Re: [patch 00/48] hrtimer,sched: General optimizations and hrtick enablement

[Christian Loehle]

On 2/24/26 16:35, Thomas Gleixner wrote:
> Peter recently posted a series tweaking the hrtimer subsystem to reduce the
> overhead of the scheduler hrtick timer so it can be enabled by default:
> 
>    https://lore.kernel.org/20260121162010.647043073@xxxxxxxxxxxxx
> 
> That turned out to be incomplete and led to a deeper investigation of the
> related bits and pieces.
> 
> The problem is that the hrtick deadline changes on every context switch and
> is also modified by wakeups and balancing. On a hackbench run this results
> in about 2500 clockevent reprogramming cycles per second, which is
> especially hurtful in a VM as accessing the clockevent device implies a
> VM-Exit.
> 
> The following series addresses various aspects of the overall related
> problem space:
> 
>     1) Scheduler
> 
>        Aside of some trivial fixes the handling of the hrtick timer in
>        the scheduler is suboptimal:
> 
>         - schedule() modifies the hrtick when picking the next task
> 
> 	- schedule() can modify the hrtick when the balance callback runs
>           before releasing rq:lock
> 
> 	- the expiry time is unfiltered and can result in really tiny
>           changes of the expiry time, which are functionally completely
>           irrelevant
> 
>        Solve this by deferring the hrtick update to the end of schedule()
>        and filtering out tiny changes.
> 
> 
>     2) Clocksource, clockevents, timekeeping
> 
>         - Reading the current clocksource involves an indirect call, which
>           is expensive especially for clocksources where the actual read is
>           a single instruction like the TSC read on x86.
> 
> 	  This could be solved with a static call, but the architecture
> 	  coverage for static calls is meager and that still has the
> 	  overhead of a function call and in the worst case a return
> 	  speculation mitigation.
> 
> 	  As x86 and other architectures like S390 have one preferred
> 	  clocksource which is normally used on all contemporary systems,
> 	  this begs for a fully inlined solution.
> 
> 	  This is achieved by a config option which tells the core code to
> 	  use the architecture provided inline guarded by a static branch.
> 
> 	  If the branch is disabled, the indirect function call is used as
> 	  before. If enabled the inlined read is utilized.
> 
> 	  The branch is disabled by default and only enabled after a
> 	  clocksource is installed which has the INLINE feature flag
> 	  set. When the clocksource is replaced the branch is disabled
> 	  before the clocksource change happens.
> 
> 
>         - Programming clock events is based on calculating a relative
>           expiry time, converting it to the clock cycles corresponding to
>           the clockevent device frequency and invoking the set_next_event()
>           callback of the clockevent device.
> 
> 	  That works perfectly fine as most hardware timers are count down
> 	  implementations which require a relative time for programming.
> 
> 	  But clockevent devices which are coupled to the clocksource and
> 	  provide a less than equal comparator suffer from this scheme. The
> 	  core calculates the relative expiry time based on a clock read
> 	  and the set_next_event() callback has to read the same clock
> 	  again to convert it back to a absolute time which can be
> 	  programmed into the comparator.
> 
> 	  The other issue is that the conversion factor of the clockevent
> 	  device is calculated at boot time and does not take the NTP/PTP
> 	  adjustments of the clocksource frequency into account. Depending
> 	  on the direction of the adjustment this can cause timers to fire
> 	  early or late. Early is the more problematic case as the timer
> 	  interrupt has to reprogram the device with a very short delta as
> 	  it can't expire timers early.
> 
> 	  This can be optimized by introducing a 'coupled' mode for the
> 	  clocksource and the clockevent device.
> 
> 	    A) If the clocksource indicates support for 'coupled' mode, the
> 	       timekeeping core calculates a (NTP adjusted) reverse
> 	       conversion factor from the clocksource to nanoseconds
> 	       conversion. This takes NTP adjustments into account and
> 	       keeps the conversion in sync.
> 
> 	    B) The timekeeping core provides a function to convert an
> 	       absolute CLOCK_MONOTONIC expiry time into a absolute time in
> 	       clocksource cycles which can be programmed directly into the
> 	       comparator without reading the clocksource at all.
> 
> 	       This is possible because timekeeping keeps a time pair of
> 	       the base cycle count and the corresponding CLOCK_MONOTONIC base
> 	       time at the last update of the timekeeper.
> 
> 	       So the absolute cycle time can be calculated by calculating
> 	       the relative time to the CLOCK_MONOTONIC base time,
> 	       converting the delta into cycles with the help of #A and
> 	       adding the base cycle count. Pure math, no hardware access.
> 
> 	    C) The clockevent reprogramming code invokes this conversion
> 	       function when the clockevent device indicates 'coupled'
> 	       mode.  The function returns false when the corresponding
> 	       clocksource is not the current system clocksource (based on
> 	       a clocksource ID check) and true if the clocksource matches
> 	       and the conversion is successful.
> 
> 	       If false, the regular relative set_next_event() mechanism is
> 	       used, otherwise a new set_next_coupled() callback which
> 	       takes the calculated absolute expiry time as argument.
> 
> 	       Similar to the clocksource, this new callback can optionally
> 	       be inlined.
> 
> 
>     3) hrtimers
> 
>        It turned out that the hrtimer code needed a long overdue spring
>        cleaning independent of the problem at hand. That was conducted
>        before tackling the actual performance issues:
> 
>        - Timer locality
> 
>        	 The handling of timer locality is suboptimal and results often in
> 	 pointless invocations of switch_hrtimer_base() which end up
> 	 keeping the CPU base unchanged.
> 
> 	 Aside of the pointless overhead, this prevents further
> 	 optimizations for the common local case.
> 
> 	 Address this by improving the decision logic for keeping the clock
> 	 base local and splitting out the (re)arm handling into a unified
> 	 operation.
> 
> 
>        - Evalutation of the clock base expiries
> 
>        	 The clock bases (MONOTONIC, REALTIME, BOOT, TAI) cache the first
>        	 expiring timer, but not the corresponding expiry time, which means
>        	 a re-evaluation of the clock bases for the next expiring timer on
>        	 the CPU requires to touch up to for extra cache lines.
> 
> 	 Trivial to solve by caching the earliest expiry time in the clock
> 	 base itself.
> 
> 
>        - Reprogramming of the clock event device
> 
>        	 The hrtimer interrupt already deferres reprogramming until the
>        	 interrupt handler completes, but in case of the hrtick timer
>        	 that's not sufficient because the hrtick timer callback only sets
>        	 the NEED_RESCHED flag but has no information about the next hrtick
>        	 timer expiry time, which can only be determined in the scheduler.
> 
> 	 Expand the deferred reprogramming so it can ideally be handled in
> 	 the subsequent schedule() after the new hrtick value has been
> 	 established. If there is no schedule, soft interrupts have to be
> 	 processed on return from interrupt or a nested interrupt hits
> 	 before reaching schedule, the deferred programming is handled in
> 	 those contexts.
> 
> 
>        - Modification of queued timers
> 
>        	 If a timer is already queued modifying the expiry time requires
>        	 dequeueing from the RB tree and requeuing after the new expiry
>        	 value has been updated. It turned out that the hrtick timer
>        	 modification end up very often at the same spot in the RB tree as
>        	 they have been before, which means the dequeue/enqueue cycle along
>        	 with the related rebalancing could have been avoided. The timer
>        	 wheel timers have a similar mechanism by checking upfront whether
>        	 the resulting expiry time keeps them in the same hash bucket.
> 
> 	 It was tried to check this by using rb_prev() and rb_next() to
> 	 evaluate whether the modification keeps the timer in the same
> 	 spot, but that turned out to be really inefficent.
> 
> 	 Solve this by providing a RB tree variant which extends the node
> 	 with links to the previous and next nodes, which is established
> 	 when the node is linked into the tree or adjusted when it is
> 	 removed. These links allow a quick peek into the previous and next
> 	 expiry time and if the new expiry stays in the boundary the whole
> 	 RB tree operation can be avoided.
> 
> 	 This also simplifies the caching and update of the leftmost node
> 	 as on remove the rb_next() walk can be completely avoided. It
> 	 would obviously provide a cached rightmost pointer too, but there
> 	 is not use case for that (yet).
> 
> 	 On a hackbench run this results in about 35% of the updates being
> 	 handled that way, which cuts the execution time of
> 	 hrtimer_start_range_ns() down to 50ns on a 2GHz machine.
> 
> 
>        - Cancellation of queued timers
> 
>        	 Cancelling a timer or moving its expiry time past the programmed
>        	 time can result in reprogramming the clock event device.
>        	 Especially with frequent modifications of a queued timer this
>        	 results in substantial overhead especially in VMs.
> 
> 	 Provide an option for hrtimers to tell the core to handle
> 	 reprogramming lazy in those cases, which means it trades frequent
> 	 reprogramming against an occasional pointless hrtimer interrupt.
> 
> 	 But it turned out for the hrtick timer this is a reasonable
> 	 tradeoff. It's especially valuable when transitioning to idle,
> 	 where the timer has to be cancelled but then the NOHZ idle code
> 	 will reprogram it in case of a long idle sleep anyway. But also in
> 	 high frequency scheduling scenarios this turned out to be
> 	 beneficial.
> 
> 
> With all the above modifications in place enabling hrtick does not longer
> result in regressions compared to the hrtick disabled mode.
> 
> The reprogramming frequency of the clockevent device got down from
> ~2500/sec to ~100/sec for a hackbench run with a spurious hrtimer interrupt
> ratio of about 25%.
> 
> What's interesting is the astonishing improvement of a hackbench run with
> the following command line parameters: '-l$LOOPS -p -s8'. That uses pipes
> with a message size of 8 bytes. On a 112 CPU SKL machine this results in:
> 
>        	   NO HRTICK[_DL]		HRTICK[_DL]
> runtime:   0.840s			0.481s		~-42%
> 
> With other message sizes up to 256, HRTICK still results in improvements,
> but not in that magnitude. Haven't investigated the cause of that yet.
> 
> While quite some parts of the series are independent enhancements, I've
> decided to keep them together in one big pile for now as all of the
> components are required to actually achieve the overall goal.
> 
> The patches have been already structured in a way that they can be
> distributed to different subsystem branches without causing major cross
> subsystem contamination or merge conflict headaches.
> 
> The series applies on v7.0-rc1 and is also available from git:
> 
>    git://git.kernel.org/pub/scm/linux/kernel/git/tglx/devel.git sched/hrtick
> 
> Thanks,
> 
> 	tglx
> ---
>  arch/x86/Kconfig                      |    2 
>  arch/x86/include/asm/clock_inlined.h  |   22 
>  arch/x86/kernel/apic/apic.c           |   41 -
>  arch/x86/kernel/tsc.c                 |    4 
>  include/asm-generic/thread_info_tif.h |    5 
>  include/linux/clockchips.h            |    8 
>  include/linux/clocksource.h           |    3 
>  include/linux/hrtimer.h               |   59 -
>  include/linux/hrtimer_defs.h          |   79 +-
>  include/linux/hrtimer_rearm.h         |   83 ++
>  include/linux/hrtimer_types.h         |   19 
>  include/linux/irq-entry-common.h      |   25 
>  include/linux/rbtree.h                |   81 ++
>  include/linux/rbtree_types.h          |   16 
>  include/linux/rseq_entry.h            |   14 
>  include/linux/timekeeper_internal.h   |    8 
>  include/linux/timerqueue.h            |   56 +
>  include/linux/timerqueue_types.h      |   15 
>  include/trace/events/timer.h          |   35 -
>  kernel/entry/common.c                 |    4 
>  kernel/sched/core.c                   |   89 ++
>  kernel/sched/deadline.c               |    2 
>  kernel/sched/fair.c                   |   55 -
>  kernel/sched/features.h               |    5 
>  kernel/sched/sched.h                  |   41 -
>  kernel/softirq.c                      |   15 
>  kernel/time/Kconfig                   |   16 
>  kernel/time/clockevents.c             |   48 +
>  kernel/time/hrtimer.c                 | 1116 +++++++++++++++++++---------------
>  kernel/time/tick-broadcast-hrtimer.c  |    1 
>  kernel/time/tick-sched.c              |   27 
>  kernel/time/timekeeping.c             |  184 +++++
>  kernel/time/timekeeping.h             |    2 
>  kernel/time/timer_list.c              |   12 
>  lib/rbtree.c                          |   17 
>  lib/timerqueue.c                      |   14 
>  36 files changed, 1497 insertions(+), 728 deletions(-)
> 
> 
> 

FWIW I tested various workloads for this on an arm64 rk3399 comparing
mainline NO_HRTICK
mainline HRTICK
rearm NO_HRTICK
rearm HRTICK
rearm being $SUBJECT + arm64 generic entry + enabling generic TIF bits.
https://lore.kernel.org/lkml/20260203133728.848283-1-ruanjinjie@xxxxxxxxxx/

There's nothing statistically significant with 1000HZ (it has 6 CPUs, so base
slice granularity is 2.1ms).
With 250HZ I get at least something, a selection:
+-------------+---------------------+---------------------+----------------------+----------------------+----------------------+----------------------+
| Test        | mainline NO_HRTICK  | mainline HRTICK     | rearm NO_HRTICK      | rearm HRTICK         | subject NO_HRTICK    | subject HRTICK       |
+-------------+---------------------+---------------------+----------------------+----------------------+----------------------+----------------------+
| schbench    | 306.83 ± 3.10       | 301.81 ± 1.07       | 298.67 ± 3.33        | (304.87 ± 3.29)      | (305.79 ± 3.64)      | (307.07 ± 1.05)      |
| ebizzy      | 10664 ± 19          | (10565 ± 285)       | (10510 ± 245)        | (10580 ± 240)        | (10674 ± 259)        | 10816 ± 27           |
| hackbench   | 19.715 ± 0.11       | (19.707 ± 0.10)     | (19.826 ± 0.15)      | (19.81 ± 0.12)       | 19.98 ± 0.10         | (19.74 ± 0.11)       |
| nullb0 IOPS | 102525 ± 367        | (101850 ± 262)      | 92209 ± 7624         | (103385 ± 422)       | (101854 ± 473)       | (102141 ± 149)       |
+-------------+----------------------+--------------------+----------------------+----------------------+----------------------+----------------------+
(subject is $SUBJECT only, so no REARM_DEFERRED on arm64).
but at least no regression with sched_feat HRTICK.