[知其然不知其所以然-29] Keep system as idle as possible

Previously someone reported to me that, his system can not stay in low power state for long time,

thus woken up from S0ix.

According to  reporter, most of the wake up which break S0ix on his platform

are triggered by timer interrupt, and based on the output

of /proc/timer_list, the most active timers are illustrated below as:

tick_sched_timer

watchdog_timer_fn

several other hrtimer_wakeup by applications

So here's my ponit of  view on reducing such timer interrupts:

1. tick_sched_timer

    Although the system is config with CONFIG_NOHZ,

which means if the CPU is in idle then there would be no

tick triggered. But a more complete solution is always set

CONFIG_NO_HZ_FULL, which means that, besides cpu idle,

if there is only one task running on the CPU, there would be

no tick triggered, thus trigger defered softirqd/kevend, thus make

the cpu less likely to be in idle, so better set CONFIG_NO_HZ_FULL and

CONFIG_NO_HZ_FULL_ALL=y

2.watchdog_timer_fn

This one is to track whether there is deadlock on single CPU,

but in our case I don't think we need to enable softlockup/nmi detector

so better disable this watchdog by:

echo 0 > /proc/sys/kernel/watchdog_thresh

3.hrtimer_wakeup

This one is usually set up by user mode applications, such as

nanosleep, select, etc, according to OH's log,

we might need to kill or stop the following services:

avahi-daemon, tcf-agent, rpcbind,connmand

But a more straightfoward way is to boot with a mininmal

system, and see what's the status, for example, how about booting up

with init=/bin/bash?