Re: rcu_process_callbacks irqsoff latency caused by taking spinlock with irqs disabled

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Apr 05, 2018 at 09:34:14AM +1000, Nicholas Piggin wrote:
> 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.

Well, that 3 milliseconds would cause serious problems for some workloads...

> # 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.

If I recall correctly, the issue is that an unsynchronized (due to
interrupts being enabled) check in the "if" statement can cause extra
unneeded grace periods.

I am guessing that the long latency is caused by lots of CPUs suddenly
needing a new grace period at about the same time.  If so, this is
a bottleneck that I have been expecting for some time, and one that
I would resolve by introducing funnel locking, sort of like SRCU and
expedited grace periods already use.

Or am I confused about the source of the contention?

							Thanx, Paul