Dan Williams 5c29506449 libata/ahci: accommodate tag ordered controllers ... commit 8a4aeec8d2 upstream. 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: 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> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>		2014-05-13 13:32:51 +02:00
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acpi	Merge branches 'acpi-processor', 'acpi-hotplug', 'acpi-init', 'acpi-pm' and 'acpica'	2014-01-29 11:47:18 +01:00
asm-generic	word-at-a-time: avoid undefined behaviour in zero_bytemask macro	2014-05-06 07:59:29 -07:00
clocksource	…
crypto	…
drm	Merge tag 'ttm-fixes-3.14-2014-02-18' of git://people.freedesktop.org/~thomash/linux into drm-fixes	2014-02-19 08:21:26 +10:00
dt-bindings	dt: tegra: remove non-existent clock IDs	2014-05-06 07:59:38 -07:00
keys	…
kvm	ARM: KVM: fix non-VGIC compilation	2014-03-06 09:47:42 +01:00
linux	libata/ahci: accommodate tag ordered controllers	2014-05-13 13:32:51 +02:00
math-emu	…
media	…
memory	…
misc	…
net	ipv6: move DAD and addrconf_verify processing to workqueue	2014-03-28 16:54:50 -04:00
pcmcia	…
ras	…
rdma	IB/core: Don't resolve passive side RoCE L2 address in CMA REQ handler	2014-05-06 07:59:28 -07:00
rxrpc	…
scsi	Merge branch 'for-next' of git://git.kernel.org/pub/scm/linux/kernel/git/nab/target-pending	2014-01-31 15:31:23 -08:00
sound	ASoC: dapm: Add locking to snd_soc_dapm_xxxx_pin functions	2014-02-20 18:40:07 +09:00
target	iscsi/iser-target: Fix isert_conn->state hung shutdown issues	2014-03-04 17:54:09 -08:00
trace	tracing: Fix array size mismatch in format string	2014-03-20 13:21:05 -04:00
uapi	xattr: guard against simultaneous glibc header inclusion	2014-05-06 07:59:35 -07:00
video	…
xen	Merge branch 'for-linus' of git://git.kernel.dk/linux-block	2014-02-14 10:45:18 -08:00
Kbuild	…