[libata] Disable R_OK (Early ACK) on SII 3726 PMP (4f2c7748) · Commits · 戴 / test

In 2009, While running "cache read" performance test of drives behind SII PMP we encountered a "all 5 drives" timeout on more than 30% of the machines under test. This patch reduces the rate by a factor of about 70. Low enough that we didn't care to further investigate the issue. Performance impact with any sort of "normal" use was ~2%+ CPU and less than 1% throughput degradation. Worst case impact (cached read) was 6% IOPS reduction. This is with NCQ off (q=1) but I believe FIS based switching enabled in the SATA driver. The patch disables "Early ACK" in the 3726 port multiplier. "Early ACK" is issued when device sends a FIS to the host (via PMP) and the PMP sends an ACK immediately back to the device - well before the host gets the response. Under worst case IOPs load (cached read test) and more than 2 PMPs connected to a 4-port SATA controller, I suspect the time to service all of the PMPs is exceeding the PMPs ability to keep track of outstanding FIS it owes the Host. Reducing the number of PMPs to 2 (or 1) reduces the frequency by several orders of magnitude. Kudos to Gwendal for initial debugging of this issue. [Any errors in the description are mine, not his.] Patch is currently in production on Google servers. Signed-off-by:

Grant Grundler <grundler@google.com> Signed-off-by:

Gwendal Grignou <gwendal@google.com> Acked-by:

Tejun Heo <tj@kernel.org> Signed-off-by:

Jeff Garzik <jgarzik@redhat.com>

drivers/ata/libata-pmp.c

+29 −3

Original line number	Diff line number	Diff line
		@@ -231,10 +231,14 @@ static const char sata_pmp_spec_rev_str(const u32 gscr)
		return "<unknown>";
		}

		#define PMP_GSCR_SII_POL 129

		static int sata_pmp_configure(struct ata_device *dev, int print_info)
		{
		struct ata_port *ap = dev->link->ap;
		u32 *gscr = dev->gscr;
		u16 vendor = sata_pmp_gscr_vendor(gscr);
		u16 devid = sata_pmp_gscr_devid(gscr);
		unsigned int err_mask = 0;
		const char *reason;
		int nr_ports, rc;
		@@ -260,12 +264,34 @@ static int sata_pmp_configure(struct ata_device *dev, int print_info)
		goto fail;
		}

		/* Disable sending Early R_OK.
		* With "cached read" HDD testing and multiple ports busy on a SATA
		* host controller, 3726 PMP will very rarely drop a deferred
		* R_OK that was intended for the host. Symptom will be all
		* 5 drives under test will timeout, get reset, and recover.
		*/
		if (vendor == 0x1095 && devid == 0x3726) {
		u32 reg;

		err_mask = sata_pmp_read(&ap->link, PMP_GSCR_SII_POL, &reg);
		if (err_mask) {
		rc = -EIO;
		reason = "failed to read Sil3726 Private Register";
		goto fail;
		}
		reg &= ~0x1;
		err_mask = sata_pmp_write(&ap->link, PMP_GSCR_SII_POL, reg);
		if (err_mask) {
		rc = -EIO;
		reason = "failed to write Sil3726 Private Register";
		goto fail;
		}
		}

		if (print_info) {
		ata_dev_printk(dev, KERN_INFO, "Port Multiplier %s, "
		"0x%04x:0x%04x r%d, %d ports, feat 0x%x/0x%x\n",
		sata_pmp_spec_rev_str(gscr),
		sata_pmp_gscr_vendor(gscr),
		sata_pmp_gscr_devid(gscr),
		sata_pmp_spec_rev_str(gscr), vendor, devid,
		sata_pmp_gscr_rev(gscr),
		nr_ports, gscr[SATA_PMP_GSCR_FEAT_EN],
		gscr[SATA_PMP_GSCR_FEAT]);

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