[PATCH 4/5] cpuidle-pseries : Include extended CEDE states in cpuidle framework

[Gautham R. Shenoy]

From: "Gautham R. Shenoy" <ego at linux.vnet.ibm.com>

This patch exposes those extended CEDE states to the cpuidle framework
which are responsive to external interrupts and do not need an H_PROD.

Since as per the PAPR, all the extended CEDE states are non-responsive
to timers, we indicate this to the cpuidle subsystem via the
CPUIDLE_FLAG_TIMER_STOP flag for all those extende CEDE states which
can wake up on external interrupts.

With the patch, we are able to see the extended CEDE state with
latency hint = 1 exposed via the cpuidle framework.

	$ cpupower idle-info
	CPUidle driver: pseries_idle
	CPUidle governor: menu
	analyzing CPU 0:

	Number of idle states: 3
	Available idle states: snooze CEDE XCEDE1
	snooze:
	Flags/Description: snooze
	Latency: 0
	Usage: 33429446
	Duration: 27006062
	CEDE:
	Flags/Description: CEDE
	Latency: 1
	Usage: 10272
	Duration: 110786770
	XCEDE1:
	Flags/Description: XCEDE1
	Latency: 12
	Usage: 26445
	Duration: 1436433815

Benchmark results:
TLDR: Over all we do not see any additional benefit from having XCEDE1 over
CEDE.

ebizzy :
2 threads bound to a big-core. With this patch, we see a 3.39%
regression compared to with only CEDE0 latency fixup.
x With only CEDE0 latency fixup
* With CEDE0 latency fixup + CEDE1
    N           Min           Max        Median           Avg        Stddev
x  10       2893813       5834474       5832448     5327281.3     1055941.4
*  10       2907329       5834923       5831398     5146614.6     1193874.8

context_switch2:
With the context_switch2 there are no observable regressions in the
results.

context_switch2 CPU0 CPU1 (Same Big-core, different small-cores).
No difference with and without patch.
x without_patch
* with_patch
    N           Min           Max        Median           Avg        Stddev
x 500        343644        348778        345444     345584.02     1035.1658
* 500        344310        347646        345776     345877.22     802.19501

context_switch2 CPU0 CPU8 (different big-cores). Minor 0.05% improvement
with patch
x without_patch
* with_patch
    N           Min           Max        Median           Avg        Stddev
x 500        287562        288756        288162     288134.76     262.24328
* 500        287874        288960        288306     288274.66     187.57034

schbench:
No regressions observed with schbench

Without Patch:
Latency percentiles (usec)
	50.0th: 29
	75.0th: 40
	90.0th: 50
	95.0th: 61
	*99.0th: 13648
	99.5th: 14768
	99.9th: 15664
	min=0, max=29812

With Patch:
Latency percentiles (usec)
	50.0th: 30
	75.0th: 40
	90.0th: 51
	95.0th: 59
	*99.0th: 13616
	99.5th: 14512
	99.9th: 15696
	min=0, max=15996

Signed-off-by: Gautham R. Shenoy <ego at linux.vnet.ibm.com>
---
 drivers/cpuidle/cpuidle-pseries.c | 50 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)

diff --git a/drivers/cpuidle/cpuidle-pseries.c b/drivers/cpuidle/cpuidle-pseries.c
index 502f906..6f893cd 100644
--- a/drivers/cpuidle/cpuidle-pseries.c
+++ b/drivers/cpuidle/cpuidle-pseries.c
@@ -362,9 +362,59 @@ static int add_pseries_idle_states(void)
 	for (i = 0; i < nr_xcede_records; i++) {
 		u64 latency_tb = xcede_records[i].wakeup_latency_tb_ticks;
 		u64 latency_us = tb_to_ns(latency_tb) / NSEC_PER_USEC;
+		char name[CPUIDLE_NAME_LEN];
+		unsigned int latency_hint = xcede_records[i].latency_hint;
+		u64 residency_us;
+
+		if (!xcede_records[i].responsive_to_irqs) {
+			pr_info("cpuidle : Skipping XCEDE%d. Not responsive to IRQs\n",
+				latency_hint);
+			continue;
+		}
 
 		if (latency_us < min_latency_us)
 			min_latency_us = latency_us;
+		snprintf(name, CPUIDLE_NAME_LEN, "XCEDE%d", latency_hint);
+
+		/*
+		 * As per the section 14.14.1 of PAPR version 2.8.1
+		 * says that alling H_CEDE with the value of the cede
+		 * latency specifier set greater than zero allows the
+		 * processor timer facility to be disabled (so as not
+		 * to cause gratuitous wake-ups - the use of H_PROD,
+		 * or other external interrupt is required to wake the
+		 * processor in this case).
+		 *
+		 * So, inform the cpuidle-subsystem that the timer
+		 * will be stopped for these states.
+		 *
+		 * Also, bump up the latency by 10us, since cpuidle
+		 * would use timer-offload framework which will need
+		 * to send an IPI to wakeup a CPU whose timer has
+		 * expired.
+		 */
+		if (latency_hint > 0) {
+			dedicated_states[nr_states].flags = CPUIDLE_FLAG_TIMER_STOP;
+			latency_us += 10;
+		}
+
+		/*
+		 * Thumb rule : Reside in the XCEDE state for at least
+		 * 10x the time required to enter and exit that state.
+		 */
+		residency_us = latency_us * 10;
+
+		strlcpy(dedicated_states[nr_states].name, (const char *)name,
+			CPUIDLE_NAME_LEN);
+		strlcpy(dedicated_states[nr_states].desc, (const char *)name,
+			CPUIDLE_NAME_LEN);
+		dedicated_states[nr_states].exit_latency = latency_us;
+		dedicated_states[nr_states].target_residency = residency_us;
+		dedicated_states[nr_states].enter = &dedicated_cede_loop;
+		cede_latency_hint[nr_states] = latency_hint;
+		pr_info("cpuidle : Added %s. latency-hint = %d\n",
+			name, latency_hint);
+		nr_states++;
 	}
 
 	/*
-- 
1.9.4