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SubjectRe: Query regarding synchronize_sched_expedited and resched_cpu
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:
>
> • 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())
>
>
>
> • 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
>
> • CPU5 calls rcu_idle_enter() and as need_resched is not set, goes to
> idle (schedule() is not called).
>
> • 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

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