cpufreq: powernv: Ramp-down global pstate slower than local-pstate

The frequency transition latency from pmin to pmax is observed to be in
few millisecond granurality. And it usually happens to take a performance
penalty during sudden frequency rampup requests.

This patch set solves this problem by using an entity called "global
pstates". The global pstate is a Chip-level entity, so the global entitiy
(Voltage) is managed across the cores. The local pstate is a Core-level
entity, so the local entity (frequency) is managed across threads.

This patch brings down global pstate at a slower rate than the local
pstate. Hence by holding global pstates higher than local pstate makes
the subsequent rampups faster.

A per policy structure is maintained to keep track of the global and
local pstate changes. The global pstate is brought down using a parabolic
equation. The ramp down time to pmin is set to ~5 seconds. To make sure
that the global pstates are dropped at regular interval , a timer is
queued for every 2 seconds during ramp-down phase, which eventually brings
the pstate down to local pstate.

Iozone results show fairly consistent performance boost.
YCSB on redis shows improved Max latencies in most cases.

Iozone write/rewite test were made with filesizes 200704Kb and 401408Kb
with different record sizes . The following table shows IOoperations/sec
with and without patch.

Iozone Results ( in op/sec) ( mean over 3 iterations )
---------------------------------------------------------------------
file size-                      with            without		  %
recordsize-IOtype               patch           patch		change
----------------------------------------------------------------------
200704-1-SeqWrite               1616532         1615425         0.06
200704-1-Rewrite                2423195         2303130         5.21
200704-2-SeqWrite               1628577         1602620         1.61
200704-2-Rewrite                2428264         2312154         5.02
200704-4-SeqWrite               1617605         1617182         0.02
200704-4-Rewrite                2430524         2351238         3.37
200704-8-SeqWrite               1629478         1600436         1.81
200704-8-Rewrite                2415308e         2298136         5.09
200704-16-SeqWrite              1619632         1618250         0.08
200704-16-Rewrite               2396650         2352591         1.87
200704-32-SeqWrite              1632544         1598083         2.15
200704-32-Rewrite               2425119         2329743         4.09
200704-64-SeqWrite              1617812         1617235         0.03
200704-64-Rewrite               2402021         2321080         3.48
200704-128-SeqWrite             1631998         1600256         1.98
200704-128-Rewrite              2422389         2304954         5.09
200704-256 SeqWrite             1617065         1616962         0.00
200704-256-Rewrite              2432539         2301980         5.67
200704-512-SeqWrite             1632599         1598656         2.12
200704-512-Rewrite              2429270         2323676         4.54
200704-1024-SeqWrite            1618758         1616156         0.16
200704-1024-Rewrite             2431631         2315889         4.99
401408-1-SeqWrite               1631479         1608132         1.45
401408-1-Rewrite                2501550         2459409         1.71
401408-2-SeqWrite               1617095         1626069         -0.55
401408-2-Rewrite                2507557         2443621         2.61
401408-4-SeqWrite               1629601         1611869         1.10
401408-4-Rewrite                2505909         2462098         1.77
401408-8-SeqWrite               1617110         1626968         -0.60
401408-8-Rewrite                2512244         2456827         2.25
401408-16-SeqWrite              1632609         1609603         1.42
401408-16-Rewrite               2500792         2451405         2.01
401408-32-SeqWrite              1619294         1628167         -0.54
401408-32-Rewrite               2510115         2451292         2.39
401408-64-SeqWrite              1632709         1603746         1.80
401408-64-Rewrite               2506692         2433186         3.02
401408-128-SeqWrite             1619284         1627461         -0.50
401408-128-Rewrite              2518698         2453361         2.66
401408-256-SeqWrite             1634022         1610681         1.44
401408-256-Rewrite              2509987         2446328         2.60
401408-512-SeqWrite             1617524         1628016         -0.64
401408-512-Rewrite              2504409         2442899         2.51
401408-1024-SeqWrite            1629812         1611566         1.13
401408-1024-Rewrite             2507620          2442968        2.64

Tested with YCSB workload (50% update + 50% read) over redis for 1 million
records and 1 million operation. Each test was carried out with target
operations per second and persistence disabled.

Max-latency (in us)( mean over 5 iterations )
---------------------------------------------------------------
op/s    Operation       with patch      without patch   %change
---------------------------------------------------------------
15000   Read            61480.6         50261.4         22.32
15000   cleanup         215.2           293.6           -26.70
15000   update          25666.2         25163.8         2.00

25000   Read            32626.2         89525.4         -63.56
25000   cleanup         292.2           263.0           11.10
25000   update          32293.4         90255.0         -64.22

35000   Read            34783.0         33119.0         5.02
35000   cleanup         321.2           395.8           -18.8
35000   update          36047.0         38747.8         -6.97

40000   Read            38562.2         42357.4         -8.96
40000   cleanup         371.8           384.6           -3.33
40000   update          27861.4         41547.8         -32.94

45000   Read            42271.0         88120.6         -52.03
45000   cleanup         263.6           383.0           -31.17
45000   update          29755.8         81359.0         -63.43

(test without target op/s)
47659   Read            83061.4         136440.6        -39.12
47659   cleanup         195.8           193.8           1.03
47659   update          73429.4         124971.8        -41.24
Signed-off-by: Akshay Adiga <akshay.adiga@linux.vnet.ibm.com>
Reviewed-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/cpufreq/powernv-cpufreq.c

@@ -36,12 +36,56 @@
 #include <asm/reg.h>
 #include <asm/smp.h> /* Required for cpu_sibling_mask() in UP configs */
 #include <asm/opal.h>
+#include <linux/timer.h>
 
 #define POWERNV_MAX_PSTATES	256
 #define PMSR_PSAFE_ENABLE	(1UL << 30)
 #define PMSR_SPR_EM_DISABLE	(1UL << 31)
 #define PMSR_MAX(x)		((x >> 32) & 0xFF)
 
+#define MAX_RAMP_DOWN_TIME				5120
+/*
+ * On an idle system we want the global pstate to ramp-down from max value to
+ * min over a span of ~5 secs. Also we want it to initially ramp-down slowly and
+ * then ramp-down rapidly later on.
+ *
+ * This gives a percentage rampdown for time elapsed in milliseconds.
+ * ramp_down_percentage = ((ms * ms) >> 18)
+ *			~= 3.8 * (sec * sec)
+ *
+ * At 0 ms	ramp_down_percent = 0
+ * At 5120 ms	ramp_down_percent = 100
+ */
+#define ramp_down_percent(time)		((time * time) >> 18)
+
+/* Interval after which the timer is queued to bring down global pstate */
+#define GPSTATE_TIMER_INTERVAL				2000
+
+/**
+ * struct global_pstate_info -	Per policy data structure to maintain history of
+ *				global pstates
+ * @highest_lpstate:		The local pstate from which we are ramping down
+ * @elapsed_time:		Time in ms spent in ramping down from
+ *				highest_lpstate
+ * @last_sampled_time:		Time from boot in ms when global pstates were
+ *				last set
+ * @last_lpstate,last_gpstate:	Last set values for local and global pstates
+ * @timer:			Is used for ramping down if cpu goes idle for
+ *				a long time with global pstate held high
+ * @gpstate_lock:		A spinlock to maintain synchronization between
+ *				routines called by the timer handler and
+ *				governer's target_index calls
+ */
+struct global_pstate_info {
+	int highest_lpstate;
+	unsigned int elapsed_time;
+	unsigned int last_sampled_time;
+	int last_lpstate;
+	int last_gpstate;
+	spinlock_t gpstate_lock;
+	struct timer_list timer;
+};
+
 static struct cpufreq_frequency_table powernv_freqs[POWERNV_MAX_PSTATES+1];
 static bool rebooting, throttled, occ_reset;
 
@@ -94,6 +138,17 @@ static struct powernv_pstate_info {
 	int nr_pstates;
 } powernv_pstate_info;
 
+static inline void reset_gpstates(struct cpufreq_policy *policy)
+{
+	struct global_pstate_info *gpstates = policy->driver_data;
+
+	gpstates->highest_lpstate = 0;
+	gpstates->elapsed_time = 0;
+	gpstates->last_sampled_time = 0;
+	gpstates->last_lpstate = 0;
+	gpstates->last_gpstate = 0;
+}
+
 /*
  * Initialize the freq table based on data obtained
  * from the firmware passed via device-tree
@@ -285,6 +340,7 @@ static inline void set_pmspr(unsigned long sprn, unsigned long val)
 struct powernv_smp_call_data {
 	unsigned int freq;
 	int pstate_id;
+	int gpstate_id;
 };
 
 /*
@@ -343,19 +399,21 @@ static unsigned int powernv_cpufreq_get(unsigned int cpu)
  * (struct powernv_smp_call_data *) and the pstate_id which needs to be set
  * on this CPU should be present in freq_data->pstate_id.
  */
-static void set_pstate(void *freq_data)
+static void set_pstate(void *data)
 {
 	unsigned long val;
-	unsigned long pstate_ul =
-		((struct powernv_smp_call_data *) freq_data)->pstate_id;
+	struct powernv_smp_call_data *freq_data = data;
+	unsigned long pstate_ul = freq_data->pstate_id;
+	unsigned long gpstate_ul = freq_data->gpstate_id;
 
 	val = get_pmspr(SPRN_PMCR);
 	val = val & 0x0000FFFFFFFFFFFFULL;
 
 	pstate_ul = pstate_ul & 0xFF;
+	gpstate_ul = gpstate_ul & 0xFF;
 
 	/* Set both global(bits 56..63) and local(bits 48..55) PStates */
-	val = val | (pstate_ul << 56) | (pstate_ul << 48);
+	val = val | (gpstate_ul << 56) | (pstate_ul << 48);
 
 	pr_debug("Setting cpu %d pmcr to %016lX\n",
 			raw_smp_processor_id(), val);
@@ -424,6 +482,110 @@ static void powernv_cpufreq_throttle_check(void *data)
 	}
 }
 
+/**
+ * calc_global_pstate - Calculate global pstate
+ * @elapsed_time:	Elapsed time in milliseconds
+ * @local_pstate:	New local pstate
+ * @highest_lpstate:	pstate from which its ramping down
+ *
+ * Finds the appropriate global pstate based on the pstate from which its
+ * ramping down and the time elapsed in ramping down. It follows a quadratic
+ * equation which ensures that it reaches ramping down to pmin in 5sec.
+ */
+static inline int calc_global_pstate(unsigned int elapsed_time,
+				     int highest_lpstate, int local_pstate)
+{
+	int pstate_diff;
+
+	/*
+	 * Using ramp_down_percent we get the percentage of rampdown
+	 * that we are expecting to be dropping. Difference between
+	 * highest_lpstate and powernv_pstate_info.min will give a absolute
+	 * number of how many pstates we will drop eventually by the end of
+	 * 5 seconds, then just scale it get the number pstates to be dropped.
+	 */
+	pstate_diff =  ((int)ramp_down_percent(elapsed_time) *
+			(highest_lpstate - powernv_pstate_info.min)) / 100;
+
+	/* Ensure that global pstate is >= to local pstate */
+	if (highest_lpstate - pstate_diff < local_pstate)
+		return local_pstate;
+	else
+		return highest_lpstate - pstate_diff;
+}
+
+static inline void  queue_gpstate_timer(struct global_pstate_info *gpstates)
+{
+	unsigned int timer_interval;
+
+	/*
+	 * Setting up timer to fire after GPSTATE_TIMER_INTERVAL ms, But
+	 * if it exceeds MAX_RAMP_DOWN_TIME ms for ramp down time.
+	 * Set timer such that it fires exactly at MAX_RAMP_DOWN_TIME
+	 * seconds of ramp down time.
+	 */
+	if ((gpstates->elapsed_time + GPSTATE_TIMER_INTERVAL)
+	     > MAX_RAMP_DOWN_TIME)
+		timer_interval = MAX_RAMP_DOWN_TIME - gpstates->elapsed_time;
+	else
+		timer_interval = GPSTATE_TIMER_INTERVAL;
+
+	mod_timer_pinned(&gpstates->timer, jiffies +
+			msecs_to_jiffies(timer_interval));
+}
+
+/**
+ * gpstate_timer_handler
+ *
+ * @data: pointer to cpufreq_policy on which timer was queued
+ *
+ * This handler brings down the global pstate closer to the local pstate
+ * according quadratic equation. Queues a new timer if it is still not equal
+ * to local pstate
+ */
+void gpstate_timer_handler(unsigned long data)
+{
+	struct cpufreq_policy *policy = (struct cpufreq_policy *)data;
+	struct global_pstate_info *gpstates = policy->driver_data;
+	int gpstate_id;
+	unsigned int time_diff = jiffies_to_msecs(jiffies)
+					- gpstates->last_sampled_time;
+	struct powernv_smp_call_data freq_data;
+
+	if (!spin_trylock(&gpstates->gpstate_lock))
+		return;
+
+	gpstates->last_sampled_time += time_diff;
+	gpstates->elapsed_time += time_diff;
+	freq_data.pstate_id = gpstates->last_lpstate;
+
+	if ((gpstates->last_gpstate == freq_data.pstate_id) ||
+	    (gpstates->elapsed_time > MAX_RAMP_DOWN_TIME)) {
+		gpstate_id = freq_data.pstate_id;
+		reset_gpstates(policy);
+		gpstates->highest_lpstate = freq_data.pstate_id;
+	} else {
+		gpstate_id = calc_global_pstate(gpstates->elapsed_time,
+						gpstates->highest_lpstate,
+						freq_data.pstate_id);
+	}
+
+	/*
+	 * If local pstate is equal to global pstate, rampdown is over
+	 * So timer is not required to be queued.
+	 */
+	if (gpstate_id != freq_data.pstate_id)
+		queue_gpstate_timer(gpstates);
+
+	freq_data.gpstate_id = gpstate_id;
+	gpstates->last_gpstate = freq_data.gpstate_id;
+	gpstates->last_lpstate = freq_data.pstate_id;
+
+	/* Timer may get migrated to a different cpu on cpu hot unplug */
+	smp_call_function_any(policy->cpus, set_pstate, &freq_data, 1);
+	spin_unlock(&gpstates->gpstate_lock);
+}
+
 /*
  * powernv_cpufreq_target_index: Sets the frequency corresponding to
  * the cpufreq table entry indexed by new_index on the cpus in the
@@ -433,6 +595,9 @@ static int powernv_cpufreq_target_index(struct cpufreq_policy *policy,
 					unsigned int new_index)
 {
 	struct powernv_smp_call_data freq_data;
+	unsigned int cur_msec, gpstate_id;
+	unsigned long flags;
+	struct global_pstate_info *gpstates = policy->driver_data;
 
 	if (unlikely(rebooting) && new_index != get_nominal_index())
 		return 0;
@@ -440,22 +605,70 @@ static int powernv_cpufreq_target_index(struct cpufreq_policy *policy,
 	if (!throttled)
 		powernv_cpufreq_throttle_check(NULL);
 
+	cur_msec = jiffies_to_msecs(get_jiffies_64());
+
+	spin_lock_irqsave(&gpstates->gpstate_lock, flags);
 	freq_data.pstate_id = powernv_freqs[new_index].driver_data;
 
+	if (!gpstates->last_sampled_time) {
+		gpstate_id = freq_data.pstate_id;
+		gpstates->highest_lpstate = freq_data.pstate_id;
+		goto gpstates_done;
+	}
+
+	if (gpstates->last_gpstate > freq_data.pstate_id) {
+		gpstates->elapsed_time += cur_msec -
+						 gpstates->last_sampled_time;
+
+		/*
+		 * If its has been ramping down for more than MAX_RAMP_DOWN_TIME
+		 * we should be resetting all global pstate related data. Set it
+		 * equal to local pstate to start fresh.
+		 */
+		if (gpstates->elapsed_time > MAX_RAMP_DOWN_TIME) {
+			reset_gpstates(policy);
+			gpstates->highest_lpstate = freq_data.pstate_id;
+			gpstate_id = freq_data.pstate_id;
+		} else {
+		/* Elaspsed_time is less than 5 seconds, continue to rampdown */
+			gpstate_id = calc_global_pstate(gpstates->elapsed_time,
+							gpstates->highest_lpstate,
+							freq_data.pstate_id);
+		}
+	} else {
+		reset_gpstates(policy);
+		gpstates->highest_lpstate = freq_data.pstate_id;
+		gpstate_id = freq_data.pstate_id;
+	}
+
+	/*
+	 * If local pstate is equal to global pstate, rampdown is over
+	 * So timer is not required to be queued.
+	 */
+	if (gpstate_id != freq_data.pstate_id)
+		queue_gpstate_timer(gpstates);
+
+gpstates_done:
+	freq_data.gpstate_id = gpstate_id;
+	gpstates->last_sampled_time = cur_msec;
+	gpstates->last_gpstate = freq_data.gpstate_id;
+	gpstates->last_lpstate = freq_data.pstate_id;
+
 	/*
 	 * Use smp_call_function to send IPI and execute the
 	 * mtspr on target CPU.  We could do that without IPI
 	 * if current CPU is within policy->cpus (core)
 	 */
 	smp_call_function_any(policy->cpus, set_pstate, &freq_data, 1);
-
+	spin_unlock_irqrestore(&gpstates->gpstate_lock, flags);
 	return 0;
 }
 
 static int powernv_cpufreq_cpu_init(struct cpufreq_policy *policy)
 {
-	int base, i;
+	int base, i, ret;
 	struct kernfs_node *kn;
+	struct global_pstate_info *gpstates;
 
 	base = cpu_first_thread_sibling(policy->cpu);
 
@@ -475,7 +688,34 @@ static int powernv_cpufreq_cpu_init(struct cpufreq_policy *policy)
 	} else {
 		kernfs_put(kn);
 	}
-	return cpufreq_table_validate_and_show(policy, powernv_freqs);
+
+	gpstates =  kzalloc(sizeof(*gpstates), GFP_KERNEL);
+	if (!gpstates)
+		return -ENOMEM;
+
+	policy->driver_data = gpstates;
+
+	/* initialize timer */
+	init_timer_deferrable(&gpstates->timer);
+	gpstates->timer.data = (unsigned long)policy;
+	gpstates->timer.function = gpstate_timer_handler;
+	gpstates->timer.expires = jiffies +
+				msecs_to_jiffies(GPSTATE_TIMER_INTERVAL);
+	spin_lock_init(&gpstates->gpstate_lock);
+	ret = cpufreq_table_validate_and_show(policy, powernv_freqs);
+
+	if (ret < 0)
+		kfree(policy->driver_data);
+
+	return ret;
+}
+
+static int powernv_cpufreq_cpu_exit(struct cpufreq_policy *policy)
+{
+	/* timer is deleted in cpufreq_cpu_stop() */
+	kfree(policy->driver_data);
+
+	return 0;
 }
 
 static int powernv_cpufreq_reboot_notifier(struct notifier_block *nb,
@@ -603,15 +843,19 @@ static struct notifier_block powernv_cpufreq_opal_nb = {
 static void powernv_cpufreq_stop_cpu(struct cpufreq_policy *policy)
 {
 	struct powernv_smp_call_data freq_data;
+	struct global_pstate_info *gpstates = policy->driver_data;
 
 	freq_data.pstate_id = powernv_pstate_info.min;
+	freq_data.gpstate_id = powernv_pstate_info.min;
 	smp_call_function_single(policy->cpu, set_pstate, &freq_data, 1);
+	del_timer_sync(&gpstates->timer);
 }
 
 static struct cpufreq_driver powernv_cpufreq_driver = {
 	.name		= "powernv-cpufreq",
 	.flags		= CPUFREQ_CONST_LOOPS,
 	.init		= powernv_cpufreq_cpu_init,
+	.exit		= powernv_cpufreq_cpu_exit,
 	.verify		= cpufreq_generic_frequency_table_verify,
 	.target_index	= powernv_cpufreq_target_index,
 	.get		= powernv_cpufreq_get,