Messages in this thread Patch in this message | | | Date | Tue, 19 Sep 2017 08:31:26 -0700 | From | "Paul E. McKenney" <> | Subject | Re: Query regarding synchronize_sched_expedited and resched_cpu |
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On Mon, Sep 18, 2017 at 09:24:12AM -0700, Paul E. McKenney wrote: > On Mon, Sep 18, 2017 at 12:12:13PM -0400, Steven Rostedt wrote: > > On Mon, 18 Sep 2017 09:01:25 -0700 > > "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote: > > > > > > > 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. > > > > Probably want to run this with several workloads with lockdep enabled > > first. > > As soon as I work through the backlog of lockdep complaints that > appeared in the last merge window... :-(
And this patch survived all rcutorture scenarios, including those with lockdep enabled. There were failures, but these are pre-existing issues I am chasing: Lost timeouts on TREE01 and rt_mutex trying to awaken an offline CPU in TREE03.
So I have this one queued. Objections?
Thanx, Paul
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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 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>
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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