rcu_process_callbacks irqsoff latency caused by taking spinlock with irqs disabled

[Nicholas Piggin]

Hi Paul,

Just looking at latencies, and RCU showed up as one of the maximums.
This is a 2 socket system with (176 CPU threads). Just doing a
`make -j 352` kernel build. Got a max latency of 3ms. I don't think
that's anything to worry about really, but I wanted to check the
cause.

# tracer: irqsoff
#
# irqsoff latency trace v1.1.5 on 4.16.0-01530-g43d1859f0994
# --------------------------------------------------------------------
# latency: 3055 us, #19/19, CPU#135 | (M:server VP:0, KP:0, SP:0 HP:0 #P:176)
#    -----------------
#    | task: cc1-58689 (uid:1003 nice:0 policy:0 rt_prio:0)
#    -----------------
#  => started at: rcu_process_callbacks
#  => ended at:   _raw_spin_unlock_irqrestore
#
#
#                  _------=> CPU#            
#                 / _-----=> irqs-off        
#                | / _----=> need-resched    
#                || / _---=> hardirq/softirq 
#                ||| / _--=> preempt-depth   
#                |||| /     delay            
#  cmd     pid   ||||| time  |   caller      
#     \   /      |||||  \    |   /         
   <...>-58689 135d.s.    0us : rcu_process_callbacks
   <...>-58689 135d.s.    1us : cpu_needs_another_gp <-rcu_process_callbacks
   <...>-58689 135d.s.    2us : rcu_segcblist_future_gp_needed <-cpu_needs_another_gp
   <...>-58689 135d.s.    3us#: _raw_spin_lock <-rcu_process_callbacks
   <...>-58689 135d.s. 3047us : rcu_start_gp <-rcu_process_callbacks
   <...>-58689 135d.s. 3048us : rcu_advance_cbs <-rcu_start_gp
   <...>-58689 135d.s. 3049us : rcu_segcblist_pend_cbs <-rcu_advance_cbs
   <...>-58689 135d.s. 3049us : rcu_segcblist_advance <-rcu_advance_cbs
   <...>-58689 135d.s. 3050us : rcu_accelerate_cbs <-rcu_start_gp
   <...>-58689 135d.s. 3050us : rcu_segcblist_pend_cbs <-rcu_accelerate_cbs
   <...>-58689 135d.s. 3051us : rcu_segcblist_accelerate <-rcu_accelerate_cbs
   <...>-58689 135d.s. 3052us : trace_rcu_future_gp.isra.0 <-rcu_accelerate_cbs
   <...>-58689 135d.s. 3052us : trace_rcu_future_gp.isra.0 <-rcu_accelerate_cbs
   <...>-58689 135d.s. 3053us : rcu_start_gp_advanced.isra.35 <-rcu_start_gp
   <...>-58689 135d.s. 3053us : cpu_needs_another_gp <-rcu_start_gp_advanced.isra.35
   <...>-58689 135d.s. 3054us : _raw_spin_unlock_irqrestore <-rcu_process_callbacks
   <...>-58689 135d.s. 3055us : _raw_spin_unlock_irqrestore
   <...>-58689 135d.s. 3056us : trace_hardirqs_on <-_raw_spin_unlock_irqrestore
   <...>-58689 135d.s. 3061us : <stack trace>

So it's taking a contende lock with interrupts disabled:

static void
__rcu_process_callbacks(struct rcu_state *rsp)
{
        unsigned long flags;
        bool needwake;
        struct rcu_data *rdp = raw_cpu_ptr(rsp->rda);

        WARN_ON_ONCE(!rdp->beenonline);

        /* Update RCU state based on any recent quiescent states. */
        rcu_check_quiescent_state(rsp, rdp);

        /* Does this CPU require a not-yet-started grace period? */
        local_irq_save(flags);
        if (cpu_needs_another_gp(rsp, rdp)) {
                raw_spin_lock_rcu_node(rcu_get_root(rsp)); /* irqs disabled. */
                needwake = rcu_start_gp(rsp);
                raw_spin_unlock_irqrestore_rcu_node(rcu_get_root(rsp), flags);
                if (needwake)
                        rcu_gp_kthread_wake(rsp);
        } else {
                local_irq_restore(flags);
        }

Because irqs are disabled before taking the lock, we can't spin with
interrupts enabled.

cpu_needs_another_gp needs interrupts off to prevent races with normal
callback registry, but that doesn't seem to be preventing any wider
races in this code, because we immediately re-enable interrupts anyway
if no gp is needed. So an interrupt can come in right after that and
queue something up.

So then the question is whether it's safe-albeit-racy to call with
interrupts ensabled? Would be nice to move it to a spin_lock_irqsave.

Thanks,
Nick