Re: [PATCH v10 00/20] timers: Move from a push remote at enqueue to a pull at expiry model

Christian Loehle
2024-01-30T22:07:12Z

On 15/01/2024 14:37, Anna-Maria Behnsen wrote:
> Hi,
>
> the cleanup patches are already applied and so the contains only two parts:
>
> - Patches 1 - 4: timer base idle marking rework with two preparatory
> changes. See the section below for more details.
>
> - Patches 5 - 20: Updated timer pull model on top of timer idle rework
>
>
> The queue is available here:
>
> git://git.kernel.org/pub/scm/linux/kernel/git/tglx/devel timers/pushpull
>
>
> Move marking timer bases as idle into tick_nohz_stop_tick()
> -----------------------------------------------------------
>
> The idle marking of timer bases is done in get_next_timer_interrupt()
> whenever possible. The timer bases are idle, even if the tick will not be
> stopped. This lead to an IPI when a new first timer is enqueued remote. To
> prevent this, setting timer_base->in_idle flag is postponed to
> tick_nohz_stop_tick().
>
> Furthermore this synchronizes the states of timer base is_idle and
> tick_stopped. With the timer pull model in place, also the idle state in
> the hierarchy of a CPU is synchronized with the other idle related states.
>
>
> Timer pull model
> ----------------
>
> Placing timers at enqueue time on a target CPU based on dubious heuristics
> does not make any sense:
>
> 1) Most timer wheel timers are canceled or rearmed before they expire.
>
> 2) The heuristics to predict which CPU will be busy when the timer expires
> are wrong by definition.
>
> So placing the timers at enqueue wastes precious cycles.
>
> The proper solution to this problem is to always queue the timers on the
> local CPU and allow the non pinned timers to be pulled onto a busy CPU at
> expiry time.
>
> Therefore split the timer storage into local pinned and global timers:
> Local pinned timers are always expired on the CPU on which they have been
> queued. Global timers can be expired on any CPU.
>
> As long as a CPU is busy it expires both local and global timers. When a
> CPU goes idle it arms for the first expiring local timer. If the first
> expiring pinned (local) timer is before the first expiring movable timer,
> then no action is required because the CPU will wake up before the first
> movable timer expires. If the first expiring movable timer is before the
> first expiring pinned (local) timer, then this timer is queued into a idle
> timerqueue and eventually expired by some other active CPU.
>
> To avoid global locking the timerqueues are implemented as a hierarchy. The
> lowest level of the hierarchy holds the CPUs. The CPUs are associated to
> groups of 8, which are separated per node. If more than one CPU group
> exist, then a second level in the hierarchy collects the groups. Depending
> on the size of the system more than 2 levels are required. Each group has a
> "migrator" which checks the timerqueue during the tick for remote timers to
> be expired.
>
> If the last CPU in a group goes idle it reports the first expiring event in
> the group up to the next group(s) in the hierarchy. If the last CPU goes
> idle it arms its timer for the first system wide expiring timer to ensure
> that no timer event is missed.
>
>
> Testing
> ~~~~~~~
>
> Enqueue
> ^^^^^^^
>
> The impact of wasting cycles during enqueue by using the heuristic in
> contrast to always queuing the timer on the local CPU was measured with a
> micro benchmark. Therefore a timer is enqueued and dequeued in a loop with
> 1000 repetitions on a isolated CPU. The time the loop takes is measured. A
> quarter of the remaining CPUs was kept busy. This measurement was repeated
> several times. With the patch queue the average duration was reduced by
> approximately 25%.
>
> 145ns plain v6
> 109ns v6 with patch queue
>
>
> Furthermore the impact of residence in deep idle states of an idle system
> was investigated. The patch queue doesn't downgrade this behavior.
>
> dbench test
> ^^^^^^^^^^^
>
> A dbench test starting X pairs of client servers are used to create load on
> the system. The measurable value is the throughput. The tests were executed
> on a zen3 machine. The base is the tip tree branch timers/core which is
> based on a v6.6-rc1.
>
> governor menu
>
> NR timers/core pull-model impact
> ----------------------------------------------
> 1 353.19 (0.19) 353.45 (0.30) 0.07%
> 2 700.10 (0.96) 687.00 (0.20) -1.87%
> 4 1329.37 (0.63) 1282.91 (0.64) -3.49%
> 8 2561.16 (1.28) 2493.56 (1.76) -2.64%
> 16 4959.96 (0.80) 4914.59 (0.64) -0.91%
> 32 9741.92 (3.44) 8979.83 (1.13) -7.82%
> 64 16535.40 (2.84) 16388.47 (4.02) -0.89%
> 128 22136.83 (2.42) 23174.50 (1.43) 4.69%
> 256 39256.77 (4.48) 38994.00 (0.39) -0.67%
> 512 36799.03 (1.83) 38091.10 (0.63) 3.51%
> 1024 32903.03 (0.86) 35370.70 (0.89) 7.50%
>
>
> governor teo
>
> NR timers/core pull-model impact
> ----------------------------------------------
> 1 350.83 (1.27) 352.45 (0.96) 0.46%
> 2 699.52 (0.85) 690.10 (0.54) -1.35%
> 4 1339.53 (1.99) 1294.71 (2.71) -3.35%
> 8 2574.10 (0.76) 2495.46 (1.97) -3.06%
> 16 4898.50 (1.74) 4783.06 (1.64) -2.36%
> 32 9115.50 (4.63) 9037.83 (1.58) -0.85%
> 64 16663.90 (3.80) 16042.00 (1.72) -3.73%
> 128 25044.93 (1.11) 23250.03 (1.08) -7.17%
> 256 38059.53 (1.70) 39658.57 (2.98) 4.20%
> 512 36369.30 (0.39) 38890.13 (0.36) 6.93%
> 1024 33956.83 (1.14) 35514.83 (0.29) 4.59%
>
>
>
> Ping Pong Oberservation
> ^^^^^^^^^^^^^^^^^^^^^^^
>
> During testing on a mostly idle machine a ping pong game could be observed:
> a process_timeout timer is expired remotely on a non idle CPU. Then the CPU
> where the schedule_timeout() was executed to enqueue the timer comes out of
> idle and restarts the timer using schedule_timeout() and goes back to idle
> again. This is due to the fair scheduler which tries to keep the task on
> the CPU which it previously executed on.
>
>
>
>
> Possible Next Steps
> ~~~~~~~~~~~~~~~~~~~
>
> Simple deferrable timers are no longer required as they can be converted to
> global timers. If a CPU goes idle, a formerly deferrable timer will not
> prevent the CPU to sleep as long as possible. Only the last migrator CPU
> has to take care of them. Deferrable timers with timer pinned flags needs
> to be expired on the specified CPU but must not prevent CPU from going
> idle. They require their own timer base which is never taken into account
> when calculating the next expiry time. This conversation and required
> cleanup will be done in a follow up series.
>
>
> v9..v10: https://lore.kernel.org/r/20231201092654.34614-1-anna-maria@linutronix.de/
> - Address review Feedback of Bigeasy
>
>
> v8..v9: https://lore.kernel.org/r/20231004123454.15691-1-anna-maria@linutronix.de
> - Address review feedback
> - Add more minor cleanup fixes
> - fixes inconsistent idle related states
>
>
> v7..v8: https://lore.kernel.org/r/20230524070629.6377-1-anna-maria@linutronix.de
> - Address review feedback
> - Move marking timer base idle into tick_nohz_stop_tick()
> - Look ahead function to determine possible sleep lenght
>
>
> v6..v7:
> - Address review feedback of Frederic and bigeasy
> - Change lock, unlock fetch next timer interrupt logic after remote expiry
> - Move timer_expire_remote() into tick-internal.h
> - Add documentation section about "Required event and timerqueue update
> after remote expiry"
> - Fix fallout of kernel test robot
>
>
> v5..v6:
>
> - Address review of Frederic Weisbecker and Peter Zijlstra (spelling,
> locking, race in tmigr_handle_remote_cpu())
>
> - unconditionally set TIMER_PINNED flag in add_timer_on(); introduce
> add_timer() variants which set/unset TIMER_PINNED flag; drop fixing
> add_timer_on() call sites, as TIMER_PINNED flag is set implicitly;
> Fixing workqueue to use add_timer_global() instead of simply
> add_timer() for unbound work.
>
> - Drop support for siblings to end up in the same level 0 group (could be
> added again in a better way as an improvement later on)
>
> - Do not send IPI for new first deferrable timers
>
> v4..v5:
> - address review feedback of Frederic Weisbecker
> - fix issue with group timer update after remote expiry
>
> v3..v4:
> - address review feedback of Frederic Weisbecker
> - address kernel test robot fallout
> - Move patch 16 "add_timer_on(): Make sure callers have TIMER_PINNED
> flag" at the begin of the queue to prevent timers to end up in global
> timer base when they were queued using add_timer_on()
> - Fix some comments and typos
>
> v2..v3: https://lore.kernel.org/r/20170418111102.490432548@linutronix.de/
> - Minimize usage of locks by storing data using atomic_cmpxchg() for
> migrator information and information about active cpus.
>
>
> Thanks,
>
> Anna-Maria
>
>
>
>
> Anna-Maria Behnsen (18):
> timers: Restructure get_next_timer_interrupt()
> timers: Split out get next timer interrupt
> timers: Move marking timer bases idle into tick_nohz_stop_tick()
> timers: Optimization for timer_base_try_to_set_idle()
> timers: Introduce add_timer() variants which modify timer flags
> workqueue: Use global variant for add_timer()
> timers: add_timer_on(): Make sure TIMER_PINNED flag is set
> timers: Ease code in run_local_timers()
> timers: Split next timer interrupt logic
> timers: Keep the pinned timers separate from the others
> timers: Retrieve next expiry of pinned/non-pinned timers separately
> timers: Split out "get next timer interrupt" functionality
> timers: Add get next timer interrupt functionality for remote CPUs
> timers: Check if timers base is handled already
> timers: Introduce function to check timer base is_idle flag
> timers: Implement the hierarchical pull model
> timer_migration: Add tracepoints
> timers: Always queue timers on the local CPU
>
> Richard Cochran (linutronix GmbH) (2):
> timers: Restructure internal locking
> tick/sched: Split out jiffies update helper function
>
> MAINTAINERS | 1 +
> include/linux/cpuhotplug.h | 1 +
> include/linux/timer.h | 16 +-
> include/trace/events/timer_migration.h | 297 +++++
> kernel/time/Makefile | 3 +
> kernel/time/tick-internal.h | 14 +
> kernel/time/tick-sched.c | 65 +-
> kernel/time/timer.c | 505 +++++--
> kernel/time/timer_migration.c | 1693 ++++++++++++++++++++++++
> kernel/time/timer_migration.h | 147 ++
> kernel/workqueue.c | 2 +-
> 11 files changed, 2629 insertions(+), 115 deletions(-)
> create mode 100644 include/trace/events/timer_migration.h
> create mode 100644 kernel/time/timer_migration.c
> create mode 100644 kernel/time/timer_migration.h

Hi Anna-Maria,
I did some quick measurements on a pixel6 Android 14 with 6.6 kernel baseline.
The workload is 5 iterations of uibenchjanktests (~70 Min runtime total).
Backport of timers/pushpull up to:
6b7e23d1f495 ("timers: Always queue timers on the local CPU").

Power:
+------------+--------+------------+-------+-----------+
| channel | metric | tag | value | perc_diff |
+------------+--------+------------+-------+-----------+
| CPU | gmean | mainline_5 | 196.6 | 0.0% |
| CPU-Big | gmean | mainline_5 | 65.3 | 0.0% |
| CPU-Little | gmean | mainline_5 | 99.6 | 0.0% |
| CPU-Mid | gmean | mainline_5 | 31.6 | 0.0% |
| GPU | gmean | mainline_5 | 36.7 | 0.0% |
| Total | gmean | mainline_5 | 233.3 | 0.0% |
| CPU | gmean | pushpull_5 | 195.9 | -0.35% |
| CPU-Big | gmean | pushpull_5 | 64.8 | -0.85% |
| CPU-Little | gmean | pushpull_5 | 98.5 | -1.12% |
| CPU-Mid | gmean | pushpull_5 | 32.6 | 3.13% |
| GPU | gmean | pushpull_5 | 36.8 | 0.19% |
| Total | gmean | pushpull_5 | 232.6 | -0.26% |
+------------+--------+------------+-------+-----------+
(Slightly skewed in favor of mainline because of starting
temperature.)

Idle residency:
+------------+---------+------------+--------+
| tag | cluster | idle_state | time |
+------------+---------+------------+--------+
| mainline_5 | little | -1.0 | 518.42 |
| mainline_5 | little | 0.0 | 238.28 |
| mainline_5 | little | 1.0 | 19.7 |
| mainline_5 | mid | -1.0 | 201.0 |
| mainline_5 | mid | 0.0 | 335.26 |
| mainline_5 | mid | 1.0 | 240.15 |
| mainline_5 | big | -1.0 | 173.86 |
| mainline_5 | big | 0.0 | 330.93 |
| mainline_5 | big | 1.0 | 271.61 |
| pushpull_5 | little | -1.0 | 526.45 |
| pushpull_5 | little | 0.0 | 257.77 |
| pushpull_5 | little | 1.0 | 5.18 |
| pushpull_5 | mid | -1.0 | 220.98 |
| pushpull_5 | mid | 0.0 | 347.43 |
| pushpull_5 | mid | 1.0 | 220.98 |
| pushpull_5 | big | -1.0 | 177.36 |
| pushpull_5 | big | 0.0 | 331.61 |
| pushpull_5 | big | 1.0 | 280.42 |
+------------+---------+------------+--------+

We can see the improvement we were hoping for:
Longer idle times on the big cores.

For completeness here are the idle misses:
+------------+-------+--------------------+
| tag | type | count_perc |
+------------+-------+--------------------+
| mainline_5 | False | 3.4829999999999997 |
| mainline_5 | True | 15.639000000000001 |
| pushpull_5 | False | 3.487 |
| pushpull_5 | True | 15.881 |
+------------+-------+--------------------+

If there is anything else you would like to see some data on, please
let me know.

Kind Regards,
Christian