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SubjectRe: panic with CPU hotplug + blk-mq + scsi-mq
On 21.04.2015 00:48, Ming Lei wrote:
> Thanks for providing that.
> The trick is just in CPU number and virito-scsi hw queue number,
> and that is why I asked that, :-)
> Now the problem is quite clear, before CPU1 online, suppose
> CPU3 is mapped hw queue 6, and CPU 3 will map to hw queue 5
> after CPU1 is offline, unfortunately current code can't allocate
> tags for hw queue 5 even it becomes mapped.
> The following updated patch(include original patch 2) will fix
> the problem, and patch 1 is required too.
> So the following patch should fix your hotplug issue.

Yes, it works indeed. Thanks a lot! :-)
You can add:

Tested-by: Dongsu Park <dongsu.park@profitbricks.com>

As the original patch didn't apply, I had to change some nitpicks though.
(see below)

Cheers,
Dongsu

----

From 8c0edcbbdfbab67dc8ae2fd46cca6a86e0cadcba Mon Sep 17 00:00:00 2001
From: Ming Lei <ming.lei@canonical.com>
Date: Sun, 19 Apr 2015 23:32:46 +0800
Subject: [PATCH v1 2/2] blk-mq: fix CPU hotplug handling

Firstly the hctx->tags have to be set as NULL if it is to be disabled
no matter if set->tags[i] is NULL or not in blk_mq_map_swqueue() because
shared tags can be freed already from another request queue.

The same situation has to be considered in blk_mq_hctx_cpu_online() too.

Finally one unmapped hw queue can be remapped after CPU topo is changed,
we need to allocate tags for the hw queue in blk_mq_map_swqueue() too.
Then tags allocation for hw queue can be removed in hctx cpu online
notifier, and it is reasonable to do that after remapping is done.

Cc: <stable@vger.kernel.org>
Reported-by: Dongsu Park <dongsu.park@profitbricks.com>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
---
block/blk-mq.c | 34 +++++++++++++---------------------
1 file changed, 13 insertions(+), 21 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 078840ce8670..df4b9597e477 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1573,22 +1573,6 @@ static int blk_mq_hctx_cpu_offline(struct blk_mq_hw_ctx *hctx, int cpu)
return NOTIFY_OK;
}

-static int blk_mq_hctx_cpu_online(struct blk_mq_hw_ctx *hctx, int cpu)
-{
- struct request_queue *q = hctx->queue;
- struct blk_mq_tag_set *set = q->tag_set;
-
- if (set->tags[hctx->queue_num])
- return NOTIFY_OK;
-
- set->tags[hctx->queue_num] = blk_mq_init_rq_map(set, hctx->queue_num);
- if (!set->tags[hctx->queue_num])
- return NOTIFY_STOP;
-
- hctx->tags = set->tags[hctx->queue_num];
- return NOTIFY_OK;
-}
-
static int blk_mq_hctx_notify(void *data, unsigned long action,
unsigned int cpu)
{
@@ -1596,8 +1580,11 @@ static int blk_mq_hctx_notify(void *data, unsigned long action,

if (action == CPU_DEAD || action == CPU_DEAD_FROZEN)
return blk_mq_hctx_cpu_offline(hctx, cpu);
- else if (action == CPU_ONLINE || action == CPU_ONLINE_FROZEN)
- return blk_mq_hctx_cpu_online(hctx, cpu);
+
+ /*
+ * In case of CPU online, tags will be reallocated
+ * after new mapping is done in blk_mq_map_swqueue().
+ */

return NOTIFY_OK;
}
@@ -1779,6 +1766,7 @@ static void blk_mq_map_swqueue(struct request_queue *q)
unsigned int i;
struct blk_mq_hw_ctx *hctx;
struct blk_mq_ctx *ctx;
+ struct blk_mq_tag_set *set = q->tag_set;

queue_for_each_hw_ctx(q, hctx, i) {
cpumask_clear(hctx->cpumask);
@@ -1805,16 +1793,20 @@ static void blk_mq_map_swqueue(struct request_queue *q)
* disable it and free the request entries.
*/
if (!hctx->nr_ctx) {
- struct blk_mq_tag_set *set = q->tag_set;
-
if (set->tags[i]) {
blk_mq_free_rq_map(set, set->tags[i], i);
set->tags[i] = NULL;
- hctx->tags = NULL;
}
+ hctx->tags = NULL;
continue;
}

+ /* unmapped hw queue can be remapped after CPU topo changed */
+ if (!set->tags[i])
+ set->tags[i] = blk_mq_init_rq_map(set, hctx->queue_num);
+ hctx->tags = set->tags[i];
+ WARN_ON(!hctx->tags);
+
/*
* Initialize batch roundrobin counts
*/
--
2.1.0


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