linux-toradex.git/kernel/time/tick-sched.c, branch v4.8

tick/nohz: Fix softlockup on scheduler stalls in kvm guest

2016-09-02T08:25:40+00:00

tick_nohz_start_idle() is prevented to be called if the idle tick can't 
be stopped since commit 1f3b0f8243cb934 ("tick/nohz: Optimize nohz idle 
enter"). As a result, after suspend/resume the host machine, full dynticks 
kvm guest will softlockup:

 NMI watchdog: BUG: soft lockup - CPU#0 stuck for 26s! [swapper/0:0]
 Call Trace:
  default_idle+0x31/0x1a0
  arch_cpu_idle+0xf/0x20
  default_idle_call+0x2a/0x50
  cpu_startup_entry+0x39b/0x4d0
  rest_init+0x138/0x140
  ? rest_init+0x5/0x140
  start_kernel+0x4c1/0x4ce
  ? set_init_arg+0x55/0x55
  ? early_idt_handler_array+0x120/0x120
  x86_64_start_reservations+0x24/0x26
  x86_64_start_kernel+0x142/0x14f

In addition, cat /proc/stat | grep cpu in guest or host:

cpu  398 16 5049 15754 5490 0 1 46 0 0
cpu0 206 5 450 0 0 0 1 14 0 0
cpu1 81 0 3937 3149 1514 0 0 9 0 0
cpu2 45 6 332 6052 2243 0 0 11 0 0
cpu3 65 2 328 6552 1732 0 0 11 0 0

The idle and iowait states are weird 0 for cpu0(housekeeping). 

The bug is present in both guest and host kernels, and they both have 
cpu0's idle and iowait states issue, however, host kernel's suspend/resume 
path etc will touch watchdog to avoid the softlockup.

- The watchdog will not be touched in tick_nohz_stop_idle path (need be 
  touched since the scheduler stall is expected) if idle_active flags are 
  not detected.
- The idle and iowait states will not be accounted when exit idle loop 
  (resched or interrupt) if idle start time and idle_active flags are 
  not set. 

This patch fixes it by reverting commit 1f3b0f8243cb934 since can't stop 
idle tick doesn't mean can't be idle.

Fixes: 1f3b0f8243cb934 ("tick/nohz: Optimize nohz idle enter")
Signed-off-by: Wanpeng Li 
Cc: Sanjeev Yadav
Cc: Gaurav Jindal
Cc: stable@vger.kernel.org
Cc: kvm@vger.kernel.org
Cc: Radim Krčmář 
Cc: Peter Zijlstra 
Cc: Paolo Bonzini 
Link: http://lkml.kernel.org/r/1472798303-4154-1-git-send-email-wanpeng.li@hotmail.com
Signed-off-by: Thomas Gleixner

tick/nohz: Optimize nohz idle enter

2016-07-19T11:48:24+00:00

tick_nohz_start_idle is called before checking whether the idle tick can be
stopped. If the tick cannot be stopped, calling tick_nohz_start_idle() is
pointless and just wasting CPU cycles.

Only invoke tick_nohz_start_idle() when can_stop_idle_tick() returns true. A
short one minute observation of the effect on ARM64 shows a reduction of calls
by 1.5% thus optimizing the idle entry sequence.

[tglx: Massaged changelog ]

Co-developed-by: Sanjeev Yadav
Signed-off-by: Gaurav Jindal
Link: http://lkml.kernel.org/r/20160714120416.GB21099@gaurav.jindal@spreadtrum.com
Signed-off-by: Thomas Gleixner

Merge branch 'timers/fast-wheel' into timers/core

2016-07-07T08:35:28+00:00

timers: Forward the wheel clock whenever possible

2016-07-07T08:35:11+00:00

The wheel clock is stale when a CPU goes into a long idle sleep. This has the
side effect that timers which are queued end up in the outer wheel levels.
That results in coarser granularity.

To solve this, we keep track of the idle state and forward the wheel clock
whenever possible.

Signed-off-by: Thomas Gleixner 
Cc: Arjan van de Ven 
Cc: Chris Mason 
Cc: Eric Dumazet 
Cc: Frederic Weisbecker 
Cc: George Spelvin 
Cc: Josh Triplett 
Cc: Len Brown 
Cc: Linus Torvalds 
Cc: Paul E. McKenney 
Cc: Peter Zijlstra 
Cc: Rik van Riel 
Cc: rt@linutronix.de
Link: http://lkml.kernel.org/r/20160704094342.512039360@linutronix.de
Signed-off-by: Ingo Molnar

timers/nohz: Remove pointless tick_nohz_kick_tick() function

2016-07-07T08:35:10+00:00

This was a failed attempt to optimize the timer expiry in idle, which was
disabled and never revisited. Remove the cruft.

Signed-off-by: Thomas Gleixner 
Cc: Arjan van de Ven 
Cc: Chris Mason 
Cc: Eric Dumazet 
Cc: Frederic Weisbecker 
Cc: George Spelvin 
Cc: Josh Triplett 
Cc: Len Brown 
Cc: Linus Torvalds 
Cc: Paul E. McKenney 
Cc: Peter Zijlstra 
Cc: Rik van Riel 
Cc: rt@linutronix.de
Link: http://lkml.kernel.org/r/20160704094342.431073782@linutronix.de
Signed-off-by: Ingo Molnar

timers/nohz: Capitalize 'CPU' consistently

2016-07-01T10:45:34+00:00

While reviewing another patch I noticed that kernel/time/tick-sched.c
had a charmingly (confusingly, annoyingly) rich set of variants for
spelling 'CPU':

  cpu
  cpus
  CPU
  CPUs
  per CPU
  per-CPU
  per cpu

... sometimes these were mixed even within the same comment block!

Compress these variants down to a single consistent set of:

  CPU
  CPUs
  per-CPU

Cc: Frederic Weisbecker 
Cc: Thomas Gleixner 
Cc: Linus Torvalds 
Cc: Peter Zijlstra 
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar

timers/nohz: Fix several typos

2016-07-01T10:39:22+00:00

Signed-off-by: Wei Jiangang 
Cc: Linus Torvalds 
Cc: Peter Zijlstra 
Cc: Thomas Gleixner 
Cc: fenghua.yu@intel.com
Link: http://lkml.kernel.org/r/1467175910-2966-2-git-send-email-weijg.fnst@cn.fujitsu.com
Signed-off-by: Ingo Molnar

Merge branch 'sched-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

2016-05-16T21:47:16+00:00

Pull scheduler updates from Ingo Molnar:

 - massive CPU hotplug rework (Thomas Gleixner)

 - improve migration fairness (Peter Zijlstra)

 - CPU load calculation updates/cleanups (Yuyang Du)

 - cpufreq updates (Steve Muckle)

 - nohz optimizations (Frederic Weisbecker)

 - switch_mm() micro-optimization on x86 (Andy Lutomirski)

 - ... lots of other enhancements, fixes and cleanups.

* 'sched-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (66 commits)
  ARM: Hide finish_arch_post_lock_switch() from modules
  sched/core: Provide a tsk_nr_cpus_allowed() helper
  sched/core: Use tsk_cpus_allowed() instead of accessing ->cpus_allowed
  sched/loadavg: Fix loadavg artifacts on fully idle and on fully loaded systems
  sched/fair: Correct unit of load_above_capacity
  sched/fair: Clean up scale confusion
  sched/nohz: Fix affine unpinned timers mess
  sched/fair: Fix fairness issue on migration
  sched/core: Kill sched_class::task_waking to clean up the migration logic
  sched/fair: Prepare to fix fairness problems on migration
  sched/fair: Move record_wakee()
  sched/core: Fix comment typo in wake_q_add()
  sched/core: Remove unused variable
  sched: Make hrtick_notifier an explicit call
  sched/fair: Make ilb_notifier an explicit call
  sched/hotplug: Make activate() the last hotplug step
  sched/hotplug: Move migration CPU_DYING to sched_cpu_dying()
  sched/migration: Move CPU_ONLINE into scheduler state
  sched/migration: Move calc_load_migrate() into CPU_DYING
  sched/migration: Move prepare transition to SCHED_STARTING state
  ...

locking/atomics: Flip atomic_fetch_or() arguments

2016-05-05T07:58:52+00:00

All the atomic operations have their arguments the wrong way around;
make atomic_fetch_or() consistent and flip them.

Signed-off-by: Peter Zijlstra (Intel) 
Cc: Andrew Morton 
Cc: Linus Torvalds 
Cc: Paul E. McKenney 
Cc: Peter Zijlstra 
Cc: Thomas Gleixner 
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar

sched/fair: Correctly handle nohz ticks CPU load accounting

2016-04-23T12:20:42+00:00

Ticks can happen while the CPU is in dynticks-idle or dynticks-singletask
mode. In fact "nohz" or "dynticks" only mean that we exit the periodic
mode and we try to minimize the ticks as much as possible. The nohz
subsystem uses a confusing terminology with the internal state
"ts->tick_stopped" which is also available through its public interface
with tick_nohz_tick_stopped(). This is a misnomer as the tick is instead
reduced with the best effort rather than stopped. In the best case the
tick can indeed be actually stopped but there is no guarantee about that.
If a timer needs to fire one second later, a tick will fire while the
CPU is in nohz mode and this is a very common scenario.

Now this confusion happens to be a problem with CPU load updates:
cpu_load_update_active() doesn't handle nohz ticks correctly because it
assumes that ticks are completely stopped in nohz mode and that
cpu_load_update_active() can't be called in dynticks mode. When that
happens, the whole previous tickless load is ignored and the function
just records the load for the current tick, ignoring potentially long
idle periods behind.

In order to solve this, we could account the current load for the
previous nohz time but there is a risk that we account the load of a
task that got freshly enqueued for the whole nohz period.

So instead, lets record the dynticks load on nohz frame entry so we know
what to record in case of nohz ticks, then use this record to account
the tickless load on nohz ticks and nohz frame end.

Signed-off-by: Frederic Weisbecker 
Signed-off-by: Peter Zijlstra (Intel) 
Cc: Byungchul Park 
Cc: Chris Metcalf 
Cc: Christoph Lameter 
Cc: Luiz Capitulino 
Cc: Mike Galbraith 
Cc: Paul E . McKenney 
Cc: Peter Zijlstra 
Cc: Rik van Riel 
Cc: Thomas Gleixner 
Link: http://lkml.kernel.org/r/1460555812-25375-3-git-send-email-fweisbec@gmail.com
Signed-off-by: Ingo Molnar