On Mon, Jan 22, 2024 at 03:32:44PM +0100, Leon Woestenberg wrote:
> Hi Frederic, Paul, all,
>
> On Sat, Jan 20, 2024 at 1:37 AM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> > On Fri, Jan 19, 2024 at 11:08:23PM +0100, Frederic Weisbecker wrote:
> > > Le Fri, Jan 19, 2024 at 04:38:23PM +0100, Leon Woestenberg a écrit :
> > > > The isolated core should run a single non-kernel user-space-busy-loop
> > > > thread (SCHED_OTHER, not FIFO or RR).
> > > >
> > > > tsc=reliable isolcpus=domain,managed_irq,4-7 nohz_full=4-7
> > > > rcu_nocbs=all rcu_nocb_poll
> > > > /proc/irq/*/smp_affinity and cpumasks in
> > > > /sys/devices/virtual/workqueue/ all show 000f or less (CPUs 0-3)
> > > >
> > > > Using osnoise I see rcuc/4 popping up as a cause of noise, but my
> > > > kernel command line should prevent that, right?
> >
> > Well, your choice is rcuc or ksoftirqd. Either way, there is some per-CPU
> > work that must be done. But to Frederic's later point, not very much
> > work. In particular, RCU callbacks should be invoked on an rcuoc kthread.
> >
> Yes, that's what I expected (and what I got to work on an earlier
> system with an older kernel).
>
> I went back to running ftrace against the isolated CPU, and I see some
> kworker work.
> Sometimes blk_mq_run_work_func().
>
> (I think seeing rcuc in my osnoise trace was coincidental.)
>
> What would be the method to find the root cause more clearly? Should I
> deepen my ftrace function call depth?
Is the application thread running on CPU 4 doing any kernel work?
Alternatively, is there more than one runnable task on CPU 4?
> I am getting one interruption per 8 minutes on the isolated CPU. Here
> is an example trace that ran 30 minutes:
>
> cat trace.4
> # tracer: function_graph
> #
> # CPU DURATION FUNCTION CALLS
> # | | | | | | |
> 4) | vtime_user_exit() {
> 4) 0.634 us | get_vtime_delta();
> 4) 1.799 us | account_user_time();
> 4) 5.808 us | }
> 4) | irq_enter_rcu() {
> 4) 0.640 us | tick_irq_enter();
> 4) 0.914 us | }
> 4) | __sysvec_call_function_single() {
> 4) | /* call_function_single_entry: vector=251 */
> 4) 0.513 us | generic_smp_call_function_single_interrupt();
> 4) | /* call_function_single_exit: vector=251 */
> 4) 2.136 us | }
CPU 4 was the target of an smp_call_function_single(). Which just
means that some other CPU needs CPU 4 to do something for it. The
csd_function_entry trace event, if enabled, will give you the name of the
function called, which might help you work out what needed to be adjusted.
> 4) | irq_exit_rcu() {
> 4) 0.203 us | idle_cpu();
> 4) | tick_nohz_irq_exit() {
> 4) | /* tick_stop: success=0 dependency=SCHED */
> 4) 7.564 us | }
> 4) 8.285 us | }
> 4) | exit_to_user_mode_prepare() {
> 4) 0.217 us | rcu_nocb_flush_deferred_wakeup();
> 4) 0.187 us | __rcu_irq_enter_check_tick();
> 4) | /* reschedule_entry: vector=253 */
> 4) | /* reschedule_exit: vector=253 */
> 4) | raw_irqentry_exit_cond_resched() {
> 4) | /* local_timer_entry: vector=236 */
> 4) | /* local_timer_exit: vector=236 */
> 4) | /* tick_stop: success=0 dependency=SCHED */
> 4) | /* sched_switch: prev_comm=hiccups_x86-64
> prev_pid=1335 prev_prio=120 prev_state=R+ ==> next_comm=kworker/4:0H
> next_pid=1160 next_prio=100 */
> ------------------------------------------
> 4) hiccups-1335 => kworker-1160
> ------------------------------------------
>
> 4) | finish_task_switch.isra.0() {
> 4) 0.475 us | vtime_task_switch_generic();
> 4) 0.126 us | __tick_nohz_task_switch();
> 4) 1.069 us | }
> 4) | wq_worker_running() {
> 4) 0.223 us | kthread_data();
> 4) 0.547 us | }
> 4) 0.180 us | _raw_spin_lock_irq();
> 4) 0.115 us | _raw_spin_unlock_irq();
> 4) | mod_timer() {
> 4) 0.241 us | lock_timer_base();
> 4) 0.104 us | detach_if_pending();
> 4) 0.123 us | _raw_spin_lock();
> 4) 0.120 us | calc_wheel_index();
> 4) 0.115 us | enqueue_timer();
> 4) 0.134 us | _raw_spin_unlock_irqrestore();
> 4) 1.815 us | }
> 4) | create_worker() {
> 4) 0.238 us | rt_spin_lock();
> 4) 0.288 us | rt_spin_unlock();
> 4) 0.144 us | kfree();
> 4) 1.216 us | alloc_worker();
> 4) | kthread_create_on_node() {
> 4) | /* sched_switch: prev_comm=kworker/4:0H
The workqueue_execute_start trace event will tell you which function
is the workqueue handler, though that would not have helped here.
> prev_pid=1160 prev_prio=100 prev_state=D ==> next_comm=hiccups_x86-64
> next_pid=1335 next_prio=120 */
> ------------------------------------------
> 4) kworker-1160 => hiccups-1335
> ------------------------------------------
>
> 4) + 33.354 us | } /* raw_irqentry_exit_cond_resched */
> 4) 1.334 us | schedule();
> 4) 0.151 us | rcu_nocb_flush_deferred_wakeup();
> 4) 0.133 us | mem_cgroup_handle_over_high();
> 4) 0.203 us | blkcg_maybe_throttle_current();
> 4) 0.300 us | __rseq_handle_notify_resume();
> 4) 0.113 us | rcu_nocb_flush_deferred_wakeup();
> 4) 0.114 us | fpregs_assert_state_consistent();
> 4) 0.142 us | switch_fpu_return();
> 4) + 39.502 us | } /* exit_to_user_mode_prepare */
> 4) | vtime_user_enter() {
> 4) 0.177 us | get_vtime_delta();
> 4) 0.390 us | }
> 4) | vtime_user_exit() {
> 4) 0.171 us | get_vtime_delta();
> 4) 0.437 us | }
> 4) | irq_enter_rcu() {
> 4) 0.167 us | tick_irq_enter();
> 4) 0.393 us | }
> 4) | __sysvec_call_function_single() {
> 4) | /* call_function_single_entry: vector=251 */
> 4) 0.348 us | generic_smp_call_function_single_interrupt();
> 4) | /* call_function_single_exit: vector=251 */
Another smp_call_function_single(), same tracepoint.
> 4) 0.908 us | }
> 4) | irq_exit_rcu() {
> 4) 0.304 us | idle_cpu();
> 4) | tick_nohz_irq_exit() {
> 4) | /* tick_stop: success=0 dependency=SCHED */
> 4) 0.646 us | }
> 4) 2.154 us | }
> 4) | exit_to_user_mode_prepare() {
> 4) 0.129 us | rcu_nocb_flush_deferred_wakeup();
> 4) 0.219 us | __rcu_irq_enter_check_tick();
> 4) | /* reschedule_entry: vector=253 */
> 4) | /* reschedule_exit: vector=253 */
> 4) | raw_irqentry_exit_cond_resched() {
> 4) | /* sched_switch: prev_comm=hiccups_x86-64
> prev_pid=1335 prev_prio=120 prev_state=R+ ==> next_comm=kworker/4:0H
> next_pid=1160 next_prio=100 */
> ------------------------------------------
> 4) hiccups-1335 => kworker-1160
> ------------------------------------------
>
> 4) + 96.177 us | } /* kthread_create_on_node */
> 4) 1.071 us | set_user_nice();
> 4) 0.999 us | kthread_bind_mask();
> 4) 0.436 us | worker_attach_to_pool();
> 4) 0.112 us | _raw_spin_lock_irq();
> 4) 0.146 us | worker_enter_idle();
> 4) 2.932 us | wake_up_process();
> 4) 0.114 us | _raw_spin_unlock_irq();
> 4) ! 107.063 us | } /* create_worker */
> 4) | del_timer_sync() {
> 4) 0.154 us | lock_timer_base();
> 4) 0.132 us | detach_if_pending();
> 4) 0.099 us | _raw_spin_unlock_irqrestore();
> 4) 0.854 us | }
> 4) 0.103 us | _raw_spin_lock_irq();
> 4) | rcuwait_wake_up() {
> 4) 0.097 us | __rcu_read_lock();
> 4) 0.099 us | __rcu_read_unlock();
> 4) 0.511 us | }
> 4) | process_one_work() {
> 4) 0.100 us | _raw_spin_unlock_irq();
> 4) 8.334 us | blk_mq_run_work_fn();
And in this case the workqueue handler is blk_mq_run_work_fn(), as
you noted. Is I/O being initiated on CPU 4? If so, it might help
to move that I/O to one of CPUs 0-3.
> 4) 0.133 us | _raw_spin_lock_irq();
> 4) 0.118 us | pwq_dec_nr_in_flight();
> 4) 9.496 us | }
> 4) 0.114 us | worker_enter_idle();
> 4) 0.108 us | _raw_spin_unlock_irq();
> 4) | schedule() {
> 4) 0.148 us | wq_worker_sleeping();
> 4) 0.114 us | rcu_note_context_switch();
> 4) 0.100 us | _raw_spin_lock();
> 4) 0.125 us | update_rq_clock();
> 4) 0.817 us | dequeue_task_fair();
> 4) 0.550 us | pick_next_task_fair();
> 4) 0.435 us | psi_task_switch();
> 4) | __traceiter_sched_switch() {
> 4) | /* sched_switch: prev_comm=kworker/4:0H
> prev_pid=1160 prev_prio=100 prev_state=I ==> next_comm=kworker/4:1H
> next_pid=1348 next_prio=100 */
> 4) 0.315 us | }
> 4) 0.114 us | enter_lazy_tlb();
> ------------------------------------------
> 4) kworker-1160 => kworker-1348
> ------------------------------------------
>
> 4) | finish_task_switch.isra.0() {
> 4) 0.289 us | vtime_task_switch_generic();
> 4) 0.111 us | __tick_nohz_task_switch();
> 4) 0.791 us | }
> 4) | __kthread_parkme() {
> 4) 0.106 us | _raw_spin_lock_irqsave();
> 4) 0.098 us | _raw_spin_unlock_irqrestore();
> 4) 0.611 us | }
> 4) | worker_thread() {
> 4) 0.107 us | mutex_lock();
> 4) 0.107 us | mutex_unlock();
> 4) 0.101 us | _raw_spin_lock_irq();
> 4) 0.128 us | worker_enter_idle();
> 4) 0.104 us | _raw_spin_unlock_irq();
> 4) | schedule() {
> 4) | /* sched_switch: prev_comm=kworker/4:1H
> prev_pid=1348 prev_prio=100 prev_state=I ==> next_comm=hiccups_x86-64
> next_pid=1335 next_prio=120 */
> ------------------------------------------
> 4) kworker-1348 => hiccups-1335
> ------------------------------------------
>
> 4) + 36.544 us | } /* raw_irqentry_exit_cond_resched */
> 4) 1.160 us | schedule();
> 4) 0.138 us | rcu_nocb_flush_deferred_wakeup();
> 4) 0.125 us | mem_cgroup_handle_over_high();
> 4) 0.113 us | blkcg_maybe_throttle_current();
> 4) 0.262 us | __rseq_handle_notify_resume();
> 4) 0.112 us | rcu_nocb_flush_deferred_wakeup();
> 4) 0.111 us | fpregs_assert_state_consistent();
> 4) 0.124 us | switch_fpu_return();
> 4) + 41.539 us | } /* exit_to_user_mode_prepare */
> 4) | vtime_user_enter() {
> 4) 0.189 us | get_vtime_delta();
> 4) 0.390 us | }
> 4) | vtime_user_exit() {
> 4) 0.178 us | get_vtime_delta();
> 4) 0.413 us | }
> 4) | irq_enter_rcu() {
> 4) 0.155 us | tick_irq_enter();
> 4) 0.428 us | }
> 4) | __sysvec_call_function_single() {
> 4) | /* call_function_single_entry: vector=251 */
> 4) 0.329 us | generic_smp_call_function_single_interrupt();
> 4) | /* call_function_single_exit: vector=251 */
> 4) 1.158 us | }
> 4) | irq_exit_rcu() {
> 4) 0.190 us | idle_cpu();
> 4) | tick_nohz_irq_exit() {
> 4) | /* tick_stop: success=0 dependency=SCHED */
> 4) 0.518 us | }
> 4) 1.128 us | }
> 4) | exit_to_user_mode_prepare() {
> 4) 0.128 us | rcu_nocb_flush_deferred_wakeup();
> 4) 0.109 us | __rcu_irq_enter_check_tick();
> 4) | /* reschedule_entry: vector=253 */
> 4) | /* reschedule_exit: vector=253 */
> 4) | raw_irqentry_exit_cond_resched() {
> 4) | /* sched_switch: prev_comm=hiccups_x86-64
> prev_pid=1335 prev_prio=120 prev_state=R+ ==> next_comm=kworker/4:1H
> next_pid=1348 next_prio=100 */
> ------------------------------------------
> 4) hiccups-1335 => kworker-1348
> ------------------------------------------
>
> 4) ! 277.107 us | } /* schedule */
> 4) 0.190 us | _raw_spin_lock_irq();
> 4) 6.380 us | process_one_work();
> 4) 0.128 us | worker_enter_idle();
> 4) 0.115 us | _raw_spin_unlock_irq();
> 4) | schedule() {
> 4) | /* sched_switch: prev_comm=kworker/4:1H
> prev_pid=1348 prev_prio=100 prev_state=I ==> next_comm=hiccups_x86-64
> next_pid=1335 next_prio=120 */
> ------------------------------------------
> 4) kworker-1348 => hiccups-1335
> ------------------------------------------
>
> 4) + 15.222 us | } /* raw_irqentry_exit_cond_resched */
> 4) 1.086 us | schedule();
> 4) 0.115 us | rcu_nocb_flush_deferred_wakeup();
> 4) 0.100 us | mem_cgroup_handle_over_high();
> 4) 0.106 us | blkcg_maybe_throttle_current();
> 4) 0.148 us | __rseq_handle_notify_resume();
> 4) 0.116 us | rcu_nocb_flush_deferred_wakeup();
> 4) 0.119 us | fpregs_assert_state_consistent();
> 4) 0.123 us | switch_fpu_return();
> 4) + 19.265 us | }
> 4) | vtime_user_enter() {
> 4) 0.160 us | get_vtime_delta();
> 4) 0.366 us | }
> 4) | vtime_user_exit() {
> 4) 0.173 us | get_vtime_delta();
> 4) 0.399 us | }
> 4) | irq_enter_rcu() {
> 4) 0.152 us | tick_irq_enter();
> 4) 0.380 us | }
> 4) | __sysvec_apic_timer_interrupt() {
> 4) | /* local_timer_entry: vector=236 */
> 4) 8.901 us | hrtimer_interrupt();
> 4) | /* local_timer_exit: vector=236 */
> 4) 9.422 us | }
> 4) | irq_exit_rcu() {
> 4) 0.394 us | wake_up_process();
> 4) 0.137 us | idle_cpu();
> 4) 0.677 us | tick_nohz_irq_exit();
> 4) 1.797 us | }
> 4) | exit_to_user_mode_prepare() {
> 4) 0.112 us | rcu_nocb_flush_deferred_wakeup();
> 4) | schedule() {
> 4) | /* sched_switch: prev_comm=hiccups_x86-64
> prev_pid=1335 prev_prio=120 prev_state=R+ ==> next_comm=ktimers/4
> next_pid=1157 next_prio=98 */
> ------------------------------------------
> 4) hiccups-1335 => ktimers-1157
> ------------------------------------------
>
> 4) | finish_task_switch.isra.0() {
> 4) 0.564 us | vtime_task_switch_generic();
> 4) 0.210 us | __tick_nohz_task_switch();
> 4) 1.321 us | }
> 4) 0.213 us | kthread_should_stop();
> 4) 0.135 us | kthread_should_park();
> 4) 0.118 us | timersd_should_run();
> 4) | run_timersd() {
> 4) 0.409 us | __local_bh_disable_ip();
> 4) | __do_softirq() {
> 4) | /* timer_expire_entry:
> timer=00000000b281d363 function=delayed_work_timer_fn now=4295134337
> baseclk=4295016960 */
> 4) 4.507 us | }
> 4) 0.355 us | __local_bh_enable();
> 4) 5.864 us | }
> 4) 0.129 us | kthread_should_stop();
> 4) 0.102 us | kthread_should_park();
> 4) 0.099 us | timersd_should_run();
> 4) | schedule() {
> 4) 0.132 us | rcu_note_context_switch();
> 4) 0.114 us | _raw_spin_lock();
> 4) 0.122 us | update_rq_clock();
> 4) 1.191 us | dequeue_task_rt();
> 4) 0.184 us | balance_rt();
> 4) 0.120 us | balance_fair();
> 4) 0.233 us | put_prev_task_rt();
> 4) 0.121 us | pick_next_task_stop();
> 4) 0.121 us | pick_next_task_dl();
> 4) 0.175 us | pick_next_task_rt();
> 4) 0.442 us | __pick_next_task_fair();
> 4) 0.396 us | psi_task_switch();
> 4) | __traceiter_sched_switch() {
> 4) | /* sched_switch: prev_comm=ktimers/4
> prev_pid=1157 prev_prio=98 prev_state=S ==> next_comm=kworker/4:1
> next_pid=1163 next_prio=120 */
> 4) 0.269 us | }
> 4) 0.114 us | enter_lazy_tlb();
> ------------------------------------------
> 4) ktimers-1157 => kworker-1163
> ------------------------------------------
>
> 4) | finish_task_switch.isra.0() {
> 4) 0.288 us | vtime_task_switch_generic();
> 4) 0.116 us | __tick_nohz_task_switch();
> 4) 0.111 us | __rcu_irq_enter_check_tick();
> 4) 0.176 us | irq_enter_rcu();
> 4) | __sysvec_irq_work() {
> 4) | /* irq_work_entry: vector=246 */
> 4) | /* irq_work_exit: vector=246 */
> 4) 0.907 us | }
> 4) | irq_exit_rcu() {
> 4) | /* tick_stop: success=0 dependency=SCHED */
> 4) 0.554 us | }
> 4) 0.123 us | raw_irqentry_exit_cond_resched();
> 4) 3.596 us | }
> 4) | wq_worker_running() {
> 4) 0.101 us | kthread_data();
> 4) 0.344 us | }
> 4) 0.121 us | _raw_spin_lock_irq();
> 4) | process_one_work() {
> 4) 0.112 us | _raw_spin_unlock_irq();
> 4) 1.246 us | vmstat_update();
> 4) 0.107 us | _raw_spin_lock_irq();
> 4) 0.108 us | pwq_dec_nr_in_flight();
> 4) 2.356 us | }
This is your kworker doing work, if I am not too confused.
Preceded by a context switch.
> 4) 0.099 us | worker_enter_idle();
> 4) 0.097 us | _raw_spin_unlock_irq();
> 4) | schedule() {
> 4) 0.153 us | wq_worker_sleeping();
> 4) 0.194 us | rcu_note_context_switch();
> 4) 0.413 us | _raw_spin_lock();
> 4) 0.134 us | update_rq_clock();
> 4) 0.702 us | dequeue_task_fair();
> 4) 1.123 us | pick_next_task_fair();
> 4) 0.687 us | psi_task_switch();
> 4) | __traceiter_sched_switch() {
> 4) | /* sched_switch: prev_comm=kworker/4:1
> prev_pid=1163 prev_prio=120 prev_state=I ==> next_comm=hiccups_x86-64
> next_pid=1335 next_prio=120 */
> 4) 0.246 us | }
> 4) 0.139 us | switch_mm_irqs_off();
> ------------------------------------------
> 4) kworker-1163 => hiccups-1335
> ------------------------------------------
>
> 4) + 32.824 us | } /* schedule */
> 4) 0.113 us | rcu_nocb_flush_deferred_wakeup();
> 4) 0.110 us | mem_cgroup_handle_over_high();
> 4) 0.104 us | blkcg_maybe_throttle_current();
> 4) 0.151 us | __rseq_handle_notify_resume();
> 4) 0.116 us | rcu_nocb_flush_deferred_wakeup();
> 4) 0.097 us | fpregs_assert_state_consistent();
> 4) 0.102 us | switch_fpu_return();
> 4) + 34.991 us | } /* exit_to_user_mode_prepare */
> 4) | vtime_user_enter() {
> 4) 0.174 us | get_vtime_delta();
> 4) 0.388 us | }
> 4) | vtime_user_exit() {
> 4) 0.188 us | get_vtime_delta();
> 4) 0.456 us | account_user_time();
> 4) 1.118 us | }
> 4) | irq_enter_rcu() {
> 4) 0.158 us | tick_irq_enter();
> 4) 0.391 us | }
> 4) | __sysvec_apic_timer_interrupt() {
> 4) | /* local_timer_entry: vector=236 */
> 4) 5.066 us | hrtimer_interrupt();
> 4) | /* local_timer_exit: vector=236 */
> 4) 5.596 us | }
> 4) | irq_exit_rcu() {
> 4) 0.111 us | idle_cpu();
> 4) | tick_nohz_irq_exit() {
> 4) | /* tick_stop: success=1 dependency=NONE */
> 4) 5.546 us | }
> 4) 6.018 us | }
> 4) | exit_to_user_mode_prepare() {
> 4) 0.118 us | rcu_nocb_flush_deferred_wakeup();
> 4) 0.128 us | fpregs_assert_state_consistent();
> 4) 0.623 us | }
> 4) | vtime_user_enter() {
> 4) 0.158 us | get_vtime_delta();
> 4) 0.364 us | }
This looks like the kernel exiting to userspace?
> > Unless CPU 4 spends absolutely no time in the kernel, and never ever
> > has more than one runnable task at any given time. And doesn't have
> > any other excuse to take a scheduling-clock interrupt. Otherwise, CPU
> > 4's rcuc kthread will burn at least a little bit of time. The more time
> > CPU 4 spends in the kernel and the more time CPU 4 spends with more than
> > one runnable task, the more time CPU 4's rcuc kthread will burn.
> >
> I am exactly trying to achieve this (again, but now on 6.1.*-rt* latest).
>
> > > > I see this in the kernel boot log, and I wonder if this is related to my issue:
> > > >
> > > > [ 0.063518] rcu: RCU_SOFTIRQ processing moved to rcuc kthreads.
> >
> > This simply means that the processing that would have been done in
> > softirq (for example, by ksoftirqd) will now be done by rcuc.
> >
> Note that my kernel has the PREEMPT_RT patch, so I would expect any
> softirq to be a kthread anyway.
>
> > > > tree.c:102:static bool use_softirq = !IS_ENABLED(CONFIG_PREEMPT_RT);
> > > >
> > > > Could this cause the rcuc threads to become active, even with the
> > > > "rcu_nocbs=all rcu_nocb_poll" setting?
> >
> > You can override this with rcutree.use_softirq=1 if you wish. But that
> > will simply move the processing from the rcuc kthread to softirq context.
> >
> ...which is a kthread after all in PREEMPT_RT, right?
>
> > > That's unexpected. The RCU load should be moved from rcuc to rcuo* kthreads.
> > >
> > > Do you have CONFIG_RCU_STRICT_GRACE_PERIOD=y perhaps?
> > >
> No. I do have some _TIMEOUT related settings, that I am not sure of if
> they are rcuc side or rcuo* side:
>
> 162:CONFIG_RCU_EXPERT=y
> 172:CONFIG_RCU_STALL_COMMON=y
> 173:CONFIG_RCU_NEED_SEGCBLIST=y
> 174:CONFIG_RCU_FANOUT=64
> 175:CONFIG_RCU_FANOUT_LEAF=16
> 176:CONFIG_RCU_BOOST=y
> 177:CONFIG_RCU_BOOST_DELAY=500
> 178:# CONFIG_RCU_EXP_KTHREAD is not set
> 179:CONFIG_RCU_NOCB_CPU=y
This combined with your nohz_full=4-7 will offload handling of RCU
callbacks to kthreads, but you should also check to make sure that
the resulting rcuo* kthreads are never running on CPUs 4-7.
> 180:# CONFIG_RCU_NOCB_CPU_DEFAULT_ALL is not set
> 181:CONFIG_RCU_NOCB_CPU_CB_BOOST=y
> 10445:# CONFIG_RCU_SCALE_TEST is not set
> 10446:# CONFIG_RCU_TORTURE_TEST is not set
> 10447:# CONFIG_RCU_REF_SCALE_TEST is not set
> 10448:CONFIG_RCU_CPU_STALL_TIMEOUT=21
> 10449:CONFIG_RCU_EXP_CPU_STALL_TIMEOUT=0
> 10450:# CONFIG_RCU_TRACE is not set
> 10451:# CONFIG_RCU_EQS_DEBUG is not set
>
> > > Also do you really need rcu_nocb_poll? I would expect this option to be useful
> > > for debugging only, though I'm not aware of every usecases.
> >
> "Setting rcu_nocb_poll to relieve each CPU from the responsibility
> awakening their RCU offload threads".
>
> I hoped this would decrease the CPU specific RCU (rcuc) side to become
> more silent on the timer. Or is it purely synchronous on the event
> anyway?
Without rcu_nocb_poll, when your kernel code does call_rcu(), for example,
when userspace closes a file, that call_rcu() will do a wakeup with
attendant overhead. With rcu_nocb_poll, call_rcu() won't do wakeups
(unless your application (say) closes tens of thousands of files all at
once on a single CPU), but the rcuog* kthreads will wake up periodically
to poll. Depending on what is pinned where, you might want either
setting of rcu_nocb_poll.
> > I have the same questions. ;-)
> >
> Good, that means -EUSER is even more likely. (five nines? :-) )
This file gives some information on per-CPU kthreads, a bit dated, but
still likely helpful:
Documentation/admin-guide/kernel-per-CPU-kthreads.rst
Thanx, Paul
