Commit 8a4aeec8 authored Apr 17, 2014 by Dan Williams Committed by Tejun Heo Apr 18, 2014

libata/ahci: accommodate tag ordered controllers



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>

parent 2cf532f5

drivers/ata/libata-core.c

+13 −8

Original line number	Diff line number	Diff line
		@@ -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;

		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. */
		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);
		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;
		}

include/linux/libata.h

+1 −0

Original line number	Diff line number	Diff line
		@@ -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() */

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