linux-kernel - [PATCH] [v3] tracing: use %ps format string to print symbols

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Message-Id: <20211019153337.294790-1-arnd@kernel.org>
Date:   Tue, 19 Oct 2021 17:33:13 +0200
From:   Arnd Bergmann <arnd@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>
Cc:     Arnd Bergmann <arnd@...db.de>, Tom Zanussi <zanussi@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: [PATCH] [v3] tracing: use %ps format string to print symbols

From: Arnd Bergmann <arnd@...db.de>

clang started warning about excessive stack usage in
hist_trigger_print_key()

kernel/trace/trace_events_hist.c:4723:13: error: stack frame size (1336) exceeds limit (1024) in function 'hist_trigger_print_key' [-Werror,-Wframe-larger-than]

The problem is that there are two 512-byte arrays on the stack if
hist_trigger_stacktrace_print() gets inlined. I don't think this has
changed in the past five years, but something probably changed the
inlining decisions made by the compiler, so the problem is now made
more obvious.

Rather than printing the symbol names into separate buffers, it
seems we can simply use the special %ps format string modifier
to print the pointers symbolically and get rid of both buffers.

Marking hist_trigger_stacktrace_print() would be a simpler
way of avoiding the warning, but that would not address the
excessive stack usage.

Fixes: 69a0200c2e25 ("tracing: Add hist trigger support for stacktraces as keys")
Link: https://lore.kernel.org/all/20211015095704.49a99859@gandalf.local.home/
Reviewed-by: Tom Zanussi <zanussi@...nel.org>
Tested-by: Tom Zanussi <zanussi@...nel.org>
Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
Changes in v3:
 - Fix 32-bit cast warning

Changes in v2:
 - Use %pS instead of %ps to include offset in third string
 - add (void*) cast to avoid compile-time warnings
---
 kernel/trace/trace_events_hist.c | 15 +++++----------
 1 file changed, 5 insertions(+), 10 deletions(-)

diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index f01e442716e2..632386b73fc3 100644
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -4706,7 +4706,6 @@ static void hist_trigger_stacktrace_print(struct seq_file *m,
 					  unsigned long *stacktrace_entries,
 					  unsigned int max_entries)
 {
-	char str[KSYM_SYMBOL_LEN];
 	unsigned int spaces = 8;
 	unsigned int i;
 
@@ -4715,8 +4714,7 @@ static void hist_trigger_stacktrace_print(struct seq_file *m,
 			return;
 
 		seq_printf(m, "%*c", 1 + spaces, ' ');
-		sprint_symbol(str, stacktrace_entries[i]);
-		seq_printf(m, "%s\n", str);
+		seq_printf(m, "%pS\n", (void*)stacktrace_entries[i]);
 	}
 }
 
@@ -4726,7 +4724,6 @@ static void hist_trigger_print_key(struct seq_file *m,
 				   struct tracing_map_elt *elt)
 {
 	struct hist_field *key_field;
-	char str[KSYM_SYMBOL_LEN];
 	bool multiline = false;
 	const char *field_name;
 	unsigned int i;
@@ -4747,14 +4744,12 @@ static void hist_trigger_print_key(struct seq_file *m,
 			seq_printf(m, "%s: %llx", field_name, uval);
 		} else if (key_field->flags & HIST_FIELD_FL_SYM) {
 			uval = *(u64 *)(key + key_field->offset);
-			sprint_symbol_no_offset(str, uval);
-			seq_printf(m, "%s: [%llx] %-45s", field_name,
-				   uval, str);
+			seq_printf(m, "%s: [%llx] %-45ps", field_name,
+				   uval, (void *)(uintptr_t)uval);
 		} else if (key_field->flags & HIST_FIELD_FL_SYM_OFFSET) {
 			uval = *(u64 *)(key + key_field->offset);
-			sprint_symbol(str, uval);
-			seq_printf(m, "%s: [%llx] %-55s", field_name,
-				   uval, str);
+			seq_printf(m, "%s: [%llx] %-55pS", field_name,
+				   uval, (void *)(uintptr_t)uval);
 		} else if (key_field->flags & HIST_FIELD_FL_EXECNAME) {
 			struct hist_elt_data *elt_data = elt->private_data;
 			char *comm;
-- 
2.29.2