linux-kernel - [PATCH] x86, UV: Fix RTC latency bug by reading replicated cachelines

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Date:	Tue, 19 Jan 2010 08:41:37 -0600
From:	Dimitri Sivanich <sivanich@....com>
To:	linux-kernel@...r.kernel.org
Cc:	Ingo Molnar <mingo@...e.hu>, Jack Steiner <steiner@....com>,
	"H. Peter Anvin" <hpa@...or.com>, tglx@...utronix.de
Subject: [PATCH] x86, UV: Fix RTC latency bug by reading replicated
	cachelines

Fix an RTC read latency bug by using replicated cachelines to read the RTC
timer on SGI UV node controllers (HUB) rev 2.0 or greater.

Signed-off-by: Dimitri Sivanich <sivanich@....com>

---

Patch requires "Add function retrieving node controller revision number" patch.

 arch/x86/kernel/uv_time.c |   13 ++++++++++++-
 drivers/char/uv_mmtimer.c |   17 ++++++++++++-----
 2 files changed, 24 insertions(+), 6 deletions(-)

Index: linux/arch/x86/kernel/uv_time.c
===================================================================
--- linux.orig/arch/x86/kernel/uv_time.c	2010-01-14 11:52:44.000000000 -0600
+++ linux/arch/x86/kernel/uv_time.c	2010-01-14 11:56:57.000000000 -0600
@@ -282,10 +282,21 @@ static int uv_rtc_unset_timer(int cpu, i
 
 /*
  * Read the RTC.
+ *
+ * Starting with HUB rev 2.0, the UV RTC register is replicated across all
+ * cachelines of it's own page.  This allows faster simultaneous reads
+ * from a given socket.
  */
 static cycle_t uv_read_rtc(struct clocksource *cs)
 {
-	return (cycle_t)uv_read_local_mmr(UVH_RTC);
+	unsigned long offset;
+
+	if (uv_get_min_hub_revision_id() == 1)
+		offset = 0;
+	else
+		offset = (uv_blade_processor_id() * L1_CACHE_BYTES) % PAGE_SIZE;
+
+	return (cycle_t)uv_read_local_mmr(UVH_RTC | offset);
 }
 
 /*
Index: linux/drivers/char/uv_mmtimer.c
===================================================================
--- linux.orig/drivers/char/uv_mmtimer.c	2010-01-14 11:52:45.000000000 -0600
+++ linux/drivers/char/uv_mmtimer.c	2010-01-14 11:56:57.000000000 -0600
@@ -89,12 +89,19 @@ static long uv_mmtimer_ioctl(struct file
 	switch (cmd) {
 	case MMTIMER_GETOFFSET:	/* offset of the counter */
 		/*
-		 * UV RTC register is on its own page
+		 * Starting with HUB rev 2.0, the UV RTC register is
+		 * replicated across all cachelines of it's own page.
+		 * This allows faster simultaneous reads from a given socket.
+		 *
+		 * The offset returned is in 64 bit units.
 		 */
-		if (PAGE_SIZE <= (1 << 16))
-			ret = ((UV_LOCAL_MMR_BASE | UVH_RTC) & (PAGE_SIZE-1))
-				/ 8;
-		else
+		if (PAGE_SIZE <= (1 << 16)) {
+			if (uv_get_min_hub_revision_id() == 1)
+				ret = 0;
+			else
+				ret = ((uv_blade_processor_id() *
+					L1_CACHE_BYTES) % PAGE_SIZE) / 8;
+		} else
 			ret = -ENOSYS;
 		break;
 
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