linux-kernel - [021/145] ACPI: Be in TS_POLLING state during mwait based C-state entry

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Date:	Fri, 12 Mar 2010 16:25:54 -0800
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...nel.org
Cc:	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>,
	Len Brown <len.brown@...el.com>
Subject: [021/145] ACPI: Be in TS_POLLING state during mwait based C-state entry

2.6.32-stable review patch.  If anyone has any objections, please let me know.

----------------
From: Pallipadi, Venkatesh <venkatesh.pallipadi@...el.com>

commit d306ebc28649b89877a22158fe0076f06cc46f60 upstream.

ACPI deep C-state entry had a long standing bug/missing feature, wherein we were sending
resched IPIs when an idle CPU is in mwait based deep C-state. Only mwait based C1 was using
the write to the monitored address to wake up mwait'ing CPU.

This patch changes the code to retain TS_POLLING bit if we are entering an mwait based
deep C-state.

The patch has been verified to reduce the number of resched IPIs in general and also
improves the performance/power on workloads with low system utilization (i.e., when mwait based
deep C-states are being used).

Fixes "netperf ~50% regression with 2.6.33-rc1, bisect to 1b9508f"
http://marc.info/?l=linux-kernel&m=126441481427331&w=4

Reported-by: Lin Ming <ming.m.lin@...el.com>
Tested-by: Alex Shi <alex.shi@...el.com>
Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>
Signed-off-by: Len Brown <len.brown@...el.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>

---
 drivers/acpi/processor_idle.c |   28 ++++++++++++++++------------
 1 file changed, 16 insertions(+), 12 deletions(-)

--- a/drivers/acpi/processor_idle.c
+++ b/drivers/acpi/processor_idle.c
@@ -888,12 +888,14 @@ static int acpi_idle_enter_simple(struct
 		return(acpi_idle_enter_c1(dev, state));
 
 	local_irq_disable();
-	current_thread_info()->status &= ~TS_POLLING;
-	/*
-	 * TS_POLLING-cleared state must be visible before we test
-	 * NEED_RESCHED:
-	 */
-	smp_mb();
+	if (cx->entry_method != ACPI_CSTATE_FFH) {
+		current_thread_info()->status &= ~TS_POLLING;
+		/*
+		 * TS_POLLING-cleared state must be visible before we test
+		 * NEED_RESCHED:
+		 */
+		smp_mb();
+	}
 
 	if (unlikely(need_resched())) {
 		current_thread_info()->status |= TS_POLLING;
@@ -973,12 +975,14 @@ static int acpi_idle_enter_bm(struct cpu
 	}
 
 	local_irq_disable();
-	current_thread_info()->status &= ~TS_POLLING;
-	/*
-	 * TS_POLLING-cleared state must be visible before we test
-	 * NEED_RESCHED:
-	 */
-	smp_mb();
+	if (cx->entry_method != ACPI_CSTATE_FFH) {
+		current_thread_info()->status &= ~TS_POLLING;
+		/*
+		 * TS_POLLING-cleared state must be visible before we test
+		 * NEED_RESCHED:
+		 */
+		smp_mb();
+	}
 
 	if (unlikely(need_resched())) {
 		current_thread_info()->status |= TS_POLLING;


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