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From:   Alexey Brodkin <alexey.brodkin@synopsys.com>
To:     linux-kernel@vger.kernel.org
Cc:     linux-snps-arc@lists.infradead.org,
        Alexey Brodkin <alexey.brodkin@synopsys.com>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        Vineet Gupta <vineet.gupta1@synopsys.com>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: [PATCH v2] clocksource/drivers/arc_timer: Utilize generic sched_clock
Date:   Wed, 17 Oct 2018 14:30:20 +0300
Message-Id: <20181017113020.7551-1-abrodkin@synopsys.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

It turned out we used to use default implementation of sched_clock()
from kernel/sched/clock.c which was as precise as 1/HZ, i.e.
by default we had 10 msec granularity of time measurement.

Now given ARC built-in timers are clocked with the same frequency as
CPU cores we may get much higher precision of time tracking.

Thus we switch to generic sched_clock which really reads ARC hardware
counters.

This is especially helpful for measuring short events.
That's what we used to have:
------------------------------>8------------------------
$ perf stat /bin/sh -c /root/lmbench-master/bin/arc/hello > /dev/null

 Performance counter stats for '/bin/sh -c /root/lmbench-master/bin/arc/hello':

         10.000000      task-clock (msec)         #    2.832 CPUs utilized
                 1      context-switches          #    0.100 K/sec
                 1      cpu-migrations            #    0.100 K/sec
                63      page-faults               #    0.006 M/sec
           3049480      cycles                    #    0.305 GHz
           1091259      instructions              #    0.36  insn per cycle
            256828      branches                  #   25.683 M/sec
             27026      branch-misses             #   10.52% of all branches

       0.003530687 seconds time elapsed

       0.000000000 seconds user
       0.010000000 seconds sys
------------------------------>8------------------------

And now we'll see:
------------------------------>8------------------------
$ perf stat /bin/sh -c /root/lmbench-master/bin/arc/hello > /dev/null

 Performance counter stats for '/bin/sh -c /root/lmbench-master/bin/arc/hello':

          3.004322      task-clock (msec)         #    0.865 CPUs utilized
                 1      context-switches          #    0.333 K/sec
                 1      cpu-migrations            #    0.333 K/sec
                63      page-faults               #    0.021 M/sec
           2986734      cycles                    #    0.994 GHz
           1087466      instructions              #    0.36  insn per cycle
            255209      branches                  #   84.947 M/sec
             26002      branch-misses             #   10.19% of all branches

       0.003474829 seconds time elapsed

       0.003519000 seconds user
       0.000000000 seconds sys
------------------------------>8------------------------

Note how much more meaningful is the second output - time spent for
execution pretty much matches number of cycles spent (we're running
@ 1GHz here).

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Vineet Gupta <vgupta@synopsys.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
---

Changes v1 -> v2:

 * Timer read callbacks marked as "notrace"
 * ARC Timer1 explicitly described as 32-bit one on
   sched_clock_register() invocation

 arch/arc/Kconfig                |  1 +
 drivers/clocksource/Kconfig     |  1 +
 drivers/clocksource/arc_timer.c | 22 ++++++++++++++++++++++
 3 files changed, 24 insertions(+)

diff --git a/arch/arc/Kconfig b/arch/arc/Kconfig
index 5151d81476a1..714f769389a4 100644
--- a/arch/arc/Kconfig
+++ b/arch/arc/Kconfig
@@ -9,6 +9,7 @@
 config ARC
 	def_bool y
 	select ARC_TIMERS
+	select GENERIC_SCHED_CLOCK
 	select ARCH_HAS_SYNC_DMA_FOR_CPU
 	select ARCH_HAS_SYNC_DMA_FOR_DEVICE
 	select ARCH_HAS_SG_CHAIN
diff --git a/drivers/clocksource/Kconfig b/drivers/clocksource/Kconfig
index dec0dd88ec15..3268dad4effe 100644
--- a/drivers/clocksource/Kconfig
+++ b/drivers/clocksource/Kconfig
@@ -290,6 +290,7 @@ config CLKSRC_MPS2
 
 config ARC_TIMERS
 	bool "Support for 32-bit TIMERn counters in ARC Cores" if COMPILE_TEST
+	depends on GENERIC_SCHED_CLOCK
 	select TIMER_OF
 	help
 	  These are legacy 32-bit TIMER0 and TIMER1 counters found on all ARC cores
diff --git a/drivers/clocksource/arc_timer.c b/drivers/clocksource/arc_timer.c
index 20da9b1d7f7d..b28970ca4a7a 100644
--- a/drivers/clocksource/arc_timer.c
+++ b/drivers/clocksource/arc_timer.c
@@ -23,6 +23,7 @@
 #include <linux/cpu.h>
 #include <linux/of.h>
 #include <linux/of_irq.h>
+#include <linux/sched_clock.h>
 
 #include <soc/arc/timers.h>
 #include <soc/arc/mcip.h>
@@ -88,6 +89,11 @@ static u64 arc_read_gfrc(struct clocksource *cs)
 	return (((u64)h) << 32) | l;
 }
 
+static notrace u64 arc_gfrc_clock_read(void)
+{
+	return arc_read_gfrc(NULL);
+}
+
 static struct clocksource arc_counter_gfrc = {
 	.name   = "ARConnect GFRC",
 	.rating = 400,
@@ -111,6 +117,8 @@ static int __init arc_cs_setup_gfrc(struct device_node *node)
 	if (ret)
 		return ret;
 
+	sched_clock_register(arc_gfrc_clock_read, 64, arc_timer_freq);
+
 	return clocksource_register_hz(&arc_counter_gfrc, arc_timer_freq);
 }
 TIMER_OF_DECLARE(arc_gfrc, "snps,archs-timer-gfrc", arc_cs_setup_gfrc);
@@ -139,6 +147,11 @@ static u64 arc_read_rtc(struct clocksource *cs)
 	return (((u64)h) << 32) | l;
 }
 
+static notrace u64 arc_rtc_clock_read(void)
+{
+	return arc_read_rtc(NULL);
+}
+
 static struct clocksource arc_counter_rtc = {
 	.name   = "ARCv2 RTC",
 	.rating = 350,
@@ -170,6 +183,8 @@ static int __init arc_cs_setup_rtc(struct device_node *node)
 
 	write_aux_reg(AUX_RTC_CTRL, 1);
 
+	sched_clock_register(arc_rtc_clock_read, 64, arc_timer_freq);
+
 	return clocksource_register_hz(&arc_counter_rtc, arc_timer_freq);
 }
 TIMER_OF_DECLARE(arc_rtc, "snps,archs-timer-rtc", arc_cs_setup_rtc);
@@ -185,6 +200,11 @@ static u64 arc_read_timer1(struct clocksource *cs)
 	return (u64) read_aux_reg(ARC_REG_TIMER1_CNT);
 }
 
+static notrace u64 arc_timer1_clock_read(void)
+{
+	return arc_read_timer1(NULL);
+}
+
 static struct clocksource arc_counter_timer1 = {
 	.name   = "ARC Timer1",
 	.rating = 300,
@@ -209,6 +229,8 @@ static int __init arc_cs_setup_timer1(struct device_node *node)
 	write_aux_reg(ARC_REG_TIMER1_CNT, 0);
 	write_aux_reg(ARC_REG_TIMER1_CTRL, TIMER_CTRL_NH);
 
+	sched_clock_register(arc_timer1_clock_read, 32, arc_timer_freq);
+
 	return clocksource_register_hz(&arc_counter_timer1, arc_timer_freq);
 }
 
-- 
2.17.2