[RFC PATCH] seq_file: Use seq_puts when seq_printf has only a format with no args

[Joe Perches <joe@perches.com>]

Instead of converting the 800 or so uses of seq_printf with
a constant format (without a % substitution) to seq_puts,
maybe there's another way to slightly speed up these outputs.

Taking a similar approach to commit abd84d60eb
("tracing: Optimize trace_printk() with one arg to use trace_puts()")
use the preprocessor to convert seq_printf(seq, "string constant")
to seq_puts(seq, "string constant")

By stringifying __VA_ARGS__, we can, at compile time, determine
the number of args that are being passed to seq_printf() and
call seq_puts or seq_printf appropriately.

The actual function definition for seq_printf must now
be enclosed in parenthesis to avoid further macro expansion.

Signed-off-by: Joe Perches <joe@perches.com>
---
 fs/seq_file.c            |  7 ++++++-
 include/linux/seq_file.h | 24 ++++++++++++++++++++++++
 2 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/fs/seq_file.c b/fs/seq_file.c
index 38bb59f..d3a957d 100644
--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -405,7 +405,12 @@ int seq_vprintf(struct seq_file *m, const char *f, va_list args)
 }
 EXPORT_SYMBOL(seq_vprintf);
 
-int seq_printf(struct seq_file *m, const char *f, ...)
+/*
+ * seq_printf is also a macro that expands to seq_printf or seq_puts.
+ * This means that the actual function definition must be in parenthesis
+ * to prevent the preprocessor from expanding this function name again.
+ */
+int (seq_printf)(struct seq_file *m, const char *f, ...)
 {
 	int ret;
 	va_list args;
diff --git a/include/linux/seq_file.h b/include/linux/seq_file.h
index 68a04a3..7255f01 100644
--- a/include/linux/seq_file.h
+++ b/include/linux/seq_file.h
@@ -7,6 +7,7 @@
 #include <linux/mutex.h>
 #include <linux/cpumask.h>
 #include <linux/nodemask.h>
+#include <linux/stringify.h>
 
 struct seq_operations;
 struct file;
@@ -92,6 +93,29 @@ int seq_write(struct seq_file *seq, const void *data, size_t len);
 __printf(2, 3) int seq_printf(struct seq_file *, const char *, ...);
 __printf(2, 0) int seq_vprintf(struct seq_file *, const char *, va_list args);
 
+/*
+ * A little optimization trick is done here. If there's only one
+ * argument, there's no need to scan the string for printf formats.
+ * seq_puts() will suffice. But how can we take advantage of
+ * using seq_puts() when seq_printf() has only one argument?
+ * By stringifying the args and checking the size we can tell
+ * whether or not there are args. __stringify(__VA_ARGS__) will
+ * turn into "" with a size of 1 when there are no args, anything
+ * else will be bigger. All we need to do is define a string to this,
+ * and then take its size and compare to 1. If it's bigger, use
+ * seq_printf() otherwise, optimize it to seq_puts(). Then just
+ * let gcc optimize the rest.  The actual function definition of
+ * seq_printf must be (seq_printf) to prevent further macro expansion.
+ */
+#define seq_printf(seq, fmt, ...)				\
+do {								\
+	char va_args[] = __stringify(__VA_ARGS__);		\
+	if (sizeof(va_args) > 1)				\
+		seq_printf(seq, fmt, ##__VA_ARGS__);		\
+	else							\
+		seq_puts(seq, fmt);				\
+} while (0)
+
 int seq_path(struct seq_file *, const struct path *, const char *);
 int seq_dentry(struct seq_file *, struct dentry *, const char *);
 int seq_path_root(struct seq_file *m, const struct path *path,