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SubjectRe: Query regarding synchronize_sched_expedited and resched_cpu
On Tue, Sep 19, 2017 at 11:58:59AM -0400, Steven Rostedt wrote:
> On Tue, 19 Sep 2017 08:31:26 -0700
> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
>
> > commit bc43e2e7e08134e6f403ac845edcf4f85668d803
> > Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > Date: Mon Sep 18 08:54:40 2017 -0700
> >
> > sched: Make resched_cpu() unconditional
> >
> > The current implementation of synchronize_sched_expedited() incorrectly
> > assumes that resched_cpu() is unconditional, which it is not. This means
> > that synchronize_sched_expedited() can hang when resched_cpu()'s trylock
> > fails as follows (analysis by Neeraj Upadhyay):
> >
> > o 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())
> >
> > o 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
> >
> > o CPU5 calls rcu_idle_enter() and as need_resched is not set, goes to
> > idle (schedule() is not called).
> >
> > o CPU 1 reports RCU stall.
> >
> > Given that resched_cpu() is used only by RCU, this commit fixes the
>
> "is now only used by RCU", as it was created for another purpose.

Good catch, fixed.

> > assumption by making resched_cpu() unconditional.
> >
> > Reported-by: Neeraj Upadhyay <neeraju@codeaurora.org>
> > Suggested-by: Neeraj Upadhyay <neeraju@codeaurora.org>
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Steven Rostedt <rostedt@goodmis.org>
>
> Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

Applied, thank you!

Thanx, Paul

> -- Steve
>
> >
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index cab8c5ec128e..b2281971894c 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -505,8 +505,7 @@ void resched_cpu(int cpu)
> > struct rq *rq = cpu_rq(cpu);
> > unsigned long flags;
> >
> > - if (!raw_spin_trylock_irqsave(&rq->lock, flags))
> > - return;
> > + raw_spin_lock_irqsave(&rq->lock, flags);
> > resched_curr(rq);
> > raw_spin_unlock_irqrestore(&rq->lock, flags);
> > }
>

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