0;
+	goto out;
+#endif
+#endif
+
 	if (!mod)
 		local_irq_save(flags);
 	ftrace_update_code(mod, start_pg);
diff --git a/scripts/recordmcount.h b/scripts/recordmcount.h
index 1e9baa5c4fc6..152311639a0b 100644
--- a/scripts/recordmcount.h
+++ b/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);
 	}
-- 
2.14.4.44.g2045bb6