[PATCH v6 2/2] x86, mwaitt: introduce mwaitx delay with a configurable timer

[Huang Rui]

MWAITX can enable a timer and a corresponding timer value specified in
SW P0 clocks. The SW P0 frequency is the same as TSC. The timer
provides an upper bound on how long the instruction waits before
exiting.

The implementation of delay function in kernel can leverage the timer
of MWAITX. This patch provides a new method (delay_mwaitx) to measure
delay time.

In MWAITX delay, the CPU core will be quiesced in a waiting phase,
diminishing its power consumption.

Run a simple test to measure power consumption:

cat /sys/bus/pci/devices/0000\:00\:18.4/hwmon/hwmon0/power1_acc;
sleep 10000s;
cat /sys/bus/pci/devices/0000\:00\:18.4/hwmon/hwmon0/power1_acc;

* TSC-based default delay:      485115 uWatts average power
* MWAITX-based delay:           252738 uWatts average power

Thus, that's about 240 milliWatts less power consumption. The test
method relies on the support of AMD CPU accumulated power algorithm in
fam15_power for which patches are forthcoming.

Suggested-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Suggested-by: Borislav Petkov <bp@xxxxxxx>
Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Huang Rui <ray.huang@xxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Andreas Herrmann <herrmann.der.user@xxxxxxxxx>
---
 arch/x86/include/asm/delay.h |  1 +
 arch/x86/kernel/cpu/amd.c    |  4 ++++
 arch/x86/lib/delay.c         | 48 +++++++++++++++++++++++++++++++++++++++++++-
 3 files changed, 52 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/delay.h b/arch/x86/include/asm/delay.h
index 9b3b4f2..36a760b 100644
--- a/arch/x86/include/asm/delay.h
+++ b/arch/x86/include/asm/delay.h
@@ -4,5 +4,6 @@
 #include <asm-generic/delay.h>
 
 void use_tsc_delay(void);
+void use_mwaitx_delay(void);
 
 #endif /* _ASM_X86_DELAY_H */
diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index 51ad2af..730e620 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -11,6 +11,7 @@
 #include <asm/cpu.h>
 #include <asm/smp.h>
 #include <asm/pci-direct.h>
+#include <asm/delay.h>
 
 #ifdef CONFIG_X86_64
 # include <asm/mmconfig.h>
@@ -506,6 +507,9 @@ static void bsp_init_amd(struct cpuinfo_x86 *c)
 		/* A random value per boot for bit slice [12:upper_bit) */
 		va_align.bits = get_random_int() & va_align.mask;
 	}
+
+	if (cpu_has(c, X86_FEATURE_MWAITT))
+		use_mwaitx_delay();
 }
 
 static void early_init_amd(struct cpuinfo_x86 *c)
diff --git a/arch/x86/lib/delay.c b/arch/x86/lib/delay.c
index 4453d52..f8236cb 100644
--- a/arch/x86/lib/delay.c
+++ b/arch/x86/lib/delay.c
@@ -20,6 +20,7 @@
 #include <asm/processor.h>
 #include <asm/delay.h>
 #include <asm/timer.h>
+#include <asm/mwait.h>
 
 #ifdef CONFIG_SMP
 # include <asm/smp.h>
@@ -84,6 +85,45 @@ static void delay_tsc(unsigned long __loops)
 }
 
 /*
+ * On AMD platforms MWAITX has a configurable 32-bit timer, that
+ * counts with TSC frequency. And the input value is the loop of the
+ * counter, it will exit when the timer expires.
+ */
+static void delay_mwaitx(unsigned long __loops)
+{
+	u32 delay, loops = __loops;
+	u64 end, start;
+
+	start = rdtsc_ordered();
+
+	for (;;) {
+		delay = min(MWAITX_MAX_LOOPS, loops);
+
+		/*
+		 * Use cpu_tss as a cacheline-aligned, seldomly
+		 * accessed per-cpu variable as the monitor target.
+		 */
+		__monitorx(this_cpu_ptr(&cpu_tss), 0, 0);
+		/*
+		 * AMD, like Intel, supports the EAX hint and EAX=0xf
+		 * means, do not enter any deep C-state and we use it
+		 * here in delay() to minimize wakeup latency.
+		 */
+		__mwaitx(MWAITX_DISABLE_CSTATES, delay,
+			 MWAITX_ECX_TIMER_ENABLE);
+
+		end = rdtsc_ordered();
+
+		if (loops <= end - start)
+			break;
+
+		loops -= end - start;
+
+		start = end;
+	}
+}
+
+/*
  * Since we calibrate only once at boot, this
  * function should be set once at boot and not changed
  */
@@ -91,7 +131,13 @@ static void (*delay_fn)(unsigned long) = delay_loop;
 
 void use_tsc_delay(void)
 {
-	delay_fn = delay_tsc;
+	if (delay_fn == delay_loop)
+		delay_fn = delay_tsc;
+}
+
+void use_mwaitx_delay(void)
+{
+	delay_fn = delay_mwaitx;
 }
 
 int read_current_timer(unsigned long *timer_val)
-- 
1.9.1

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