Messages in this thread Patch in this message | | | Date | Thu, 21 Sep 2017 09:00:48 -0700 | From | "Paul E. McKenney" <> | Subject | Re: Query regarding synchronize_sched_expedited and resched_cpu |
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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?
Thanx, Paul
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commit c21c9b78182e35eb0e72ef4e3bba3054f26eaaea 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, CPU 5 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 now used only by RCU, this commit fixes the 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> Acked-by: Steven Rostedt (VMware) <rostedt@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); }
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