linux/arch/loongarch
Linus Torvalds d08c407f71 A large set of updates and features for timers and timekeeping:
- The hierarchical timer pull model
 
     When timer wheel timers are armed they are placed into the timer wheel
     of a CPU which is likely to be busy at the time of expiry. This is done
     to avoid wakeups on potentially idle CPUs.
 
     This is wrong in several aspects:
 
      1) The heuristics to select the target CPU are wrong by
         definition as the chance to get the prediction right is close
         to zero.
 
      2) Due to #1 it is possible that timers are accumulated on a
         single target CPU
 
      3) The required computation in the enqueue path is just overhead for
      	dubious value especially under the consideration that the vast
      	majority of timer wheel timers are either canceled or rearmed
      	before they expire.
 
     The timer pull model avoids the above by removing the target
     computation on enqueue and queueing timers always on the CPU on which
     they get armed.
 
     This is achieved by having separate wheels for CPU pinned timers and
     global timers which do not care about where they expire.
 
     As long as a CPU is busy it handles both the pinned and the global
     timers which are queued on the CPU local timer wheels.
 
     When a CPU goes idle it evaluates its own timer wheels:
 
       - If the first expiring timer is a pinned timer, then the global
       	timers can be ignored as the CPU will wake up before they expire.
 
       - If the first expiring timer is a global timer, then the expiry time
         is propagated into the timer pull hierarchy and the CPU makes sure
         to wake up for the first pinned timer.
 
     The timer pull hierarchy organizes CPUs in groups of eight at the
     lowest level and at the next levels groups of eight groups up to the
     point where no further aggregation of groups is required, i.e. the
     number of levels is log8(NR_CPUS). The magic number of eight has been
     established by experimention, but can be adjusted if needed.
 
     In each group one busy CPU acts as the migrator. It's only one CPU to
     avoid lock contention on remote timer wheels.
 
     The migrator CPU checks in its own timer wheel handling whether there
     are other CPUs in the group which have gone idle and have global timers
     to expire. If there are global timers to expire, the migrator locks the
     remote CPU timer wheel and handles the expiry.
 
     Depending on the group level in the hierarchy this handling can require
     to walk the hierarchy downwards to the CPU level.
 
     Special care is taken when the last CPU goes idle. At this point the
     CPU is the systemwide migrator at the top of the hierarchy and it
     therefore cannot delegate to the hierarchy. It needs to arm its own
     timer device to expire either at the first expiring timer in the
     hierarchy or at the first CPU local timer, which ever expires first.
 
     This completely removes the overhead from the enqueue path, which is
     e.g. for networking a true hotpath and trades it for a slightly more
     complex idle path.
 
     This has been in development for a couple of years and the final series
     has been extensively tested by various teams from silicon vendors and
     ran through extensive CI.
 
     There have been slight performance improvements observed on network
     centric workloads and an Intel team confirmed that this allows them to
     power down a die completely on a mult-die socket for the first time in
     a mostly idle scenario.
 
     There is only one outstanding ~1.5% regression on a specific overloaded
     netperf test which is currently investigated, but the rest is either
     positive or neutral performance wise and positive on the power
     management side.
 
   - Fixes for the timekeeping interpolation code for cross-timestamps:
 
     cross-timestamps are used for PTP to get snapshots from hardware timers
     and interpolated them back to clock MONOTONIC. The changes address a
     few corner cases in the interpolation code which got the math and logic
     wrong.
 
   - Simplifcation of the clocksource watchdog retry logic to automatically
     adjust to handle larger systems correctly instead of having more
     incomprehensible command line parameters.
 
   - Treewide consolidation of the VDSO data structures.
 
   - The usual small improvements and cleanups all over the place.
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Merge tag 'timers-core-2024-03-10' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull timer updates from Thomas Gleixner:
 "A large set of updates and features for timers and timekeeping:

   - The hierarchical timer pull model

     When timer wheel timers are armed they are placed into the timer
     wheel of a CPU which is likely to be busy at the time of expiry.
     This is done to avoid wakeups on potentially idle CPUs.

     This is wrong in several aspects:

       1) The heuristics to select the target CPU are wrong by
          definition as the chance to get the prediction right is
          close to zero.

       2) Due to #1 it is possible that timers are accumulated on
          a single target CPU

       3) The required computation in the enqueue path is just overhead
          for dubious value especially under the consideration that the
          vast majority of timer wheel timers are either canceled or
          rearmed before they expire.

     The timer pull model avoids the above by removing the target
     computation on enqueue and queueing timers always on the CPU on
     which they get armed.

     This is achieved by having separate wheels for CPU pinned timers
     and global timers which do not care about where they expire.

     As long as a CPU is busy it handles both the pinned and the global
     timers which are queued on the CPU local timer wheels.

     When a CPU goes idle it evaluates its own timer wheels:

       - If the first expiring timer is a pinned timer, then the global
         timers can be ignored as the CPU will wake up before they
         expire.

       - If the first expiring timer is a global timer, then the expiry
         time is propagated into the timer pull hierarchy and the CPU
         makes sure to wake up for the first pinned timer.

     The timer pull hierarchy organizes CPUs in groups of eight at the
     lowest level and at the next levels groups of eight groups up to
     the point where no further aggregation of groups is required, i.e.
     the number of levels is log8(NR_CPUS). The magic number of eight
     has been established by experimention, but can be adjusted if
     needed.

     In each group one busy CPU acts as the migrator. It's only one CPU
     to avoid lock contention on remote timer wheels.

     The migrator CPU checks in its own timer wheel handling whether
     there are other CPUs in the group which have gone idle and have
     global timers to expire. If there are global timers to expire, the
     migrator locks the remote CPU timer wheel and handles the expiry.

     Depending on the group level in the hierarchy this handling can
     require to walk the hierarchy downwards to the CPU level.

     Special care is taken when the last CPU goes idle. At this point
     the CPU is the systemwide migrator at the top of the hierarchy and
     it therefore cannot delegate to the hierarchy. It needs to arm its
     own timer device to expire either at the first expiring timer in
     the hierarchy or at the first CPU local timer, which ever expires
     first.

     This completely removes the overhead from the enqueue path, which
     is e.g. for networking a true hotpath and trades it for a slightly
     more complex idle path.

     This has been in development for a couple of years and the final
     series has been extensively tested by various teams from silicon
     vendors and ran through extensive CI.

     There have been slight performance improvements observed on network
     centric workloads and an Intel team confirmed that this allows them
     to power down a die completely on a mult-die socket for the first
     time in a mostly idle scenario.

     There is only one outstanding ~1.5% regression on a specific
     overloaded netperf test which is currently investigated, but the
     rest is either positive or neutral performance wise and positive on
     the power management side.

   - Fixes for the timekeeping interpolation code for cross-timestamps:

     cross-timestamps are used for PTP to get snapshots from hardware
     timers and interpolated them back to clock MONOTONIC. The changes
     address a few corner cases in the interpolation code which got the
     math and logic wrong.

   - Simplifcation of the clocksource watchdog retry logic to
     automatically adjust to handle larger systems correctly instead of
     having more incomprehensible command line parameters.

   - Treewide consolidation of the VDSO data structures.

   - The usual small improvements and cleanups all over the place"

* tag 'timers-core-2024-03-10' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (62 commits)
  timer/migration: Fix quick check reporting late expiry
  tick/sched: Fix build failure for CONFIG_NO_HZ_COMMON=n
  vdso/datapage: Quick fix - use asm/page-def.h for ARM64
  timers: Assert no next dyntick timer look-up while CPU is offline
  tick: Assume timekeeping is correctly handed over upon last offline idle call
  tick: Shut down low-res tick from dying CPU
  tick: Split nohz and highres features from nohz_mode
  tick: Move individual bit features to debuggable mask accesses
  tick: Move got_idle_tick away from common flags
  tick: Assume the tick can't be stopped in NOHZ_MODE_INACTIVE mode
  tick: Move broadcast cancellation up to CPUHP_AP_TICK_DYING
  tick: Move tick cancellation up to CPUHP_AP_TICK_DYING
  tick: Start centralizing tick related CPU hotplug operations
  tick/sched: Don't clear ts::next_tick again in can_stop_idle_tick()
  tick/sched: Rename tick_nohz_stop_sched_tick() to tick_nohz_full_stop_tick()
  tick: Use IS_ENABLED() whenever possible
  tick/sched: Remove useless oneshot ifdeffery
  tick/nohz: Remove duplicate between lowres and highres handlers
  tick/nohz: Remove duplicate between tick_nohz_switch_to_nohz() and tick_setup_sched_timer()
  hrtimer: Select housekeeping CPU during migration
  ...
2024-03-11 14:38:26 -07:00
..
boot LoongArch: dts: Minor whitespace cleanup 2024-02-23 14:36:31 +08:00
configs LoongArch changes for v6.8 2024-01-19 13:30:49 -08:00
crypto crypto: loongarch/crc32 - remove redundant setting of alignmask to 0 2023-10-27 18:04:24 +08:00
include work around gcc bugs with 'asm goto' with outputs 2024-02-09 15:57:48 -08:00
kernel A large set of updates and features for timers and timekeeping: 2024-03-11 14:38:26 -07:00
kvm LoongArch: KVM: Streamline kvm_check_cpucfg() and improve comments 2024-02-23 14:36:31 +08:00
lib LoongArch: Add KASAN (Kernel Address Sanitizer) support 2023-09-06 22:54:16 +08:00
mm LoongArch: Fix earlycon parameter if KASAN enabled 2024-02-06 12:32:05 +08:00
net LoongArch: BPF: Prevent out-of-bounds memory access 2024-01-17 12:43:13 +08:00
pci LoongArch: Add FDT booting support from efi system table 2022-12-14 08:41:53 +08:00
power LoongArch: Export some arch-specific pm interfaces 2023-06-29 20:58:44 +08:00
vdso LoongArch: vDSO: Disable UBSAN instrumentation 2024-02-06 12:32:05 +08:00
Kbuild LoongArch: Allow device trees be built into the kernel 2024-01-17 12:43:00 +08:00
Kconfig LoongArch: Select HAVE_ARCH_SECCOMP to use the common SECCOMP menu 2024-02-06 12:32:05 +08:00
Kconfig.debug LoongArch: Add prologue unwinder support 2022-08-12 13:10:11 +08:00
Makefile LoongArch: Allow device trees be built into the kernel 2024-01-17 12:43:00 +08:00