Re: RFC: reduce CONFIG_SCSI_CONSTANTS impact by 4k

[Rasmus Villemoes]

On Sat, Oct 03 2015, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:

> Hi Rasmus,
>
> I like this idea.  But maybe it's also time to just move the constants
> to a plain text file and auto-generate C headers from them?  That way
> the format in which they can be edited is decoupled from the
> representation in the kernel image.

Well, I don't really have an opinion on that part.

In the meantime, I got another idea for doubling the saving to 8k. It
requires a few more code changes and is perhaps also more hacky. 2/2
would be something like below. Please let me know which version you'd
prefer, and I'll send both patches properly.

Thanks,
Rasmus

Subject: [PATCH 2/2] scsi: reduce CONFIG_SCSI_CONSTANTS=y impact by 8k

On 64 bit, struct error_info has 6 bytes of padding, which amounts to
over 4k of wasted space in the additional[] array. We could easily get
rid of that by instead using separate arrays for the codes and the
pointers. However, we can do even better than that and save an
additional 6 bytes per entry: In the table, just store the sizeof()
the corresponding string literal. The cumulative sum of these is then
the appropriate offset into additional_text, which is built from the
concatenation (with '\0's inbetween) of the strings.

$ scripts/bloat-o-meter /tmp/vmlinux vmlinux
add/remove: 0/0 grow/shrink: 1/1 up/down: 24/-8488 (-8464)
function                                     old     new   delta
scsi_extd_sense_format                       136     160     +24
additional                                 11312    2824   -8488

Signed-off-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
---
 drivers/scsi/constants.c   | 25 +++++++++++++++++++++----
 drivers/scsi/sense_codes.h |  2 --
 2 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/drivers/scsi/constants.c b/drivers/scsi/constants.c
index 47aaccd5e68e..ccd34b0481cd 100644
--- a/drivers/scsi/constants.c
+++ b/drivers/scsi/constants.c
@@ -292,17 +292,31 @@ bool scsi_opcode_sa_name(int opcode, int service_action,
 
 struct error_info {
 	unsigned short code12;	/* 0x0302 looks better than 0x03,0x02 */
-	const char * text;
+	unsigned short size;
 };
 
 
+/*
+ * There are 700+ entries in this table. To save space, we don't store
+ * (code, pointer) pairs, which would make sizeof(struct
+ * error_info)==16 on 64 bits. Rather, the second element just stores
+ * the size (including \0) of the corresponding string, and we use the
+ * sum of these to get the appropriate offset into additional_text
+ * defined below. This approach saves 12 bytes per entry.
+ */
 static const struct error_info additional[] =
 {
-#define SENSE_CODE(c, s) {c, s},
+#define SENSE_CODE(c, s) {c, sizeof(s)},
 #include "sense_codes.h"
 #undef SENSE_CODE
 };
 
+static const char *additional_text =
+#define SENSE_CODE(c, s) s "\0"
+#include "sense_codes.h"
+#undef SENSE_CODE
+	;
+
 struct error_info2 {
 	unsigned char code1, code2_min, code2_max;
 	const char * str;
@@ -364,11 +378,14 @@ scsi_extd_sense_format(unsigned char asc, unsigned char ascq, const char **fmt)
 {
 	int i;
 	unsigned short code = ((asc << 8) | ascq);
+	unsigned offset = 0;
 
 	*fmt = NULL;
-	for (i = 0; additional[i].text; i++)
+	for (i = 0; i < ARRAY_SIZE(additional); i++) {
 		if (additional[i].code12 == code)
-			return additional[i].text;
+			return additional_text + offset;
+		offset += additional[i].size;
+	}
 	for (i = 0; additional2[i].fmt; i++) {
 		if (additional2[i].code1 == asc &&
 		    ascq >= additional2[i].code2_min &&
diff --git a/drivers/scsi/sense_codes.h b/drivers/scsi/sense_codes.h
index 54b3939d6309..da84d53b3379 100644
--- a/drivers/scsi/sense_codes.h
+++ b/drivers/scsi/sense_codes.h
@@ -833,5 +833,3 @@ SENSE_CODE(0x746E, "External data encryption control timeout")
 SENSE_CODE(0x746F, "External data encryption control error")
 SENSE_CODE(0x7471, "Logical unit access not authorized")
 SENSE_CODE(0x7479, "Security conflict in translated device")
-
-SENSE_CODE(0, NULL)
-- 
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