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p-fU1koxgmksVqKjGsTzEF1QQZg1aMBk Andrew Morton writes: > On Sun, 15 Mar 2026 18:36:39 -0700 Ankur Arora wrote: > >> Hi, >> >> This series adds waited variants of the smp_cond_load() primitives: >> smp_cond_load_relaxed_timeout(), and smp_cond_load_acquire_timeout(). >> >> ... >> > > How are we to determine that this change is successful, useful, etc? Good point. So this series was split off from this one here: https://lore.kernel.org/lkml/20250218213337.377987-1-ankur.a.arora@oracle.com/ The series enables ARCH_HAS_CPU_RELAX on arm64 which should allow relatively cheap polling in idle on arm64. However, it does need a few more patches from the series above to do that. > Reduced CPU consumption? Reduced energy usage? Improved latencies? With the additional patches this should improve wakeup latency: I ran the sched-pipe test with processes on VCPUs 4 and 5 with kvm-arm.wfi_trap_policy=notrap. # perf stat -r 5 --cpu 4,5 -e task-clock,cycles,instructions,sched:sched_wake_idle_without_ipi \ perf bench sched pipe -l 1000000 -c 4 # No haltpoll (and, no TIF_POLLING_NRFLAG): Performance counter stats for 'CPU(s) 4,5' (5 runs): 25,229.57 msec task-clock # 2.000 CPUs utilized ( +- 7.75% ) 45,821,250,284 cycles # 1.816 GHz ( +- 10.07% ) 26,557,496,665 instructions # 0.58 insn per cycle ( +- 0.21% ) 0 sched:sched_wake_idle_without_ipi # 0.000 /sec 12.615 +- 0.977 seconds time elapsed ( +- 7.75% ) # Haltpoll: Performance counter stats for 'CPU(s) 4,5' (5 runs): 15,131.58 msec task-clock # 2.000 CPUs utilized ( +- 10.00% ) 34,158,188,839 cycles # 2.257 GHz ( +- 6.91% ) 20,824,950,916 instructions # 0.61 insn per cycle ( +- 0.09% ) 1,983,822 sched:sched_wake_idle_without_ipi # 131.105 K/sec ( +- 0.78% ) 7.566 +- 0.756 seconds time elapsed ( +- 10.00% ) We get a decent boost just because we are executing ~20% fewer instructions. Not sure how the cpu frequency scaling works in a VM but we also run at a higher frequency. (That specifically applies to guests but that series also adds enables this with acpi-idle for baremetal.) (From: https://lore.kernel.org/lkml/877c9zhk68.fsf@oracle.com/) >> Finally update poll_idle() and resilient queued spinlocks to use them. > > Have you identified other suitable sites for conversion? Haven't found other places in the core kernel where this could be used. I think one reason is that the typical kernel wait is unbounded. There are some in drivers/ that have this pattern. For instance I think this in drivers/iommu/arm/arm-smmu-v3 could be converted: __arm_smmu_cmdq_poll_until_msi(). However, as David Laight pointed out in this thread (https://lore.kernel.org/lkml/20260214113122.70627a8b@pumpkin/) that this would be fine so long as the polling is on memory, but would need some work to handle MMIO. >> Documentation/atomic_t.txt | 14 +++-- >> arch/arm64/Kconfig | 3 + >> arch/arm64/include/asm/barrier.h | 23 +++++++ >> arch/arm64/include/asm/cmpxchg.h | 62 +++++++++++++++---- >> arch/arm64/include/asm/delay-const.h | 27 +++++++++ >> arch/arm64/include/asm/rqspinlock.h | 85 -------------------------- >> arch/arm64/lib/delay.c | 15 ++--- >> drivers/cpuidle/poll_state.c | 21 +------ >> drivers/soc/qcom/rpmh-rsc.c | 8 +-- >> include/asm-generic/barrier.h | 90 ++++++++++++++++++++++++++++ >> include/linux/atomic.h | 10 ++++ >> include/linux/atomic/atomic-long.h | 18 +++--- >> include/linux/sched/idle.h | 29 +++++++++ >> kernel/bpf/rqspinlock.c | 77 +++++++++++++++--------- >> scripts/atomic/gen-atomic-long.sh | 16 +++-- >> 15 files changed, 320 insertions(+), 178 deletions(-) >> create mode 100644 arch/arm64/include/asm/delay-const.h > > Some sort of testing in lib/tests/ would be appropriate and useful. Makes sense. Will add. Thanks -- ankur