Hi,
on a i.MX28:
Linux nxt 3.12.15-rt25+ #83 PREEMPT RT Thu Apr 17 20:54:00 CEST 2014 armv5tejl GNU/Linux
# tracer: irqsoff
#
# irqsoff latency trace v1.1.5 on 3.12.15-rt25+
# --------------------------------------------------------------------
# latency: 2224 us, #6/6, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0)
# -----------------
# | task: swapper-0 (uid:0 nice:0 policy:0 rt_prio:0)
# -----------------
# => started at: rcu_idle_enter
# => ended at: arch_cpu_idle
#
#
# _--------=> CPU#
# / _-------=> irqs-off
# | / _------=> need-resched
# || / _-----=> need-resched_lazy
# ||| / _----=> hardirq/softirq
# |||| / _---=> preempt-depth
# ||||| / _--=> preempt-lazy-depth
# |||||| / _-=> migrate-disable
# ||||||| / delay
# cmd pid |||||||| time | caller
# \ / |||||||| \ | /
<idle>-0 0d...1.. 2us+: rcu_idle_enter
<idle>-0 0d...2.. 11us+: T.796 <-rcu_idle_enter
<idle>-0 0d...2.. 17us!: arch_cpu_idle <-cpu_startup_entry
<idle>-0 0d...1.. 2219us+: arch_cpu_idle
<idle>-0 0d...1.. 2234us+: trace_hardirqs_on <-arch_cpu_idle
<idle>-0 0d...1.. 2303us : <stack trace>
=> arch_cpu_idle
=> cpu_startup_entry
=> start_kernel
Why are interrupts disabled here (and are they really disabled)?
$ egrep "IDLE|SMP|HZ" .config
CONFIG_BROKEN_ON_SMP=y
CONFIG_HZ_PERIODIC=y
# CONFIG_NO_HZ_IDLE is not set
# CONFIG_NO_HZ is not set
CONFIG_GENERIC_SMP_IDLE_THREAD=y
CONFIG_GENERIC_IDLE_POLL_SETUP=y
CONFIG_HZ_FIXED=0
# CONFIG_HZ_100 is not set
# CONFIG_HZ_200 is not set
CONFIG_HZ_250=y
# CONFIG_HZ_300 is not set
# CONFIG_HZ_500 is not set
# CONFIG_HZ_1000 is not set
CONFIG_HZ=250
# CONFIG_CPU_IDLE is not set
# CONFIG_ARCH_NEEDS_CPU_IDLE_COUPLED is not set
Configuring
CONFIG_CPU_IDLE=y
CONFIG_CPU_IDLE_GOV_LADDER=y
does not help
Maybe it is just some tracing infrastructure interference - the effect
of enabling the tracing is huge on this platform for some reason.
Just enabling the irqsoff tracing makes the machine unsuitable
for any serious load.
Thanks
--
Stano
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