I conducted real-time testing on the LicheePi 4A board using Cylictest and employed Ftrace for latency tracing. I observed that madvise_cold_or_pageout_pte_range() causes significant latency under memory pressure, which can be effectively reduced by adding cond_resched() within the loop. The script I tested is as follows: echo wakeup_rt > /sys/kernel/tracing/current_tracer echo 1 > /sys/kernel/tracing/tracing_on echo 0 > /sys/kernel/tracing/tracing_max_latency stress-ng --cpu 4 --vm 4 --vm-bytes 2G & cyclictest --mlockall --smp --priority=99 --distance=0 --duration=30m echo 0 > /sys/kernel/tracing/tracing_on cat /sys/kernel/tracing/trace The tracing results before modification are as follows: # tracer: wakeup_rt # # wakeup_rt latency trace v1.1.5 on 6.5.0-rt6-r1208-00003-g999d221864bf # -------------------------------------------------------------------- # latency: 1969 us, #6/6, CPU#1 | (M:preempt_rt VP:0, KP:0, SP:0 HP:0 #P:4) # ----------------- # | task: cyclictest-250 (uid:0 nice:0 policy:1 rt_prio:99) # ----------------- # # _--------=> CPU# # / _-------=> irqs-off/BH-disabled # | / _------=> need-resched # || / _-----=> need-resched-lazy # ||| / _----=> hardirq/softirq # |||| / _---=> preempt-depth # ||||| / _--=> preempt-lazy-depth # |||||| / _-=> migrate-disable # ||||||| / delay # cmd pid |||||||| time | caller # \ / |||||||| \ | / stress-n-264 1dn.h511 1us : 264:120:R + [001] 250: 0:R cyclictest stress-n-264 1dn.h511 6us : <stack trace> => __ftrace_trace_stack => __trace_stack => probe_wakeup => ttwu_do_activate => try_to_wake_up => wake_up_process => hrtimer_wakeup => __hrtimer_run_queues => hrtimer_interrupt => riscv_timer_interrupt => handle_percpu_devid_irq => generic_handle_domain_irq => riscv_intc_irq => handle_riscv_irq => do_irq stress-n-264 1dn.h511 8us#: 0 stress-n-264 1d...3.. 1953us : __schedule stress-n-264 1d...3.. 1956us+: 264:120:R ==> [001] 250: 0:R cyclictest stress-n-264 1d...3.. 1968us : <stack trace> => __ftrace_trace_stack => __trace_stack => probe_wakeup_sched_switch => __schedule => preempt_schedule => migrate_enable => rt_spin_unlock => madvise_cold_or_pageout_pte_range => walk_pgd_range => __walk_page_range => walk_page_range => madvise_pageout => madvise_vma_behavior => do_madvise => sys_madvise => do_trap_ecall_u => ret_from_exception The tracing results after modification are as follows: # tracer: wakeup_rt # # wakeup_rt latency trace v1.1.5 on 6.5.0-rt6-r1208-00003-g999d221864bf-dirty # -------------------------------------------------------------------- # latency: 879 us, #6/6, CPU#2 | (M:preempt_rt VP:0, KP:0, SP:0 HP:0 #P:4) # ----------------- # | task: cyclictest-212 (uid:0 nice:0 policy:1 rt_prio:99) # ----------------- # # _--------=> CPU# # / _-------=> irqs-off/BH-disabled # | / _------=> need-resched # || / _-----=> need-resched-lazy # ||| / _----=> hardirq/softirq # |||| / _---=> preempt-depth # ||||| / _--=> preempt-lazy-depth # |||||| / _-=> migrate-disable # ||||||| / delay # cmd pid |||||||| time | caller # \ / |||||||| \ | / stress-n-223 2dn.h413 1us : 223:120:R + [002] 212: 0:R cyclictest stress-n-223 2dn.h413 6us : <stack trace> => __ftrace_trace_stack => __trace_stack => probe_wakeup => ttwu_do_activate => try_to_wake_up => wake_up_process => hrtimer_wakeup => __hrtimer_run_queues => hrtimer_interrupt => riscv_timer_interrupt => handle_percpu_devid_irq => generic_handle_domain_irq => riscv_intc_irq => handle_riscv_irq => do_irq stress-n-223 2dn.h413 9us!: 0 stress-n-223 2d...3.. 850us : __schedule stress-n-223 2d...3.. 856us+: 223:120:R ==> [002] 212: 0:R cyclictest stress-n-223 2d...3.. 875us : <stack trace> => __ftrace_trace_stack => __trace_stack => probe_wakeup_sched_switch => __schedule => preempt_schedule => migrate_enable => free_unref_page_list => release_pages => free_pages_and_swap_cache => tlb_batch_pages_flush => tlb_flush_mmu => unmap_page_range => unmap_vmas => unmap_region => do_vmi_align_munmap.constprop.0 => do_vmi_munmap => __vm_munmap => sys_munmap => do_trap_ecall_u => ret_from_exception After the modification, the cause of maximum latency is no longer madvise_cold_or_pageout_pte_range(), so this modification can reduce the latency caused by madvise_cold_or_pageout_pte_range(). Jiexun Wang (1): add cond_resched() in madvise_cold_or_pageout_pte_range() mm/madvise.c | 9 +++++++++ 1 file changed, 9 insertions(+) -- 2.34.1
