[PATCH 0/1] mm/madvise: add cond_resched() in madvise_cold_or_pageout_pte_range()

[Jiexun Wang]

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