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" <paul...@linux.vnet.ibm.com> wrote: > > > commit bc43e2e7e08134e6f403ac845edcf4f85668d803 > > Author: Paul E. McKenney <paul...@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 <neer...@codeaurora.org> > > Suggested-by: Neeraj Upadhyay <neer...@codeaurora.org> > > Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com> > > Cc: Peter Zijlstra <pet...@infradead.org> > > Cc: Steven Rostedt <rost...@goodmis.org> > > Acked-by: Steven Rostedt (VMware) <rost...@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); > > } >