Hi,
After the issue reported here:
https://lore.kernel.org/all/[email protected]/
It occurs that the idle cputime accounting is a big mess that
accumulates within two concurrent statistics, each having their own
shortcomings:
* The accounting for online CPUs which is based on the delta between
tick_nohz_start_idle() and tick_nohz_stop_idle().
Pros:
- Works when the tick is off
- Has nsecs granularity
Cons:
- Account idle steal time but doesn't substract it from idle
cputime.
- Assumes CONFIG_IRQ_TIME_ACCOUNTING by not accounting IRQs but
the IRQ time is simply ignored when
CONFIG_IRQ_TIME_ACCOUNTING=n
- The windows between 1) idle task scheduling and the first call
to tick_nohz_start_idle() and 2) idle task between the last
tick_nohz_stop_idle() and the rest of the idle time are
blindspots wrt. cputime accounting (though mostly insignificant
amount)
- Relies on private fields outside of kernel stats, with specific
accessors.
* The accounting for offline CPUs which is based on ticks and the
jiffies delta during which the tick was stopped.
Pros:
- Handles steal time correctly
- Handle CONFIG_IRQ_TIME_ACCOUNTING=y and
CONFIG_IRQ_TIME_ACCOUNTING=n correctly.
- Handles the whole idle task
- Accounts directly to kernel stats, without midlayer accumulator.
Cons:
- Doesn't elapse when the tick is off, which doesn't make it
suitable for online CPUs.
- Has TICK_NSEC granularity (jiffies)
- Needs to track the dyntick-idle ticks that were accounted and
substract them from the total jiffies time spent while the tick
was stopped. This is an ugly workaround.
Having two different accounting for a single context is not the only
problem: since those accountings are of different natures, it is
possible to observe the global idle time going backward after a CPU goes
offline, as reported by Xin Zhao.
Clean up the situation with introducing a hybrid approach that stays
coherent, fixes the backward jumps and works for both online and offline
CPUs:
* Tick based or native vtime accounting operate before the tick is
stopped and resumes once the tick is restarted.
* When the idle loop starts, switch to dynticks-idle accounting as is
done currently, except that the statistics accumulate directly to the
relevant kernel stat fields.
* Private dyntick cputime accounting fields are removed.
* Works on both online and offline case.
* Move most of the relevant code to the common sched/cputime subsystem
* Handle CONFIG_IRQ_TIME_ACCOUNTING=n correctly such that the
dynticks-idle accounting still elapses while on IRQs.
* Correctly substract idle steal cputime from idle time
git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git
timers/core
HEAD: 6a3d814ef2f6142714bef862be36def5ca4c9d96
Thanks,
Frederic
---
Frederic Weisbecker (15):
sched/idle: Handle offlining first in idle loop
sched/cputime: Remove superfluous and error prone kcpustat_field()
parameter
sched/cputime: Correctly support generic vtime idle time
powerpc/time: Prepare to stop elapsing in dynticks-idle
s390/time: Prepare to stop elapsing in dynticks-idle
tick/sched: Unify idle cputime accounting
cpufreq: ondemand: Simplify idle cputime granularity test
tick/sched: Remove nohz disabled special case in cputime fetch
tick/sched: Move dyntick-idle cputime accounting to cputime code
tick/sched: Remove unused fields
tick/sched: Account tickless idle cputime only when tick is stopped
tick/sched: Consolidate idle time fetching APIs
sched/cputime: Consolidate get_cpu_[idle|iowait]_time_us()
sched/cputime: Handle idle irqtime gracefully
sched/cputime: Handle dyntick-idle steal time correctly
arch/powerpc/kernel/time.c | 41 +++++
arch/s390/include/asm/idle.h | 11 +-
arch/s390/kernel/idle.c | 13 +-
arch/s390/kernel/vtime.c | 57 ++++++-
drivers/cpufreq/cpufreq.c | 29 +---
drivers/cpufreq/cpufreq_governor.c | 6 +-
drivers/cpufreq/cpufreq_ondemand.c | 7 +-
drivers/macintosh/rack-meter.c | 2 +-
fs/proc/stat.c | 40 +----
fs/proc/uptime.c | 8 +-
include/linux/kernel_stat.h | 76 ++++++++--
include/linux/tick.h | 4 -
include/linux/vtime.h | 20 ++-
kernel/rcu/tree.c | 9 +-
kernel/rcu/tree_stall.h | 7 +-
kernel/sched/cputime.c | 302 +++++++++++++++++++++++++++++++------
kernel/sched/idle.c | 11 +-
kernel/sched/sched.h | 1 +
kernel/time/tick-sched.c | 203 +++++--------------------
kernel/time/tick-sched.h | 12 --
kernel/time/timer_list.c | 6 +-
scripts/gdb/linux/timerlist.py | 4 -
22 files changed, 505 insertions(+), 364 deletions(-)
