From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752085AbdIUOAB (ORCPT ); Thu, 21 Sep 2017 10:00:01 -0400 Received: from bombadil.infradead.org ([65.50.211.133]:44210 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751962AbdIUN7x (ORCPT ); Thu, 21 Sep 2017 09:59:53 -0400 Date: Thu, 21 Sep 2017 15:59:46 +0200 From: Peter Zijlstra To: "Paul E. McKenney" Cc: Steven Rostedt , Neeraj Upadhyay , josh@joshtriplett.org, mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com, linux-kernel@vger.kernel.org, sramana@codeaurora.org, prsood@codeaurora.org, pkondeti@codeaurora.org, markivx@codeaurora.org Subject: Re: Query regarding synchronize_sched_expedited and resched_cpu Message-ID: <20170921135946.37fhnlbplgseia53@hirez.programming.kicks-ass.net> References: <8f33e48e-ac6d-2c88-e16f-20b698c06292@codeaurora.org> <20170917010015.GW3521@linux.vnet.ibm.com> <20170918111105.15f687da@gandalf.local.home> <20170918160125.GL3521@linux.vnet.ibm.com> <20170918121213.312c82b0@gandalf.local.home> <20170918162412.GM3521@linux.vnet.ibm.com> <20170919153126.GA2955@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170919153126.GA2955@linux.vnet.ibm.com> User-Agent: NeoMutt/20170609 (1.8.3) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. > commit bc43e2e7e08134e6f403ac845edcf4f85668d803 > Author: Paul E. McKenney > 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 > Suggested-by: Neeraj Upadhyay > Signed-off-by: Paul E. McKenney > Cc: Peter Zijlstra > Cc: Steven Rostedt > > 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); > } >