Hi, everyone:
I found that folio_batch_move_lru() causes significant latency
under memory pressure. I try to add cond_resched() but get BUG message:
BUG: sleeping function called from invalid context at mm/swap.c
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 tracing results of wakeup_rt tracer are:
# 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
I obtained more detailed information with display-graph option:
...
1168 us | 0) stress--204 | .n..... | | unmap_region() {
1171 us | 0) stress--204 | .n..... | | lru_add_drain() {
1175 us | 0) stress--204 | .n..1.. | | lru_add_drain_cpu() {
1184 us | 0) stress--204 | .n..1.. | | folio_batch_move_lru() {
1187 us | 0) stress--204 | .n..1.. | | folio_lruvec_lock_irqsave() {
1190 us | 0) stress--204 | .n..1.. | 3.000 us | _raw_spin_lock_irqsave();
1196 us | 0) stress--204 | dn..2.. | 8.667 us | }
1200 us | 0) stress--204 | dn..2.. | | lru_add_fn() {
1204 us | 0) stress--204 | dn..3.. | 2.667 us | folio_mapping();
1213 us | 0) stress--204 | dn..2.. | | __mod_lruvec_state() {
1215 us | 0) stress--204 | dn..2.. | 2.667 us | __mod_node_page_state();
1221 us | 0) stress--204 | dn..2.. | | __mod_memcg_lruvec_state() {
1224 us | 0) stress--204 | dn..2.. | 2.334 us | cgroup_rstat_updated();
1229 us | 0) stress--204 | dn..2.. | 9.000 us | }
1232 us | 0) stress--204 | dn..2.. | + 19.000 us | }
1234 us | 0) stress--204 | dn..2.. | 2.667 us | __mod_zone_page_state();
1239 us | 0) stress--204 | dn..2.. | 4.334 us | mem_cgroup_update_lru_size();
1246 us | 0) stress--204 | dn..2.. | + 46.333 us | }
1249 us | 0) stress--204 | dn..2.. | | lru_add_fn() {
1252 us | 0) stress--204 | dn..3.. | 2.333 us | folio_mapping();
1258 us | 0) stress--204 | dn..2.. | | __mod_lruvec_state() {
1261 us | 0) stress--204 | dn..2.. | 2.333 us | __mod_node_page_state();
1266 us | 0) stress--204 | dn..2.. | | __mod_memcg_lruvec_state() {
1270 us | 0) stress--204 | dn..2.. | 2.333 us | cgroup_rstat_updated();
1276 us | 0) stress--204 | dn..2.. | + 10.333 us | }
1278 us | 0) stress--204 | dn..2.. | + 20.000 us | }
1281 us | 0) stress--204 | dn..2.. | 4.000 us | __mod_zone_page_state();
1287 us | 0) stress--204 | dn..2.. | 4.000 us | mem_cgroup_update_lru_size();
...
1696 us | 0) stress--204 | dn..2.. | | lru_add_fn() {
1700 us | 0) stress--204 | dn..3.. | 2.666 us | folio_mapping();
1706 us | 0) stress--204 | dn..2.. | | __mod_lruvec_state() {
1710 us | 0) stress--204 | dn..2.. | 2.667 us | __mod_node_page_state();
1716 us | 0) stress--204 | dn..2.. | | __mod_memcg_lruvec_state() {
1720 us | 0) stress--204 | dn..2.. | 2.334 us | cgroup_rstat_updated();
1726 us | 0) stress--204 | dn..2.. | 9.667 us | }
1728 us | 0) stress--204 | dn..2.. | + 22.000 us | }
1731 us | 0) stress--204 | dn..2.. | 2.666 us | __mod_zone_page_state();
1736 us | 0) stress--204 | dn..2.. | 2.333 us | mem_cgroup_update_lru_size();
1740 us | 0) stress--204 | dn..2.. | + 44.666 us | }
1744 us | 0) stress--204 | dn..2.. | 3.333 us | _raw_spin_unlock_irqrestore();
1750 us | 0) stress--204 | .n..1.. | | release_pages() {
1756 us | 0) stress--204 | .n..1.. | 3.667 us | __mem_cgroup_uncharge_list();
1762 us | 0) stress--204 | .n..1.. | 2.667 us | free_unref_page_list();
1769 us | 0) stress--204 | .n..1.. | + 18.667 us | }
1772 us | 0) stress--204 | .n..1.. | ! 588.000 us | }
1777 us | 0) stress--204 | .n..1.. | ! 600.000 us | }
1780 us | 0) stress--204 | .n..... | 2.667 us | mlock_drain_local();
1785 us | 0) stress--204 | .n..... | ! 614.000 us | }
...
I attempted to unlock and call cond_resched() within the loop:
@@ -217,6 +217,12 @@ static void folio_batch_move_lru(struct folio_batch *fbatch, move_fn_t move_fn)
move_fn(lruvec, folio);
folio_set_lru(folio);
+
+ if (i % 4 == 0) {
+ if (lruvec)
+ unlock_page_lruvec_irqrestore(lruvec, flags);
+ cond_resched();
+ lruvec = folio_lruvec_relock_irqsave(folio, lruvec, &flags);
}
if (lruvec)
Then I received BUG messages continuously:
BUG: sleeping function called from invalid context at ...
I noticed that there are also locks at the outer layer of this function.
I also attempted to release the locks, but still received the same BUG info.
I wonder if the internal of this function is non-preemptible?
Or do I have any means to reduce the latency caused by this function?
Best regards,
Jiexun Wang