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Message-ID: <8f33e48e-ac6d-2c88-e16f-20b698c06292@codeaurora.org> Date: Fri, 15 Sep 2017 16:44:38 +0530 From: Neeraj Upadhyay <neeraju@...eaurora.org> To: paulmck@...ux.vnet.ibm.com, josh@...htriplett.org, rostedt@...dmis.org, mathieu.desnoyers@...icios.com, jiangshanlai@...il.com Cc: linux-kernel@...r.kernel.org, sramana@...eaurora.org, prsood@...eaurora.org Subject: Query regarding synchronize_sched_expedited and resched_cpu Hi, We have one query regarding the behavior of RCU expedited grace period, for scenario where resched_cpu() in sync_sched_exp_handler() fails to acquire the rq lock and returns w/o setting the need_resched. In this case, how do we ensure that the CPU notify rcu about the end of sched grace period (schedule() -> __schedule() -> rcu_note_context_switch(cpu) -> rcu_sched_qs()) , for cases where tick is stopped on that CPU. Is it implied from the rq lock acquisition failure, that the owner of the rq lock will enforce context switch? For which scenarios in RCU paths (as the function is used only in RCU code), we need trylock check in resched_cpu()? void resched_cpu(int cpu) { struct rq *rq = cpu_rq(cpu); unsigned long flags; if (!raw_spin_trylock_irqsave(&rq->lock, flags)) return; resched_curr(rq); raw_spin_unlock_irqrestore(&rq->lock, flags); } This issue was observed in below scenario, where one of the CPUs (CPU1) started synchronize_sched_expedited and sent IPI to CPU5, which is in the idle path but handled sync_sched_exp_handler() IPI before rcu_idle_enter(). As resched_cpu() failed to acquire the rq lock, need_resched was not set, and CPU went to idle; resulting in expedited stall getting reported by CPU1. Below is the scenario: • 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()) • 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 • CPU5 calls rcu_idle_enter() and as need_resched is not set, goes to idle (schedule() is not called). • CPU 1 reports RCU stall. -- QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, hosted by The Linux Foundation
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