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Subject[PATCH] sched: introduce configurable delay before entering idle

Certain workloads perform poorly on KVM compared to baremetal
due to baremetal's ability to perform mwait on NEED_RESCHED
bit of task flags (therefore skipping the IPI).

This patch introduces a configurable busy-wait delay before entering the
architecture delay routine, allowing wakeup IPIs to be skipped
(if the IPI happens in that window).

The real-life workload which this patch improves performance
is SAP HANA (by 5-10%) (for which case setting idle_spin to 30
is sufficient).

This patch improves the attached server.py and client.py example
as follows:

Host: 31.814230202231556
Guest: 38.17718765199993 (83 %)
Guest, idle_spin=50us: 33.317709898000004 (95 %)
Guest, idle_spin=220us: 32.27826551499999 (98 %)

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

---
kernel/sched/idle.c | 86 ++++++++++++++++++++++++++++++++++++++++++
1 file changed, 86 insertions(+)

diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index f5516bae0c1b..bca7656a7ea0 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -216,6 +216,29 @@ static void cpuidle_idle_call(void)
rcu_idle_exit();
}

+static unsigned int spin_before_idle_us;

+static void do_spin_before_idle(void)
+{
+ ktime_t now, end_spin;
+
+ now = ktime_get();
+ end_spin = ktime_add_ns(now, spin_before_idle_us*1000);
+
+ rcu_idle_enter();
+ local_irq_enable();
+ stop_critical_timings();
+
+ do {
+ cpu_relax();
+ now = ktime_get();
+ } while (!tif_need_resched() && ktime_before(now, end_spin));
+
+ start_critical_timings();
+ rcu_idle_exit();
+ local_irq_disable();
+}
+
/*
* Generic idle loop implementation
*
@@ -259,6 +282,8 @@ static void do_idle(void)
tick_nohz_idle_restart_tick();
cpu_idle_poll();
} else {
+ if (spin_before_idle_us)
+ do_spin_before_idle();
cpuidle_idle_call();
}
arch_cpu_idle_exit();
@@ -465,3 +490,64 @@ const struct sched_class idle_sched_class = {
.switched_to = switched_to_idle,
.update_curr = update_curr_idle,
};
+
+
+static ssize_t store_idle_spin(struct kobject *kobj,
+ struct kobj_attribute *attr,
+ const char *buf, size_t count)
+{
+ unsigned int val;
+
+ if (kstrtouint(buf, 10, &val) < 0)
+ return -EINVAL;
+
+ if (val > USEC_PER_SEC)
+ return -EINVAL;
+
+ spin_before_idle_us = val;
+ return count;
+}
+
+static ssize_t show_idle_spin(struct kobject *kobj,
+ struct kobj_attribute *attr,
+ char *buf)
+{
+ ssize_t ret;
+
+ ret = sprintf(buf, "%d\n", spin_before_idle_us);
+
+ return ret;
+}
+
+static struct kobj_attribute idle_spin_attr =
+ __ATTR(idle_spin, 0644, show_idle_spin, store_idle_spin);
+
+static struct attribute *sched_attrs[] = {
+ &idle_spin_attr.attr,
+ NULL,
+};
+
+static const struct attribute_group sched_attr_group = {
+ .attrs = sched_attrs,
+};
+
+static struct kobject *sched_kobj;
+
+static int __init sched_sysfs_init(void)
+{
+ int error;
+
+ sched_kobj = kobject_create_and_add("sched", kernel_kobj);
+ if (!sched_kobj)
+ return -ENOMEM;
+
+ error = sysfs_create_group(sched_kobj, &sched_attr_group);
+ if (error)
+ goto err;
+ return 0;
+
+err:
+ kobject_put(sched_kobj);
+ return error;
+}
+postcore_initcall(sched_sysfs_init);#!/bin/python3

import socket
import sys
import struct, fcntl, os
import os, errno, time
import time

sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

server_address = ('127.0.0.1', 999)
print ("connecting to 127.0.0.1")
sock.connect(server_address)

nr_writes = 0

start_time = time.clock_gettime(time.CLOCK_MONOTONIC)

while nr_writes < 90000:
data = sock.recv(4096)
if len(data) == 0:
print("connection closed!\n");
exit(0);
# sleep 20us
time.sleep(20/1000000)
sock.send(data)
nr_writes = nr_writes+1

end_time = time.clock_gettime(time.CLOCK_MONOTONIC)
delta = end_time - start_time
print(delta)
#!/bin/python3

import socket
import sys
import struct, fcntl, os
import os, errno, time
import time

sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.bind(('127.0.0.1', 999))
sock.listen(10)
conn, addr = sock.accept()

nr_written=0
while 1:
conn.sendall(b"a response line of text")
data = conn.recv(1024)
if not data:
break
# sleep 200us
time.sleep(200/1000000)
nr_written = nr_written+1
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