mm/madvise: add cond_resched() in madvise_cold_or_pageout_pte_range()

I conducted real-time testing and 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.

I tested on the LicheePi 4A board using Cylictest for latency testing and
Ftrace for latency tracing. The board uses TH1520 processor and has a
memory size of 8GB. The kernel version is 6.5.0 with the PREEMPT_RT patch
applied.

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 --vm 8 --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: 2552 us, #6/6, CPU#3 | (M:preempt_rt VP:0, KP:0, SP:0 HP:0 #P:4)
# -----------------
# | task: cyclictest-196 (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-206 3dn.h512 2us : 206:120:R + [003] 196: 0:R cyclictest
stress-n-206 3dn.h512 7us : <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-206 3dn.h512 9us#: 0
stress-n-206 3d...3.. 2544us : __schedule
stress-n-206 3d...3.. 2545us : 206:120:R ==> [003] 196: 0:R cyclictest
stress-n-206 3d...3.. 2551us : <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-r1208-00004-gca3876fc69a6-dirty
# --------------------------------------------------------------------
# latency: 1689 us, #6/6, CPU#0 | (M:preempt_rt VP:0, KP:0, SP:0 HP:0 #P:4)
# -----------------
# | task: cyclictest-217 (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-232 0dn.h413 1us+: 232:120:R + [000] 217: 0:R cyclictest
stress-n-232 0dn.h413 12us : <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-232 0dn.h413 19us#: 0
stress-n-232 0d...3.. 1671us : __schedule
stress-n-232 0d...3.. 1676us+: 232:120:R ==> [000] 217: 0:R cyclictest
stress-n-232 0d...3.. 1687us : <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().


Currently the madvise_cold_or_pageout_pte_range() function exhibits
significant latency under memory pressure, which can be effectively
reduced by adding cond_resched() within the loop.

When the batch_count reaches SWAP_CLUSTER_MAX, we reschedule
the task to ensure fairness and avoid long lock holding times.

Link: https://lkml.kernel.org/r/85363861af65fac66c7a98c251906afc0d9c8098.1695291046.git.wangjiexun@tinylab.org
Signed-off-by: Jiexun Wang <wangjiexun@tinylab.org>
Cc: Zhangjin Wu <falcon@tinylab.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>