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 0D09BC433F5 for ; Sat, 12 Feb 2022 13:56:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235768AbiBLN4V (ORCPT ); Sat, 12 Feb 2022 08:56:21 -0500 Received: from mxb-00190b01.gslb.pphosted.com ([23.128.96.19]:55184 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230348AbiBLN4U (ORCPT ); Sat, 12 Feb 2022 08:56:20 -0500 Received: from alexa-out.qualcomm.com (alexa-out.qualcomm.com [129.46.98.28]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id BEC00216; Sat, 12 Feb 2022 05:56:16 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=quicinc.com; i=@quicinc.com; q=dns/txt; s=qcdkim; t=1644674177; x=1676210177; h=message-id:date:mime-version:subject:to:cc:references: from:in-reply-to:content-transfer-encoding; bh=EFXycOyHfDpnIEEuHp1bsP2udHkXPnsibsZZiH/Di6o=; b=RUSNUqs9hrHZC2GoLoUMWMCOMzsGolqb19hOPe8CI+ORZlx1OqKm5WBP V86VYJSXgnZk2WMDV+aLQ3b8NRGJ+J1p1UbetNQua9lEu/pYX9XucjuQy HDlWtRZaqPpKWNw4DCwsoyozclug80uMRv48Nmz4mcdAIYLaoL5fy8S57 I=; Received: from ironmsg09-lv.qualcomm.com ([10.47.202.153]) by alexa-out.qualcomm.com with ESMTP; 12 Feb 2022 05:56:16 -0800 X-QCInternal: smtphost Received: from nasanex01c.na.qualcomm.com ([10.47.97.222]) by ironmsg09-lv.qualcomm.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 12 Feb 2022 05:56:16 -0800 Received: from [10.216.46.141] (10.80.80.8) by nasanex01c.na.qualcomm.com (10.47.97.222) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.986.15; Sat, 12 Feb 2022 05:56:12 -0800 Message-ID: Date: Sat, 12 Feb 2022 19:26:08 +0530 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Thunderbird/91.5.0 Subject: Re: [PATCH rcu 3/3] rcu: Allow expedited RCU grace periods on incoming CPUs Content-Language: en-US To: Neeraj Upadhyay , 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> <73bc88ae-a1a4-99e2-0e66-e2ce553b2f46@quicinc.com> From: Mukesh Ojha In-Reply-To: <73bc88ae-a1a4-99e2-0e66-e2ce553b2f46@quicinc.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-Originating-IP: [10.80.80.8] X-ClientProxiedBy: nasanex01b.na.qualcomm.com (10.46.141.250) To nasanex01c.na.qualcomm.com (10.47.97.222) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2/12/2022 4:58 PM, Neeraj Upadhyay wrote: > 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. It is not there, will check the results with this patch. -Mukesh > > > 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);