Re: Query regarding synchronize_sched_expedited and resched_cpu

[Paul E. McKenney]

On Fri, Sep 15, 2017 at 04:44:38PM +0530, Neeraj Upadhyay wrote:
> Hi,
> 
> We have one query regarding the behavior of RCU expedited grace period,
> for scenario where resched_cpu() in sync_sched_exp_handler() fails to
> acquire the rq lock and returns w/o setting the need_resched. In this
> case, how do we ensure that the CPU notify rcu about the
> end of sched grace period (schedule() -> __schedule() ->
> rcu_note_context_switch(cpu) -> rcu_sched_qs()) , for cases where tick
> is stopped on that CPU.  Is it implied from the rq lock acquisition
> failure, that the owner of the rq lock will enforce context switch?
> For which scenarios in RCU paths (as the function is used only in RCU
> code), we need trylock check in resched_cpu()?
> 
> void resched_cpu(int cpu)
> {
>         struct rq *rq = cpu_rq(cpu);
>         unsigned long flags;
> 
>         if (!raw_spin_trylock_irqsave(&rq->lock, flags))
>                 return;
>         resched_curr(rq);
>         raw_spin_unlock_irqrestore(&rq->lock, flags);
> }
> 
> 
> This issue was observed in below scenario, where one of the CPUs (CPU1)
> started synchronize_sched_expedited and sent IPI to CPU5, which is in
> the idle path but handled sync_sched_exp_handler() IPI before
> rcu_idle_enter().
> As resched_cpu() failed to acquire the rq lock, need_resched was not set,
> and CPU went to idle; resulting in expedited stall getting reported
> by CPU1.
> 
> Below is the scenario:
> 
> â    CPU1 is waiting for expedited wait to complete:
> sync_rcu_exp_select_cpus
>     rdp->exp_dynticks_snap & 0x1   // returns 1 for CPU5
>     IPI sent to CPU5
> 
> synchronize_sched_expedited_wait
>         ret = swait_event_timeout(
>                                     rsp->expedited_wq,
>  sync_rcu_preempt_exp_done(rnp_root),
>                                     jiffies_stall);
> 
>            expmask = 0x20 , and CPU 5 is in idle path (in cpuidle_enter())
> 
> 
> 
> â    CPU5 handles IPI and fails to acquire rq lock.
> 
> Handles IPI
>     sync_sched_exp_handler
>         resched_cpu
>             returns while failing to try lock acquire rq->lock
>         need_resched is not set
> 
> â    CPU5 calls  rcu_idle_enter() and as need_resched is not set, goes to
>     idle (schedule() is not called).
> 
> â    CPU 1 reports RCU stall.

Good catch and good detective work!!!

I will be working on a fix this week, hopefully involving resched_cpu()
getting a return value so that I can track who needs a later retry.

							Thanx, Paul