Date:   Tue, 21 Nov 2017 21:12:20 +0100
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Jens Axboe <axboe@...nel.dk>,
        Bart Van Assche <Bart.VanAssche@....com>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "mst@...hat.com" <mst@...hat.com>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@....de>
Subject: Re: 4.14: WARNING: CPU: 4 PID: 2895 at block/blk-mq.c:1144 with
 virtio-blk (also 4.12 stable)



On 11/21/2017 08:30 PM, Jens Axboe wrote:
> On 11/21/2017 12:15 PM, Christian Borntraeger wrote:
>>
>>
>> On 11/21/2017 07:39 PM, Jens Axboe wrote:
>>> On 11/21/2017 11:27 AM, Jens Axboe wrote:
>>>> On 11/21/2017 11:12 AM, Christian Borntraeger wrote:
>>>>>
>>>>>
>>>>> On 11/21/2017 07:09 PM, Jens Axboe wrote:
>>>>>> On 11/21/2017 10:27 AM, Jens Axboe wrote:
>>>>>>> On 11/21/2017 03:14 AM, Christian Borntraeger wrote:
>>>>>>>> Bisect points to
>>>>>>>>
>>>>>>>> 1b5a7455d345b223d3a4658a9e5fce985b7998c1 is the first bad commit
>>>>>>>> commit 1b5a7455d345b223d3a4658a9e5fce985b7998c1
>>>>>>>> Author: Christoph Hellwig <hch@....de>
>>>>>>>> Date:   Mon Jun 26 12:20:57 2017 +0200
>>>>>>>>
>>>>>>>>     blk-mq: Create hctx for each present CPU
>>>>>>>>     
>>>>>>>>     commit 4b855ad37194f7bdbb200ce7a1c7051fecb56a08 upstream.
>>>>>>>>     
>>>>>>>>     Currently we only create hctx for online CPUs, which can lead to a lot
>>>>>>>>     of churn due to frequent soft offline / online operations.  Instead
>>>>>>>>     allocate one for each present CPU to avoid this and dramatically simplify
>>>>>>>>     the code.
>>>>>>>>     
>>>>>>>>     Signed-off-by: Christoph Hellwig <hch@....de>
>>>>>>>>     Reviewed-by: Jens Axboe <axboe@...nel.dk>
>>>>>>>>     Cc: Keith Busch <keith.busch@...el.com>
>>>>>>>>     Cc: linux-block@...r.kernel.org
>>>>>>>>     Cc: linux-nvme@...ts.infradead.org
>>>>>>>>     Link: http://lkml.kernel.org/r/20170626102058.10200-3-hch@lst.de
>>>>>>>>     Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
>>>>>>>>     Cc: Oleksandr Natalenko <oleksandr@...alenko.name>
>>>>>>>>     Cc: Mike Galbraith <efault@....de>
>>>>>>>>     Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>>>>>>>
>>>>>>> I wonder if we're simply not getting the masks updated correctly. I'll
>>>>>>> take a look.
>>>>>>
>>>>>> Can't make it trigger here. We do init for each present CPU, which means
>>>>>> that if I offline a few CPUs here and register a queue, those still show
>>>>>> up as present (just offline) and get mapped accordingly.
>>>>>>
>>>>>> From the looks of it, your setup is different. If the CPU doesn't show
>>>>>> up as present and it gets hotplugged, then I can see how this condition
>>>>>> would trigger. What environment are you running this in? We might have
>>>>>> to re-introduce the cpu hotplug notifier, right now we just monitor
>>>>>> for a dead cpu and handle that.
>>>>>
>>>>> I am not doing a hot unplug and the replug, I use KVM and add a previously
>>>>> not available CPU.
>>>>>
>>>>> in libvirt/virsh speak:
>>>>>   <vcpu placement='static' current='1'>4</vcpu>
>>>>
>>>> So that's why we run into problems. It's not present when we load the device,
>>>> but becomes present and online afterwards.
>>>>
>>>> Christoph, we used to handle this just fine, your patch broke it.
>>>>
>>>> I'll see if I can come up with an appropriate fix.
>>>
>>> Can you try the below?
>>
>>
>> It does prevent the crash but it seems that the new CPU is not "used " after the hotplug for mq:
>>
>>
>> output with 2 cpus:
>> /sys/kernel/debug/block/vda
>> /sys/kernel/debug/block/vda/hctx0
>> /sys/kernel/debug/block/vda/hctx0/cpu0
>> /sys/kernel/debug/block/vda/hctx0/cpu0/completed
>> /sys/kernel/debug/block/vda/hctx0/cpu0/merged
>> /sys/kernel/debug/block/vda/hctx0/cpu0/dispatched
>> /sys/kernel/debug/block/vda/hctx0/cpu0/rq_list
>> /sys/kernel/debug/block/vda/hctx0/active
>> /sys/kernel/debug/block/vda/hctx0/run
>> /sys/kernel/debug/block/vda/hctx0/queued
>> /sys/kernel/debug/block/vda/hctx0/dispatched
>> /sys/kernel/debug/block/vda/hctx0/io_poll
>> /sys/kernel/debug/block/vda/hctx0/sched_tags_bitmap
>> /sys/kernel/debug/block/vda/hctx0/sched_tags
>> /sys/kernel/debug/block/vda/hctx0/tags_bitmap
>> /sys/kernel/debug/block/vda/hctx0/tags
>> /sys/kernel/debug/block/vda/hctx0/ctx_map
>> /sys/kernel/debug/block/vda/hctx0/busy
>> /sys/kernel/debug/block/vda/hctx0/dispatch
>> /sys/kernel/debug/block/vda/hctx0/flags
>> /sys/kernel/debug/block/vda/hctx0/state
>> /sys/kernel/debug/block/vda/sched
>> /sys/kernel/debug/block/vda/sched/dispatch
>> /sys/kernel/debug/block/vda/sched/starved
>> /sys/kernel/debug/block/vda/sched/batching
>> /sys/kernel/debug/block/vda/sched/write_next_rq
>> /sys/kernel/debug/block/vda/sched/write_fifo_list
>> /sys/kernel/debug/block/vda/sched/read_next_rq
>> /sys/kernel/debug/block/vda/sched/read_fifo_list
>> /sys/kernel/debug/block/vda/write_hints
>> /sys/kernel/debug/block/vda/state
>> /sys/kernel/debug/block/vda/requeue_list
>> /sys/kernel/debug/block/vda/poll_stat
> 
> Try this, basically just a revert.

Yes, seems to work.

Tested-by: Christian Borntraeger <borntraeger@...ibm.com>

Do you know why the original commit made it into 4.12 stable? After all
it has no Fixes tag and no cc stable-


> 
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 11097477eeab..bc1950fa9ef6 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -37,6 +37,9 @@
>  #include "blk-wbt.h"
>  #include "blk-mq-sched.h"
> 
> +static DEFINE_MUTEX(all_q_mutex);
> +static LIST_HEAD(all_q_list);
> +
>  static bool blk_mq_poll(struct request_queue *q, blk_qc_t cookie);
>  static void blk_mq_poll_stats_start(struct request_queue *q);
>  static void blk_mq_poll_stats_fn(struct blk_stat_callback *cb);
> @@ -2114,8 +2117,8 @@ static void blk_mq_init_cpu_queues(struct request_queue *q,
>  		INIT_LIST_HEAD(&__ctx->rq_list);
>  		__ctx->queue = q;
> 
> -		/* If the cpu isn't present, the cpu is mapped to first hctx */
> -		if (!cpu_present(i))
> +		/* If the cpu isn't online, the cpu is mapped to first hctx */
> +		if (!cpu_online(i))
>  			continue;
> 
>  		hctx = blk_mq_map_queue(q, i);
> @@ -2158,7 +2161,8 @@ static void blk_mq_free_map_and_requests(struct blk_mq_tag_set *set,
>  	}
>  }
> 
> -static void blk_mq_map_swqueue(struct request_queue *q)
> +static void blk_mq_map_swqueue(struct request_queue *q,
> +			       const struct cpumask *online_mask)
>  {
>  	unsigned int i, hctx_idx;
>  	struct blk_mq_hw_ctx *hctx;
> @@ -2176,11 +2180,13 @@ static void blk_mq_map_swqueue(struct request_queue *q)
>  	}
> 
>  	/*
> -	 * Map software to hardware queues.
> -	 *
> -	 * If the cpu isn't present, the cpu is mapped to first hctx.
> +	 * Map software to hardware queues
>  	 */
> -	for_each_present_cpu(i) {
> +	for_each_possible_cpu(i) {
> +		/* If the cpu isn't online, the cpu is mapped to first hctx */
> +		if (!cpumask_test_cpu(i, online_mask))
> +			continue;
> +
>  		hctx_idx = q->mq_map[i];
>  		/* unmapped hw queue can be remapped after CPU topo changed */
>  		if (!set->tags[hctx_idx] &&
> @@ -2495,8 +2501,16 @@ struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
>  		blk_queue_softirq_done(q, set->ops->complete);
> 
>  	blk_mq_init_cpu_queues(q, set->nr_hw_queues);
> +
> +	get_online_cpus();
> +	mutex_lock(&all_q_mutex);
> +
> +	list_add_tail(&q->all_q_node, &all_q_list);
>  	blk_mq_add_queue_tag_set(set, q);
> -	blk_mq_map_swqueue(q);
> +	blk_mq_map_swqueue(q, cpu_online_mask);
> +
> +	mutex_unlock(&all_q_mutex);
> +	put_online_cpus();
> 
>  	if (!(set->flags & BLK_MQ_F_NO_SCHED)) {
>  		int ret;
> @@ -2522,12 +2536,18 @@ void blk_mq_free_queue(struct request_queue *q)
>  {
>  	struct blk_mq_tag_set	*set = q->tag_set;
> 
> +	mutex_lock(&all_q_mutex);
> +	list_del_init(&q->all_q_node);
> +	mutex_unlock(&all_q_mutex);
> +
>  	blk_mq_del_queue_tag_set(q);
> +
>  	blk_mq_exit_hw_queues(q, set, set->nr_hw_queues);
>  }
> 
>  /* Basically redo blk_mq_init_queue with queue frozen */
> -static void blk_mq_queue_reinit(struct request_queue *q)
> +static void blk_mq_queue_reinit(struct request_queue *q,
> +				const struct cpumask *online_mask)
>  {
>  	WARN_ON_ONCE(!atomic_read(&q->mq_freeze_depth));
> 
> @@ -2539,12 +2559,76 @@ static void blk_mq_queue_reinit(struct request_queue *q)
>  	 * we should change hctx numa_node according to the new topology (this
>  	 * involves freeing and re-allocating memory, worth doing?)
>  	 */
> -	blk_mq_map_swqueue(q);
> +	blk_mq_map_swqueue(q, online_mask);
> 
>  	blk_mq_sysfs_register(q);
>  	blk_mq_debugfs_register_hctxs(q);
>  }
> 
> +/*
> + * New online cpumask which is going to be set in this hotplug event.
> + * Declare this cpumasks as global as cpu-hotplug operation is invoked
> + * one-by-one and dynamically allocating this could result in a failure.
> + */
> +static struct cpumask cpuhp_online_new;
> +
> +static void blk_mq_queue_reinit_work(void)
> +{
> +	struct request_queue *q;
> +
> +	mutex_lock(&all_q_mutex);
> +	/*
> +	 * We need to freeze and reinit all existing queues.  Freezing
> +	 * involves synchronous wait for an RCU grace period and doing it
> +	 * one by one may take a long time.  Start freezing all queues in
> +	 * one swoop and then wait for the completions so that freezing can
> +	 * take place in parallel.
> +	 */
> +	list_for_each_entry(q, &all_q_list, all_q_node)
> +		blk_freeze_queue_start(q);
> +	list_for_each_entry(q, &all_q_list, all_q_node)
> +		blk_mq_freeze_queue_wait(q);
> +
> +	list_for_each_entry(q, &all_q_list, all_q_node)
> +		blk_mq_queue_reinit(q, &cpuhp_online_new);
> +
> +	list_for_each_entry(q, &all_q_list, all_q_node)
> +		blk_mq_unfreeze_queue(q);
> +
> +	mutex_unlock(&all_q_mutex);
> +}
> +
> +static int blk_mq_queue_reinit_dead(unsigned int cpu)
> +{
> +	cpumask_copy(&cpuhp_online_new, cpu_online_mask);
> +	blk_mq_queue_reinit_work();
> +	return 0;
> +}
> +
> +/*
> + * Before hotadded cpu starts handling requests, new mappings must be
> + * established.  Otherwise, these requests in hw queue might never be
> + * dispatched.
> + *
> + * For example, there is a single hw queue (hctx) and two CPU queues (ctx0
> + * for CPU0, and ctx1 for CPU1).
> + *
> + * Now CPU1 is just onlined and a request is inserted into ctx1->rq_list
> + * and set bit0 in pending bitmap as ctx1->index_hw is still zero.
> + *
> + * And then while running hw queue, blk_mq_flush_busy_ctxs() finds bit0 is set
> + * in pending bitmap and tries to retrieve requests in hctx->ctxs[0]->rq_list.
> + * But htx->ctxs[0] is a pointer to ctx0, so the request in ctx1->rq_list is
> + * ignored.
> + */
> +static int blk_mq_queue_reinit_prepare(unsigned int cpu)
> +{
> +	cpumask_copy(&cpuhp_online_new, cpu_online_mask);
> +	cpumask_set_cpu(cpu, &cpuhp_online_new);
> +	blk_mq_queue_reinit_work();
> +	return 0;
> +}
> +
>  static int __blk_mq_alloc_rq_maps(struct blk_mq_tag_set *set)
>  {
>  	int i;
> @@ -2757,7 +2841,7 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set,
>  	blk_mq_update_queue_map(set);
>  	list_for_each_entry(q, &set->tag_list, tag_set_list) {
>  		blk_mq_realloc_hw_ctxs(set, q);
> -		blk_mq_queue_reinit(q);
> +		blk_mq_queue_reinit(q, cpu_online_mask);
>  	}
> 
>  	list_for_each_entry(q, &set->tag_list, tag_set_list)
> @@ -2966,6 +3050,16 @@ static bool blk_mq_poll(struct request_queue *q, blk_qc_t cookie)
>  	return __blk_mq_poll(hctx, rq);
>  }
> 
> +void blk_mq_disable_hotplug(void)
> +{
> +	mutex_lock(&all_q_mutex);
> +}
> +
> +void blk_mq_enable_hotplug(void)
> +{
> +	mutex_unlock(&all_q_mutex);
> +}
> +
>  static int __init blk_mq_init(void)
>  {
>  	/*
> @@ -2976,6 +3070,10 @@ static int __init blk_mq_init(void)
> 
>  	cpuhp_setup_state_multi(CPUHP_BLK_MQ_DEAD, "block/mq:dead", NULL,
>  				blk_mq_hctx_notify_dead);
> +
> +	cpuhp_setup_state_nocalls(CPUHP_BLK_MQ_PREPARE, "block/mq:prepare",
> +				  blk_mq_queue_reinit_prepare,
> +				  blk_mq_queue_reinit_dead);
>  	return 0;
>  }
>  subsys_initcall(blk_mq_init);
> diff --git a/block/blk-mq.h b/block/blk-mq.h
> index 6c7c3ff5bf62..83b13ef1915e 100644
> --- a/block/blk-mq.h
> +++ b/block/blk-mq.h
> @@ -59,6 +59,11 @@ void __blk_mq_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
>  void blk_mq_request_bypass_insert(struct request *rq, bool run_queue);
>  void blk_mq_insert_requests(struct blk_mq_hw_ctx *hctx, struct blk_mq_ctx *ctx,
>  				struct list_head *list);
> +/*
> + * CPU hotplug helpers
> + */
> +void blk_mq_enable_hotplug(void);
> +void blk_mq_disable_hotplug(void);
> 
>  /*
>   * CPU -> queue mappings
> diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
> index 201ab7267986..c31d4e3bf6d0 100644
> --- a/include/linux/cpuhotplug.h
> +++ b/include/linux/cpuhotplug.h
> @@ -76,6 +76,7 @@ enum cpuhp_state {
>  	CPUHP_XEN_EVTCHN_PREPARE,
>  	CPUHP_ARM_SHMOBILE_SCU_PREPARE,
>  	CPUHP_SH_SH3X_PREPARE,
> +	CPUHP_BLK_MQ_PREPARE,
>  	CPUHP_NET_FLOW_PREPARE,
>  	CPUHP_TOPOLOGY_PREPARE,
>  	CPUHP_NET_IUCV_PREPARE,
> 

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