The current timer wheel has some drawbacks:
1) Cascading:
Cascading can be an unbound operation and is completely pointless in most
cases because the vast majority of the timer wheel timers are canceled or
rearmed before expiration. (They are used as timeout safeguards, not as
real timers to measure time.)
2) No fast lookup of the next expiring timer:
In NOHZ scenarios the first timer soft interrupt after a long NOHZ period
must fast forward the base time to the current value of jiffies. As we
have no way to find the next expiring timer fast, the code loops linearly
and increments the base time one by one and checks for expired timers
in each step. This causes unbound overhead spikes exactly in the moment
when we should wake up as fast as possible.
After a thorough analysis of real world data gathered on laptops,
workstations, webservers and other machines (thanks Chris!) I came to the
conclusion that the current 'classic' timer wheel implementation can be
modified to address the above issues.
The vast majority of timer wheel timers is canceled or rearmed before
expiry. Most of them are timeouts for networking and other I/O tasks. The
nature of timeouts is to catch the exception from normal operation (TCP ack
timed out, disk does not respond, etc.). For these kinds of timeouts the
accuracy of the timeout is not really a concern. Timeouts are very often
approximate worst-case values and in case the timeout fires, we already
waited for a long time and performance is down the drain already.
The few timers which actually expire can be split into two categories:
1) Short expiry times which expect halfways accurate expiry
2) Long term expiry times are inaccurate today already due to the
batching which is done for NOHZ automatically and also via the
set_timer_slack() API.
So for long term expiry timers we can avoid the cascading property and just
leave them in the less granular outer wheels until expiry or
cancelation. Timers which are armed with a timeout larger than the wheel
capacity are no longer cascaded. We expire them with the longest possible
timeout (6+ days). We have not observed such timeouts in our data collection,
but at least we handle them, applying the rule of the least surprise.
To avoid extending the wheel levels for HZ=1000 so we can accomodate the
longest observed timeouts (5 days in the network conntrack code) we reduce the
first level granularity on HZ=1000 to 4ms, which effectively is the same as
the HZ=250 behaviour. From our data analysis there is nothing which relies on
that 1ms granularity and as a side effect we get better batching and timer
locality for the networking code as well.
Contrary to the classic wheel the granularity of the next wheel is not the
capacity of the first wheel. The granularities of the wheels are in the
currently chosen setting 8 times the granularity of the previous wheel.
So for HZ=250 we end up with the following granularity levels:
Level Offset Granularity Range
0 0 4 ms 0 ms - 252 ms
1 64 32 ms 256 ms - 2044 ms (256ms - ~2s)
2 128 256 ms 2048 ms - 16380 ms (~2s - ~16s)
3 192 2048 ms (~2s) 16384 ms - 131068 ms (~16s - ~2m)
4 256 16384 ms (~16s) 131072 ms - 1048572 ms (~2m - ~17m)
5 320 131072 ms (~2m) 1048576 ms - 8388604 ms (~17m - ~2h)
6 384 1048576 ms (~17m) 8388608 ms - 67108863 ms (~2h - ~18h)
7 448 8388608 ms (~2h) 67108864 ms - 536870911 ms (~18h - ~6d)
That's a worst case inaccuracy of 12.5% for the timers which are queued at the
beginning of a level.
So the new wheel concept addresses the old issues:
1) Cascading is avoided completely
2) By keeping the timers in the bucket until expiry/cancelation we can track
the buckets which have timers enqueued in a bucket bitmap and therefore can
look up the next expiring timer very fast and O(1).
A further benefit of the concept is that the slack calculation which is done
on every timer start is no longer necessary because the granularity levels
provide natural batching already.
Our extensive testing with various loads did not show any performance
degradation vs. the current wheel implementation.
This patch does not address the 'fast lookup' issue as we wanted to make sure
that there is no regression introduced by the wheel redesign. The
optimizations are in follow up patches.
This patch contains fixes from Anna-Maria Gleixner and Richard Cochran.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Arjan van de Ven <arjan@infradead.org>
Cc: Chris Mason <clm@fb.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: George Spelvin <linux@sciencehorizons.net>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Len Brown <lenb@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: rt@linutronix.de
Link: http://lkml.kernel.org/r/20160704094342.108621834@linutronix.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>
We want to move the timer migration logic from a 'push' to a 'pull' model.
Under the current 'push' model pinned timers are handled via
a runtime API variant: mod_timer_pinned().
The 'pull' model requires us to store the pinned attribute of a timer
in the timer_list structure itself, as a new TIMER_PINNED bit in
timer->flags.
This flag must be set at initialization time and the timer APIs
recognize the flag.
This patch:
- Implements the new flag and associated new-style initialization
methods
- makes mod_timer() recognize new-style pinned timers,
- and adds some migration helper facility to allow
step by step conversion of old-style to new-style
pinned timers.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Arjan van de Ven <arjan@infradead.org>
Cc: Chris Mason <clm@fb.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: George Spelvin <linux@sciencehorizons.net>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Len Brown <lenb@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: rt@linutronix.de
Link: http://lkml.kernel.org/r/20160704094341.049338558@linutronix.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>
hrtimer_init_on_stack() needs a matching call to
destroy_hrtimer_on_stack(), so both need to be exported.
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
When activating a static object we need make sure that the object is
tracked in the object tracker. If it is a non-static object then the
activation is illegal.
In previous implementation, each subsystem need take care of this in
their fixup callbacks. Actually we can put it into debugobjects core.
Thus we can save duplicated code, and have *pure* fixup callbacks.
To achieve this, a new callback "is_static_object" is introduced to let
the type specific code decide whether a object is static or not. If
yes, we take it into object tracker, otherwise give warning and invoke
fixup callback.
This change has paassed debugobjects selftest, and I also do some test
with all debugobjects supports enabled.
At last, I have a concern about the fixups that can it change the object
which is in incorrect state on fixup? Because the 'addr' may not point
to any valid object if a non-static object is not tracked. Then Change
such object can overwrite someone's memory and cause unexpected
behaviour. For example, the timer_fixup_activate bind timer to function
stub_timer.
Link: http://lkml.kernel.org/r/1462576157-14539-1-git-send-email-changbin.du@intel.com
[changbin.du@intel.com: improve code comments where invoke the new is_static_object callback]
Link: http://lkml.kernel.org/r/1462777431-8171-1-git-send-email-changbin.du@intel.com
Signed-off-by: Du, Changbin <changbin.du@intel.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Josh Triplett <josh@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tejun Heo <tj@kernel.org>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
timespec64_add_safe() has been defined in time64.h for 64 bit systems.
But, 32 bit systems only have an extern function prototype defined.
Provide a definition for the above function.
The function will be necessary as part of y2038 changes. struct
timespec is not y2038 safe. All references to timespec will be replaced
by struct timespec64. The function is meant to be a replacement for
timespec_add_safe().
The implementation is similar to timespec_add_safe().
Link: http://lkml.kernel.org/r/1461947989-21926-2-git-send-email-deepa.kernel@gmail.com
Signed-off-by: Deepa Dinamani <deepa.kernel@gmail.com>
Acked-by: John Stultz <john.stultz@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Pull timer updates from Thomas Gleixner:
"A rather small set of patches from the timer departement:
- Some more y2038 work
- Yet another new clocksource driver
- The usual set of small fixes, cleanups and enhancements"
* 'timers-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
clocksource/drivers/tegra: Remove unused suspend/resume code
clockevents/driversi/mps2: add MPS2 Timer driver
dt-bindings: document the MPS2 timer bindings
clocksource/drivers/mtk_timer: Add __init attribute
clockevents/drivers/dw_apb_timer: Implement ->set_state_oneshot_stopped()
time: Introduce do_sys_settimeofday64()
security: Introduce security_settime64()
clocksource: Add missing include of of.h.
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
...
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 <fweisbec@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Byungchul Park <byungchul.park@lge.com>
Cc: Chris Metcalf <cmetcalf@ezchip.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Luiz Capitulino <lcapitulino@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul E . McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1460555812-25375-3-git-send-email-fweisbec@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
The do_sys_settimeofday() function uses a timespec, which is not year
2038 safe on 32bit systems.
Thus this patch introduces do_sys_settimeofday64(), which allows us to
transition users of do_sys_settimeofday() to using 64bit time types.
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
[jstultz: Include errno-base.h to avoid build issue on some arches]
Signed-off-by: John Stultz <john.stultz@linaro.org>
Use the more common logging method with the eventual goal of removing
pr_warning altogether.
Miscellanea:
- Realign arguments
- Coalesce formats
- Add missing space between a few coalesced formats
Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com> [kernel/power/suspend.c]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Merge second patch-bomb from Andrew Morton:
- a couple of hotfixes
- the rest of MM
- a new timer slack control in procfs
- a couple of procfs fixes
- a few misc things
- some printk tweaks
- lib/ updates, notably to radix-tree.
- add my and Nick Piggin's old userspace radix-tree test harness to
tools/testing/radix-tree/. Matthew said it was a godsend during the
radix-tree work he did.
- a few code-size improvements, switching to __always_inline where gcc
screwed up.
- partially implement character sets in sscanf
* emailed patches from Andrew Morton <akpm@linux-foundation.org>: (118 commits)
sscanf: implement basic character sets
lib/bug.c: use common WARN helper
param: convert some "on"/"off" users to strtobool
lib: add "on"/"off" support to kstrtobool
lib: update single-char callers of strtobool()
lib: move strtobool() to kstrtobool()
include/linux/unaligned: force inlining of byteswap operations
include/uapi/linux/byteorder, swab: force inlining of some byteswap operations
include/asm-generic/atomic-long.h: force inlining of some atomic_long operations
usb: common: convert to use match_string() helper
ide: hpt366: convert to use match_string() helper
ata: hpt366: convert to use match_string() helper
power: ab8500: convert to use match_string() helper
power: charger_manager: convert to use match_string() helper
drm/edid: convert to use match_string() helper
pinctrl: convert to use match_string() helper
device property: convert to use match_string() helper
lib/string: introduce match_string() helper
radix-tree tests: add test for radix_tree_iter_next
radix-tree tests: add regression3 test
...
Pull trivial tree updates from Jiri Kosina.
* 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/jikos/trivial:
drivers/rtc: broken link fix
drm/i915 Fix typos in i915_gem_fence.c
Docs: fix missing word in REPORTING-BUGS
lib+mm: fix few spelling mistakes
MAINTAINERS: add git URL for APM driver
treewide: Fix typo in printk
Thomas Gleixner
Anna-Maria Gleixner
Guenter Roeck
Du, Changbin
Deepa Dinamani
Linus Torvalds
Peter Zijlstra
Frederic Weisbecker
Baolin Wang
Andrew Morton
Joe Perches