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

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

[Arnd Bergmann]

From: Arnd Bergmann <arnd@arndb.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@kernel.org>
Tested-by: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.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

Re: [PATCH] [v3] tracing: use %ps format string to print symbols

[Masami Hiramatsu]

On Tue, 19 Oct 2021 17:33:13 +0200
Arnd Bergmann <arnd@kernel.org> wrote:

> From: Arnd Bergmann <arnd@arndb.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@kernel.org>
> Tested-by: Tom Zanussi <zanussi@kernel.org>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Looks good to me.

Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>

BTW, there are other functions under kernel/trace still allocating
"char str[KSYM_SYMBOL_LEN]". I guess those also should be changed if
this is the stack usage issue, right?

Thank you,

> ---
> 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
> 


-- 
Masami Hiramatsu <mhiramat@kernel.org>