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Date: Mon, 18 Sep 2017 09:24:12 -0700 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Steven Rostedt <rostedt@...dmis.org> Cc: Neeraj Upadhyay <neeraju@...eaurora.org>, josh@...htriplett.org, mathieu.desnoyers@...icios.com, jiangshanlai@...il.com, linux-kernel@...r.kernel.org, sramana@...eaurora.org, prsood@...eaurora.org, pkondeti@...eaurora.org, markivx@...eaurora.org, peterz@...radead.org Subject: Re: Query regarding synchronize_sched_expedited and resched_cpu 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@...ux.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... :-( sparse_irq_lock, I am looking at you!!! ;-) Thanx, Paul
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