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- 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. -----BEGIN PGP SIGNATURE----- iQJHBAABCgAxFiEEQp8+kY+LLUocC4bMphj1TA10mKEFAmXuAN0THHRnbHhAbGlu dXRyb25peC5kZQAKCRCmGPVMDXSYoVKXEADIR45rjR1Xtz32js7B53Y65O4WNoOQ 6/ycWcswuGzg/h4QUpPSJ6gOGVmKSWwZi4n0P/VadCiXGSPPm0aUKsoRUt9DZsPY mtj2wjCSXKXiyhTl9OtrZME86ZAIGO1dQXa/sOHsiP5PCjgQkD0b5CYi1+B6eHDt 1/Uo2Tb9g8VAPppq20V5Uo93GrPf642oyi3FCFrR1M112Uuak5DmqHJYiDpreNcG D5SgI+ykSiaUaVyHifvqijoJk0rYXkqEC6evl02477lJ/X0vVo2/M8XPS95BxHST s5Iruo4rP+qeAy8QvhZpoPX59fO0m/AgA7cf77XXAtOpVdLH+bs4ILsEbouAIOtv lsmRkcYt+TpvrZFHPAxks+6g3afuROiDtxD5sXXpVWxvofi8FwWqubdlqdsbw9MP ZCTNyzNyKL47QeDwBfSynYUL1RSyqsphtIwk4oeQklH9rwMAnW21hi30z15hQ0pQ FOVkmcwi79JNvl/G+jRkDzw7r8/zcHshWdSjyUM04CDjjnCDjQOFWSIjEPwbQjjz S4HXpJKJW963dBgs9Z84/Ctw1GwoBk1qedDWDJE1257Qvmo/Wpe/7GddWcazOGnN RRFMzGPbOqBDbjtErOKGU+iCisgNEvz2XK+TI16uRjWde7DxZpiTVYgNDrZ+/Pyh rQ23UBms6ZRR+A== =iQlu -----END PGP SIGNATURE----- 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 ... |
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