On 2018/08/06 13:51, Douglas Gilbert wrote:
> Re-arrange some logic to lessen the number of checks. With logical
> ANDs put the least likely first, with logical ORs put the most
> likely first. Also add conditional hints on the assumed fastpath.
>
> Signed-off-by: Douglas Gilbert <dgilbert@xxxxxxxxxxxx>
> ---
> drivers/scsi/sd.c | 43 ++++++++++++++++++++++++-------------------
> 1 file changed, 24 insertions(+), 19 deletions(-)
>
> diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
> index 9f047fd3c92d..05014054e357 100644
> --- a/drivers/scsi/sd.c
> +++ b/drivers/scsi/sd.c
> @@ -1171,9 +1171,9 @@ static int sd_setup_read_write_cmnd(struct scsi_cmnd *cmd)
>
> fua = (rq->cmd_flags & REQ_FUA) ? 0x8 : 0;
> dix = scsi_prot_sg_count(cmd);
> - dif = scsi_host_dif_capable(cmd->device->host, sdkp->protection_type);
> + dif = scsi_host_dif_capable(sdp->host, sdkp->protection_type);
>
> - if (write && dix)
> + if (dix && write)
> sd_dif_prepare(cmd);
>
> if (dif || dix)
> @@ -1181,19 +1181,27 @@ static int sd_setup_read_write_cmnd(struct scsi_cmnd *cmd)
> else
> protect = 0;
>
> - if (protect && sdkp->protection_type == T10_PI_TYPE2_PROTECTION) {
> + if (unlikely(protect &&
> + sdkp->protection_type == T10_PI_TYPE2_PROTECTION))
> ret = sd_setup_read_write_32_cmnd(cmd, write, lba, nr_blocks,
> protect | fua);
> - } else if (sdp->use_16_for_rw || (nr_blocks > 0xffff)) {
> + else if (sdp->use_16_for_rw)
> ret = sd_setup_read_write_16_cmnd(cmd, write, lba, nr_blocks,
> protect | fua);
So here, without use_16_for_rw being forced on (which is the case for most disks
I think, except ZBC disks which mandate it) or most disks, all read/write to low
LBAs will have to go through a longer chain of if/else if... Is this change
really such a gain in average ? It looks like this will be a loss for the first
small partition at the beginning of the disk.
> - } else if ((nr_blocks > 0xff) || (lba > 0x1fffff) || sdp->use_10_for_rw
> - || protect) {
> - ret = sd_setup_read_write_10_cmnd(cmd, write, lba, nr_blocks,
> - protect | fua);
> - } else {
> - ret = sd_setup_read_write_6_cmnd(cmd, write, lba, nr_blocks,
> - protect | fua);
> + else if (likely(nr_blocks < 0x100)) {
> + if (sdp->use_10_for_rw || (lba > 0x1fffff) || protect)
> + ret = sd_setup_read_write_10_cmnd(cmd, write, lba,
> + nr_blocks, protect | fua);
> + else
> + ret = sd_setup_read_write_6_cmnd(cmd, write, lba,
> + nr_blocks, protect | fua);
> + } else { /* not already done and nr_blocks > 0xff */
> + if (unlikely(nr_blocks > 0xffff))
> + ret = sd_setup_read_write_16_cmnd(cmd, write, lba,
> + nr_blocks, protect | fua);
> + else
> + ret = sd_setup_read_write_10_cmnd(cmd, write, lba,
> + nr_blocks, protect | fua);
> }
>
> if (ret != BLKPREP_OK)
> @@ -1976,7 +1984,6 @@ static int sd_done(struct scsi_cmnd *SCpnt)
> struct scsi_disk *sdkp = scsi_disk(SCpnt->request->rq_disk);
> struct request *req = SCpnt->request;
> int sense_valid = 0;
> - int sense_deferred = 0;
>
> /*
> * Assumption that REQ_OP_READ and REQ_OP_WRITE are 0 and 1 is
> @@ -2030,16 +2037,14 @@ static int sd_done(struct scsi_cmnd *SCpnt)
> }
> }
>
> - if (result) {
> + if (unlikely(result)) {
> sense_valid = scsi_command_normalize_sense(SCpnt, &sshdr);
> - if (sense_valid)
> - sense_deferred = scsi_sense_is_deferred(&sshdr);
> + if (driver_byte(result) == DRIVER_SENSE ||
> + (sense_valid && (!scsi_sense_is_deferred(&sshdr))))
> + good_bytes = sd_done_sense(SCpnt, good_bytes, &sshdr);
> }
> - sdkp->medium_access_timed_out = 0;
>
> - if (unlikely(driver_byte(result) == DRIVER_SENSE ||
> - (sense_valid && !sense_deferred)))
> - good_bytes = sd_done_sense(SCpnt, good_bytes, &sshdr);
> + sdkp->medium_access_timed_out = 0;
>
> if (sd_is_zoned(sdkp))
> sd_zbc_complete(SCpnt, good_bytes, &sshdr);
>
--
Damien Le Moal
Western Digital Research
