Re: Query regarding synchronize_sched_expedited and resched_cpu
From: Paul E. McKenney
Date: Thu Sep 21 2017 - 12:00:58 EST
On Thu, Sep 21, 2017 at 03:59:46PM +0200, Peter Zijlstra wrote:
> On Tue, Sep 19, 2017 at 08:31:26AM -0700, Paul E. McKenney wrote:
> > So I have this one queued. Objections?
> Changelog reads like its whitespace damaged.
It does, now that you mention it. How about the updated version below?
Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
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:
rdp->exp_dynticks_snap & 0x1 // returns 1 for CPU5
IPI sent to CPU5
ret = swait_event_timeout(rsp->expedited_wq,
expmask = 0x20, CPU 5 in idle path (in cpuidle_enter())
o CPU5 handles IPI and fails to acquire rq lock.
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 now used only by RCU, this commit fixes the
assumption by making resched_cpu() unconditional.
Reported-by: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
Suggested-by: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Acked-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index cab8c5ec128e..b2281971894c 100644
@@ -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))
+ raw_spin_lock_irqsave(&rq->lock, flags);