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libata/ahci: accommodate tag ordered controllers

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The AHCI spec allows implementations to issue commands in tag order
rather than FIFO order:

	5.3.2.12 P:SelectCmd
	HBA sets pSlotLoc = (pSlotLoc + 1) mod (CAP.NCS + 1)
	or HBA selects the command to issue that has had the
	PxCI bit set to '1' longer than any other command
	pending to be issued.

The result is that commands posted sequentially (time-wise) may play out
of sequence when issued by hardware.

This behavior has likely been hidden by drives that arrange for commands
to complete in issue order.  However, it appears recent drives (two from
different vendors that we have found so far) inflict out-of-order
completions as a matter of course.  So, we need to take care to maintain
ordered submission, otherwise we risk triggering a drive to fall out of
sequential-io automation and back to random-io processing, which incurs
large latency and degrades throughput.

This issue was found in simple benchmarks where QD=2 seq-write
performance was 30-50% *greater* than QD=32 seq-write performance.

Tagging for -stable and making the change globally since it has a low
risk-to-reward ratio.  Also, word is that recent versions of an unnamed
OS also does it this way now.  So, drives in the field are already
experienced with this tag ordering scheme.

Cc: <stable@vger.kernel.org>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Ed Ciechanowski <ed.ciechanowski@intel.com>
Reviewed-by: Matthew Wilcox <matthew.r.wilcox@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Dan Williams 2014-04-17 11:48:21 -07:00 committed by Tejun Heo
parent 2cf532f5e6
commit 8a4aeec8d2
2 changed files with 14 additions and 8 deletions
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21
drivers/ata/libata-core.c
View File

@ -4794,21 +4794,26 @@ void swap_buf_le16(u16 *buf, unsigned int buf_words)
static struct ata_queued_cmd *ata_qc_new(struct ata_port *ap)
{
struct ata_queued_cmd *qc = NULL;
unsigned int i;
unsigned int i, tag;
/* no command while frozen */
if (unlikely(ap->pflags & ATA_PFLAG_FROZEN))
return NULL;
/* the last tag is reserved for internal command. */
for (i = 0; i < ATA_MAX_QUEUE - 1; i++)
if (!test_and_set_bit(i, &ap->qc_allocated)) {
qc = __ata_qc_from_tag(ap, i);
for (i = 0; i < ATA_MAX_QUEUE; i++) {
tag = (i + ap->last_tag + 1) % ATA_MAX_QUEUE;
/* the last tag is reserved for internal command. */
if (tag == ATA_TAG_INTERNAL)
continue;
if (!test_and_set_bit(tag, &ap->qc_allocated)) {
qc = __ata_qc_from_tag(ap, tag);
qc->tag = tag;
ap->last_tag = tag;
break;
}
if (qc)
qc->tag = i;
}
return qc;
}

1
include/linux/libata.h
View File

@ -822,6 +822,7 @@ struct ata_port {
unsigned long qc_allocated;
unsigned int qc_active;
int nr_active_links; /* #links with active qcs */
unsigned int last_tag; /* track next tag hw expects */
struct ata_link link; /* host default link */
struct ata_link *slave_link; /* see ata_slave_link_init() */