Messages in this thread | ![/](/images/icornerl.gif) | | Date | Tue, 19 Sep 2017 11:58:59 -0400 | From | Steven Rostedt <> | Subject | Re: Query regarding synchronize_sched_expedited and resched_cpu |
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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.
> 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>
-- 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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