On Mon, Sep 18, 2017 at 11:11:05AM -0400, Steven Rostedt wrote:
> On Sun, 17 Sep 2017 11:37:06 +0530
> Neeraj Upadhyay <neer...@codeaurora.org> wrote:
>
> > Hi Paul, how about replacing raw_spin_trylock_irqsave with
> > raw_spin_lock_irqsave in resched_cpu()? Are there any paths
> > in RCU code, which depend on trylock check/spinlock recursion?
>
> It looks to me that resched_cpu() was added for nohz full sched
> balancing, but is not longer used by that. The only user is currently
> RCU. Perhaps we should change that from a trylock to a lock.
That certainly is a much simpler fix than the one I was thinking of!
So how about the following patch?
Thanx, Paul
------------------------------------------------------------------------
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
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>
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);
}
