Re: [PATCH tip/core/rcu 2/2] rcu: Tighten up affinity and check for sysidle

[Sasha Levin <sasha.levin@oracle.com>]

On 03/03/2015 12:50 PM, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> 
> If the RCU grace-period kthread invoking rcu_sysidle_check_cpu()
> happens to be running on the tick_do_timer_cpu initially,
> then rcu_bind_gp_kthread() won't bind it.  This kthread might
> then migrate before invoking rcu_gp_fqs(), which will trigger the
> WARN_ON_ONCE() in rcu_sysidle_check_cpu().  This commit therefore makes
> rcu_bind_gp_kthread() do the binding even if the kthread is currently
> on the same CPU.  Because this incurs added overhead, this commit also
> causes each RCU grace-period kthread to invoke rcu_bind_gp_kthread()
> once at boot rather than at the beginning of each grace period.
> And as long as rcu_bind_gp_kthread() is being modified, this commit
> eliminates its #ifdef.
> 
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
>  kernel/rcu/tree.c        |  2 +-
>  kernel/rcu/tree_plugin.h | 12 ++++++------
>  2 files changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 735bd7ee749a..a6972c20eaa5 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1707,7 +1707,6 @@ static int rcu_gp_init(struct rcu_state *rsp)
>  	struct rcu_node *rnp = rcu_get_root(rsp);
>  
>  	ACCESS_ONCE(rsp->gp_activity) = jiffies;
> -	rcu_bind_gp_kthread();
>  	raw_spin_lock_irq(&rnp->lock);
>  	smp_mb__after_unlock_lock();
>  	if (!ACCESS_ONCE(rsp->gp_flags)) {
> @@ -1895,6 +1894,7 @@ static int __noreturn rcu_gp_kthread(void *arg)
>  	struct rcu_state *rsp = arg;
>  	struct rcu_node *rnp = rcu_get_root(rsp);
>  
> +	rcu_bind_gp_kthread();
>  	for (;;) {
>  
>  		/* Handle grace-period start. */
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 0a571e9a0f1d..b46c92824db1 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -2763,7 +2763,8 @@ static void rcu_sysidle_exit(int irq)
>  
>  /*
>   * Check to see if the current CPU is idle.  Note that usermode execution
> - * does not count as idle.  The caller must have disabled interrupts.
> + * does not count as idle.  The caller must have disabled interrupts,
> + * and must be running on tick_do_timer_cpu.
>   */
>  static void rcu_sysidle_check_cpu(struct rcu_data *rdp, bool *isidle,
>  				  unsigned long *maxj)
> @@ -2784,8 +2785,8 @@ static void rcu_sysidle_check_cpu(struct rcu_data *rdp, bool *isidle,
>  	if (!*isidle || rdp->rsp != rcu_state_p ||
>  	    cpu_is_offline(rdp->cpu) || rdp->cpu == tick_do_timer_cpu)
>  		return;
> -	if (rcu_gp_in_progress(rdp->rsp))
> -		WARN_ON_ONCE(smp_processor_id() != tick_do_timer_cpu);
> +	/* Verify affinity of current kthread. */
> +	WARN_ON_ONCE(smp_processor_id() != tick_do_timer_cpu);

Hey Paul,

I seem to be hitting this warning:

[22408.472078] ------------[ cut here ]------------
[22408.476972] WARNING: CPU: 1 PID: 8 at kernel/rcu/tree_plugin.h:2796 rcu_sysidle_check_cpu+0x374/0x3f0()
[22408.481567] Modules linked in:
[22408.486054] CPU: 1 PID: 8 Comm: rcu_sched Tainted: G        W       4.1.0-rc1-next-20150504+ #3
[22408.490576] Hardware name: Oracle Corporation OVCA X3-2             /ASSY,MOTHERBOARD,1U   , BIOS 17021300 06/19/2012
[22408.495117]  ffff881ff2db4600 0000000044d8814b ffff881ff2f1fab8 ffffffff830f9926
[22408.499647]  0000000000000000 0000000000000000 ffff881ff2f1fb08 ffffffff8120bed0
[22408.504120]  ffff881ff2f1fb88 ffffffff81324ec4 1ffff103fe5e3f65 ffff881ff2f1fd48
[22408.508568] Call Trace:
[22408.512953] dump_stack (lib/dump_stack.c:52)
[22408.517331] warn_slowpath_common (kernel/panic.c:447)
[22408.521637] ? rcu_sysidle_check_cpu (kernel/rcu/tree_plugin.h:2796 (discriminator 53))
[22408.525899] warn_slowpath_null (kernel/panic.c:481)
[22408.530105] rcu_sysidle_check_cpu (kernel/rcu/tree_plugin.h:2796 (discriminator 53))
[22408.534287] ? console_conditional_schedule (kernel/time/timer.c:1472)
[22408.538413] dyntick_save_progress_counter (kernel/rcu/tree.c:1041)
[22408.542486] force_qs_rnp (kernel/rcu/tree.c:2793)
[22408.546504] ? wake_up_atomic_t (kernel/sched/wait.c:200)
[22408.550461] ? rcu_sysidle_check_cpu (kernel/rcu/tree.c:1039)
[22408.554374] rcu_gp_kthread (kernel/rcu/tree.c:1914 include/linux/jump_label.h:125 include/linux/context_tracking_state.h:28 include/linux/tick.h:123 kernel/rcu/tree_plugin.h:2914 kernel/rcu/tree.c:1916 kernel/rcu/tree.c:2075)
[22408.558240] ? rcu_oom_notify (kernel/rcu/tree.c:2012)
[22408.562044] ? finish_task_switch (include/linux/tick.h:179 kernel/sched/core.c:2248)
[22408.565789] ? __schedule (kernel/sched/core.c:2340 kernel/sched/core.c:2798)
[22408.569473] ? __wake_up_common (kernel/sched/wait.c:73)
[22408.573088] ? prepare_to_wait_event (kernel/sched/wait.c:292)
[22408.576665] ? rcu_oom_notify (kernel/rcu/tree.c:2012)
[22408.580197] kthread (kernel/kthread.c:207)
[22408.583652] ? arch_vtime_task_switch (kernel/sched/cputime.c:753)
[22408.587065] ? kthread_create_on_node (kernel/kthread.c:176)
[22408.590427] ? kthread_create_on_node (kernel/kthread.c:176)
[22408.593713] ret_from_fork (arch/x86/kernel/entry_64.S:640)
[22408.596972] ? kthread_create_on_node (kernel/kthread.c:176)


Thanks,
Sasha