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SubjectRe: 4.14: WARNING: CPU: 4 PID: 2895 at block/blk-mq.c:1144 with virtio-blk (also 4.12 stable)
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Date
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@lst.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@lst.de>
>>>>>>> Reviewed-by: Jens Axboe <axboe@kernel.dk>
>>>>>>> Cc: Keith Busch <keith.busch@intel.com>
>>>>>>> Cc: linux-block@vger.kernel.org
>>>>>>> Cc: linux-nvme@lists.infradead.org
>>>>>>> Link: http://lkml.kernel.org/r/20170626102058.10200-3-hch@lst.de
>>>>>>> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
>>>>>>> Cc: Oleksandr Natalenko <oleksandr@natalenko.name>
>>>>>>> Cc: Mike Galbraith <efault@gmx.de>
>>>>>>> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.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.


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,
--
Jens Axboe

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