linux-kernel - [PATCH] fs/proc/array.c: slightly improve render_sigset

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Date:   Wed, 21 Sep 2016 00:28:04 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>
Cc:     linux-kernel@...r.kernel.org,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: [PATCH] fs/proc/array.c: slightly improve render_sigset_t

format_decode and vsnprintf occasionally show up in perf top, so I
went looking for places that might not need the full printf
power. With the help of kprobes, I gathered some statistics on which
format strings we mostly pass to vsnprintf. On a trivial desktop
workload, I hit "%x" 25% of the time, so something apparently reads
/proc/pid/status (which does 5*16 printf("%x") calls) a lot.

With this patch, reading /proc/pid/status is 30% faster according to
this microbenchmark:

	char buf[4096];
	int i, fd;
	for (i = 0; i < 10000; ++i) {
		fd = open("/proc/self/status", O_RDONLY);
		read(fd, buf, sizeof(buf));
		close(fd);
	}

Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
---
 fs/proc/array.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/proc/array.c b/fs/proc/array.c
index 88c7de12197b..7f73b689a15c 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -251,7 +251,7 @@ void render_sigset_t(struct seq_file *m, const char *header,
 		if (sigismember(set, i+2)) x |= 2;
 		if (sigismember(set, i+3)) x |= 4;
 		if (sigismember(set, i+4)) x |= 8;
-		seq_printf(m, "%x", x);
+		seq_putc(m, hex_asc[x]);
 	} while (i >= 4);

 	seq_putc(m, '\n');
-- 
2.1.4