From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 15253C433EF for ; Sat, 12 Feb 2022 11:29:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234533AbiBLL3F (ORCPT ); Sat, 12 Feb 2022 06:29:05 -0500 Received: from mxb-00190b01.gslb.pphosted.com ([23.128.96.19]:42770 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230111AbiBLL3D (ORCPT ); Sat, 12 Feb 2022 06:29:03 -0500 Received: from alexa-out-sd-01.qualcomm.com (alexa-out-sd-01.qualcomm.com [199.106.114.38]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7919026AEA; Sat, 12 Feb 2022 03:29:00 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=quicinc.com; i=@quicinc.com; q=dns/txt; s=qcdkim; t=1644665340; x=1676201340; h=message-id:date:mime-version:subject:to:cc:references: from:in-reply-to:content-transfer-encoding; bh=HSi1VqgHPVJFN1TN6ZavB+WHmIz29T9/xnVFhhg06Hk=; b=kgbkgPW5adGMWG14k4MTiQYQV7LTpGOylDeG+3S0VZL+Zf+iBplxU7T0 VG4pyxv52oHuSsmQ5d+HO9AFd479H3BbeVJcyM7zzI8D3oSeP9yGeWyAS g6ITdYrigopRrerRYf7jW51AncMq/37QN3Ub7JX8z6R64MPvvQlW04Ogt E=; Received: from unknown (HELO ironmsg01-sd.qualcomm.com) ([10.53.140.141]) by alexa-out-sd-01.qualcomm.com with ESMTP; 12 Feb 2022 03:29:00 -0800 X-QCInternal: smtphost Received: from nasanex01b.na.qualcomm.com ([10.46.141.250]) by ironmsg01-sd.qualcomm.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 12 Feb 2022 03:29:00 -0800 Received: from [10.216.54.182] (10.80.80.8) by nasanex01b.na.qualcomm.com (10.46.141.250) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.922.19; Sat, 12 Feb 2022 03:28:57 -0800 Message-ID: <73bc88ae-a1a4-99e2-0e66-e2ce553b2f46@quicinc.com> Date: Sat, 12 Feb 2022 16:58:54 +0530 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Thunderbird/91.3.2 Subject: Re: [PATCH rcu 3/3] rcu: Allow expedited RCU grace periods on incoming CPUs Content-Language: en-US To: Mukesh Ojha , CC: , , , , Tejun Heo References: <20220204225409.GA4193020@paulmck-ThinkPad-P17-Gen-1> <20220204225507.4193113-3-paulmck@kernel.org> <20220209220601.GB4285@paulmck-ThinkPad-P17-Gen-1> <20220211221455.GM4285@paulmck-ThinkPad-P17-Gen-1> <2a4870e9-41ae-60cf-fef3-aeb1de6f559c@quicinc.com> From: Neeraj Upadhyay In-Reply-To: <2a4870e9-41ae-60cf-fef3-aeb1de6f559c@quicinc.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-Originating-IP: [10.80.80.8] X-ClientProxiedBy: nasanex01a.na.qualcomm.com (10.52.223.231) To nasanex01b.na.qualcomm.com (10.46.141.250) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Mukesh, On 2/12/2022 2:17 PM, Mukesh Ojha wrote: > > On 2/12/2022 3:44 AM, Paul E. McKenney wrote: >> On Sat, Feb 12, 2022 at 12:14:20AM +0530, Mukesh Ojha wrote: >>> On 2/10/2022 3:36 AM, Paul E. McKenney wrote: >>>> On Wed, Feb 09, 2022 at 11:53:33PM +0530, Mukesh Ojha wrote: >>>>> On 2/5/2022 4:25 AM, Paul E. McKenney wrote: >>>>>> Although it is usually safe to invoke synchronize_rcu_expedited() >>>>>> from a >>>>>> preemption-enabled CPU-hotplug notifier, if it is invoked from a >>>>>> notifier >>>>>> between CPUHP_AP_RCUTREE_ONLINE and CPUHP_AP_ACTIVE, its attempts to >>>>>> invoke a workqueue handler will hang due to RCU waiting on a CPU that >>>>>> the scheduler is not paying attention to.  This commit therefore >>>>>> expands >>>>>> use of the existing workqueue-independent synchronize_rcu_expedited() >>>>>> from early boot to also include CPUs that are being hotplugged. >>>>>> >>>>>> Link: >>>>>> https://lore.kernel.org/lkml/7359f994-8aaf-3cea-f5cf-c0d3929689d6@quicinc.com/ >>>>>> >>>>>> Reported-by: Mukesh Ojha >>>>>> Cc: Tejun Heo >>>>>> Signed-off-by: Paul E. McKenney >>>>>> --- >>>>>>     kernel/rcu/tree_exp.h | 14 ++++++++++---- >>>>>>     1 file changed, 10 insertions(+), 4 deletions(-) >>>>>> >>>>>> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h >>>>>> index 60197ea24ceb9..1a45667402260 100644 >>>>>> --- a/kernel/rcu/tree_exp.h >>>>>> +++ b/kernel/rcu/tree_exp.h >>>>>> @@ -816,7 +816,7 @@ static int rcu_print_task_exp_stall(struct >>>>>> rcu_node *rnp) >>>>>>      */ >>>>>>     void synchronize_rcu_expedited(void) >>>>>>     { >>>>>> -    bool boottime = (rcu_scheduler_active == RCU_SCHEDULER_INIT); >>>>>> +    bool no_wq; >>>>>>         struct rcu_exp_work rew; >>>>>>         struct rcu_node *rnp; >>>>>>         unsigned long s; >>>>>> @@ -841,9 +841,15 @@ void synchronize_rcu_expedited(void) >>>>>>         if (exp_funnel_lock(s)) >>>>>>             return;  /* Someone else did our work for us. */ >>>>>> +    /* Don't use workqueue during boot or from an incoming CPU. */ >>>>>> +    preempt_disable(); >>>>>> +    no_wq = rcu_scheduler_active == RCU_SCHEDULER_INIT || >>>>>> +        !cpumask_test_cpu(smp_processor_id(), cpu_active_mask); >>>>>> +    preempt_enable(); >>>>>> + >>>>>>         /* Ensure that load happens before action based on it. */ >>>>>> -    if (unlikely(boottime)) { >>>>>> -        /* Direct call during scheduler init and >>>>>> early_initcalls(). */ >>>>>> +    if (unlikely(no_wq)) { >>>>>> +        /* Direct call for scheduler init, early_initcall()s, and >>>>>> incoming CPUs. */ >>>>>>             rcu_exp_sel_wait_wake(s); >>>>>>         } else { >>>>>>             /* Marshall arguments & schedule the expedited grace >>>>>> period. */ >>>>>> @@ -861,7 +867,7 @@ void synchronize_rcu_expedited(void) >>>>>>         /* Let the next expedited grace period start. */ >>>>>>         mutex_unlock(&rcu_state.exp_mutex); >>>>>> -    if (likely(!boottime)) >>>>>> +    if (likely(!no_wq)) >>>>>>             destroy_work_on_stack(&rew.rew_work); >>>>>>     } >>>>>>     EXPORT_SYMBOL_GPL(synchronize_rcu_expedited); >>>>> Can we reach a condition after this change where no_wq = true and >>>>> during >>>>> rcu_stall report where exp_task = 0 list and exp_mask contain only >>>>> this cpu >>>>> ? >>>> Hello, Mukesh, and thank you for looking this over! >>>> >>>> At first glance, I do not believe that this can happen because the >>>> expedited grace-period machinery avoids waiting on the current CPU. >>>> (See sync_rcu_exp_select_node_cpus(), both the raw_smp_processor_id() >>>> early in the function and the get_cpu() later in the function.) >>>> >>>> But please let me know if I am missing something here. >>>> >>>> But suppose that we could in fact reach this condition.  What bad thing >>>> would happen?  Other than a resched_cpu() having been invoked several >>>> times on a not-yet-online CPU, of course.  ;-) >>> >>> I thought more about this, what if  synchronize_rcu_expedited thread got >>> schedule out and run on some other cpu >>> and we clear out cpu on which it ran next from exp_mask. >>> >>> Queuing the work on same cpu ensures that it will always be right cpu to >>> clear out. >>> Do you think this can happen ? >> Indeed it might. >> >> But if it did, the scheduler would invoke RCU's hook, which is named >> rcu_note_context_switch(), and do so on the pre-switch CPU.  There are >> two implementations for this function, one for CONFIG_PREEMPT=y >> and another for CONFIG_PREEMPT=n.  Both look to me like they invoke >> rcu_report_exp_rdp() when needed, one directly and the other via the >> CONFIG_PREEMPT=n variant of rcu_qs(). >> >> Am I missing something? >> >> > > There is a issue we are facing where exp_mask is not getting cleared and > rcu_stall report that > the cpu we are waiting on sometime in idle and sometime executing some > other task but > it is not clearing itself from exp_mask from a very long time and in all > the instances exp_task list is NULL. Can you please check whether [1] is present in your tree? Thanks Neeraj [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/kernel/rcu/tree_exp.h?h=v5.17-rc3&id=81f6d49cce2d2fe507e3fddcc4a6db021d9c2e7b > >    expmask = 8,     ==> cpu3 > > [80235.522440][T12441] rcu: INFO: rcu_preempt detected expedited stalls > on CPUs/tasks: { 3-... } 9163622 jiffies s: 634705 root: 0x8/. > [80235.534757][T12441] rcu: blocking rcu_node structures: > [80235.540102][T12441] Task dump for CPU 3: > [80235.540118][T12441] task:core_ctl        state:D stack:    0 pid: 172 > ppid:     2 flags:0x00000008 > [80235.540150][T12441] Call trace: > [80235.540178][T12441]  __switch_to+0x2a8/0x3ac > [80235.540207][T12441]  rcu_state+0x11b0/0x1480 > > > [80299.010105][T12441] rcu: INFO: rcu_preempt detected expedited stalls > on CPUs/tasks: { 3-... } 9179494 jiffies s: 634705 root: 0x8/. > [80299.022623][T12441] rcu: blocking rcu_node structures: > [80299.027924][T12441] Task dump for CPU 3: > [80299.027942][T12441] task:swapper/3       state:R  running task > stack:    0 pid:    0 ppid:     1 flags:0x00000008 > [80299.027993][T12441] Call trace: > [80299.028025][T12441]  __switch_to+0x2a8/0x3ac > [80299.028051][T12441]  0xffffffc010113eb4 > > > As we were not seeing this earlier. > Below is compile tested patch, can we do something like this  ? > > ==========================================><==================================================== > > > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h > index 6453ac5..f0332e4 100644 > --- a/kernel/rcu/tree_exp.h > +++ b/kernel/rcu/tree_exp.h > @@ -812,10 +812,12 @@ static int rcu_print_task_exp_stall(struct > rcu_node *rnp) >   */ >  void synchronize_rcu_expedited(void) >  { > -    bool no_wq; > +    bool no_wq = (rcu_scheduler_active == RCU_SCHEDULER_INIT); > +    bool is_active; >      struct rcu_exp_work rew; >      struct rcu_node *rnp; >      unsigned long s; > +    int next_cpu; > >      RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map) || >               lock_is_held(&rcu_lock_map) || > @@ -837,19 +839,28 @@ void synchronize_rcu_expedited(void) >      if (exp_funnel_lock(s)) >          return;  /* Someone else did our work for us. */ > > -    /* Don't use workqueue during boot or from an incoming CPU. */ > -    preempt_disable(); > -    no_wq = rcu_scheduler_active == RCU_SCHEDULER_INIT || > -        !cpumask_test_cpu(smp_processor_id(), cpu_active_mask); > -    preempt_enable(); > - >      /* Ensure that load happens before action based on it. */ >      if (unlikely(no_wq)) { > -        /* Direct call during scheduler init, early_initcalls() and > incoming CPUs. */ > +        /* Direct call during scheduler init, early_initcalls(). */ >          rcu_exp_sel_wait_wake(s); > +        mutex_unlock(&rcu_state.exp_mutex); > +        return; > +    } > + > +    preempt_disable(); > +    is_active = cpumask_test_cpu(smp_processor_id(), cpu_active_mask); > +    preempt_enable(); > + > +    rew.rew_s = s; > +    if (!is_active) { > +        INIT_WORK(&rew.rew_work, wait_rcu_exp_gp); > +        next_cpu = cpumask_next(smp_processor_id(), cpu_active_mask); > +        if (next_cpu >= nr_cpu_ids) > +            next_cpu = cpumask_first(cpu_active_mask); > + > +        queue_work_on(next_cpu, rcu_gp_wq, &rew.rew_work); >      } else { >          /* Marshall arguments & schedule the expedited grace period. */ > -        rew.rew_s = s; >          INIT_WORK_ONSTACK(&rew.rew_work, wait_rcu_exp_gp); >          queue_work(rcu_gp_wq, &rew.rew_work); >      } > @@ -863,7 +874,9 @@ void synchronize_rcu_expedited(void) >      /* Let the next expedited grace period start. */ >      mutex_unlock(&rcu_state.exp_mutex); > > -    if (likely(!no_wq)) > +    if (likely(is_active)) >          destroy_work_on_stack(&rew.rew_work); > +    else > +        flush_work(&rew.rew_work); >  } >  EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);