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[PATCH] tracing: Move trace_handle_return() out of line




Currently trace_handle_return() looks like this:

 static inline enum print_line_t trace_handle_return(struct trace_seq *s)
 {
        return trace_seq_has_overflowed(s) ?
                TRACE_TYPE_PARTIAL_LINE : TRACE_TYPE_HANDLED;
 }

Where trace_seq_overflowed(s) is:

 static inline bool trace_seq_has_overflowed(struct trace_seq *s)
 {
	return s->full || seq_buf_has_overflowed(&s->seq);
 }

And seq_buf_has_overflowed(&s->seq) is:

 static inline bool
 seq_buf_has_overflowed(struct seq_buf *s)
 {
	return s->len > s->size;
 }

Making trace_handle_return() into:

 return (s->full || (s->seq->len > s->seq->size)) ?
           TRACE_TYPE_PARTIAL_LINE :
           TRACE_TYPE_HANDLED;

One would think this is not an issue to keep as an inline. But because this
is used in the TRACE_EVENT() macro, it is extended for every tracepoint in
the system. Taking a look at a single tracepoint x86_irq_vector (was the
first one I randomly chosen). As trace_handle_return is used in the
TRACE_EVENT() macro of trace_raw_output_##call() we disassemble
trace_raw_output_x86_irq_vector and do a diff:

- is the original
+ is the out-of-line code

I removed identical lines that were different just due to different
addresses.

--- /tmp/irq-vec-orig	2017-03-16 09:12:48.569384851 -0400
+++ /tmp/irq-vec-ool	2017-03-16 09:13:39.378153385 -0400
@@ -6,27 +6,23 @@
        53                      push   %rbx
        48 89 fb                mov    %rdi,%rbx
        4c 8b a7 c0 20 00 00    mov    0x20c0(%rdi),%r12
        e8 f7 72 13 00          callq  ffffffff81155c80 <trace_raw_output_prep>
        83 f8 01                cmp    $0x1,%eax
        74 05                   je     ffffffff8101e993 <trace_raw_output_x86_irq_vector+0x23>
        5b                      pop    %rbx
        41 5c                   pop    %r12
        5d                      pop    %rbp
        c3                      retq
        41 8b 54 24 08          mov    0x8(%r12),%edx
-       48 8d bb 98 10 00 00    lea    0x1098(%rbx),%rdi
+       48 81 c3 98 10 00 00    add    $0x1098,%rbx
-       48 c7 c6 7b 8a a0 81    mov    $0xffffffff81a08a7b,%rsi
+       48 c7 c6 ab 8a a0 81    mov    $0xffffffff81a08aab,%rsi
-       e8 c5 85 13 00          callq  ffffffff81156f70 <trace_seq_printf>

 === here's the start of the main difference ===

+       48 89 df                mov    %rbx,%rdi
+       e8 62 7e 13 00          callq  ffffffff81156810 <trace_seq_printf>
-       8b 93 b8 20 00 00       mov    0x20b8(%rbx),%edx
-       31 c0                   xor    %eax,%eax
-       85 d2                   test   %edx,%edx
-       75 11                   jne    ffffffff8101e9c8 <trace_raw_output_x86_irq_vector+0x58>
-       48 8b 83 a8 20 00 00    mov    0x20a8(%rbx),%rax
-       48 39 83 a0 20 00 00    cmp    %rax,0x20a0(%rbx)
-       0f 93 c0                setae  %al
+       48 89 df                mov    %rbx,%rdi
+       e8 4a c5 12 00          callq  ffffffff8114af00 <trace_handle_return>
        5b                      pop    %rbx
-       0f b6 c0                movzbl %al,%eax

 === end ===

        41 5c                   pop    %r12
        5d                      pop    %rbp
        c3                      retq

If you notice, the original has 22 bytes of text more than the out of line
version. As this is for every TRACE_EVENT() defined in the system, this can
become quite large.

   text	   data	    bss	    dec	    hex	filename
8690305	5450490	1298432	15439227	 eb957b	vmlinux-orig
8681725	5450490	1298432	15430647	 eb73f7	vmlinux-handle

This change has a total of 8580 bytes in savings.

 $ objdump -dr /tmp/vmlinux-orig | grep '^[0-9a-f]* <trace_raw_output' | wc -l
324

That's 324 tracepoints. But this does not include modules (which contain
many more tracepoints). For an allyesconfig build:

 $ objdump -dr vmlinux-allyes-orig | grep '^[0-9a-f]* <trace_raw_output' | wc -l
1401

That's 1401 tracepoints giving us:

   text    data     bss     dec     hex filename
137827709       140221067       53264384        331313160       13bf7008 vmlinux-allyes-handle
137920629       140221067       53264384        331406080       13c0db00 vmlinux-allyes-orig

92920 bytes in savings!!!

Link: http://lkml.kernel.org/r/20170315021431.13107-2-andi@xxxxxxxxxxxxxx

Reported-by: Andi Kleen <andi@xxxxxxxxxxxxxx>
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
---
 include/linux/trace_events.h | 11 +----------
 kernel/trace/trace.c         | 12 ++++++++++++
 2 files changed, 13 insertions(+), 10 deletions(-)

diff --git a/include/linux/trace_events.h b/include/linux/trace_events.h
index 0af63c4..a556805 100644
--- a/include/linux/trace_events.h
+++ b/include/linux/trace_events.h
@@ -138,16 +138,7 @@ enum print_line_t {
 	TRACE_TYPE_NO_CONSUME	= 3	/* Handled but ask to not consume */
 };
 
-/*
- * Several functions return TRACE_TYPE_PARTIAL_LINE if the trace_seq
- * overflowed, and TRACE_TYPE_HANDLED otherwise. This helper function
- * simplifies those functions and keeps them in sync.
- */
-static inline enum print_line_t trace_handle_return(struct trace_seq *s)
-{
-	return trace_seq_has_overflowed(s) ?
-		TRACE_TYPE_PARTIAL_LINE : TRACE_TYPE_HANDLED;
-}
+enum print_line_t trace_handle_return(struct trace_seq *s);
 
 void tracing_generic_entry_update(struct trace_entry *entry,
 				  unsigned long flags,
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index f351095..5ad52b1 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -1927,6 +1927,18 @@ void tracing_record_cmdline(struct task_struct *tsk)
 		__this_cpu_write(trace_cmdline_save, false);
 }
 
+/*
+ * Several functions return TRACE_TYPE_PARTIAL_LINE if the trace_seq
+ * overflowed, and TRACE_TYPE_HANDLED otherwise. This helper function
+ * simplifies those functions and keeps them in sync.
+ */
+enum print_line_t trace_handle_return(struct trace_seq *s)
+{
+	return trace_seq_has_overflowed(s) ?
+		TRACE_TYPE_PARTIAL_LINE : TRACE_TYPE_HANDLED;
+}
+EXPORT_SYMBOL_GPL(trace_handle_return);
+
 void
 tracing_generic_entry_update(struct trace_entry *entry, unsigned long flags,
 			     int pc)
-- 
2.9.3