linux-kernel - Re: [PATCH v2 3/3] scsi: reduce CONFIG_SCSI

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Date:	Tue, 22 Dec 2015 15:25:33 -0500
From:	Douglas Gilbert <dgilbert@...erlog.com>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Christoph Hellwig <hch@...radead.org>,
	Hannes Reinecke <hare@...e.de>,
	Bart Van Assche <bart.vanassche@...disk.com>,
	"James E.J. Bottomley" <JBottomley@...n.com>,
	Julian Calaby <julian.calaby@...il.com>
Cc:	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] scsi: reduce CONFIG_SCSI_CONSTANTS=y impact by 8k

On 15-11-24 04:42 AM, Rasmus Villemoes wrote:
> 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@...musvillemoes.dk>

Tested-by: Douglas Gilbert <dgilbert@...erlog.com>

> ---
>   drivers/scsi/constants.c | 26 +++++++++++++++++++++-----
>   1 file changed, 21 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/scsi/constants.c b/drivers/scsi/constants.c
> index 6e813eec4f8d..83458f7a2824 100644
> --- a/drivers/scsi/constants.c
> +++ b/drivers/scsi/constants.c
> @@ -292,17 +292,30 @@ 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
> -	{0, NULL}
>   };
>
> +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 +377,14 @@ 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 &&
>

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