On 10/25/12 18:41, Jun'ichi Nomura wrote:
> With 749fefe677 ("block: lift the initial queue bypass mode on
> blk_register_queue() instead of blk_init_allocated_queue()"),
> add_disk() eventually calls blk_queue_bypass_end().
> This change invokes the following warning when multipath is used.
...
> The warning means during queue initialization blk_queue_bypass_start()
> calls sleeping function (synchronize_rcu) while dm holds md->type_lock.
>
> dm device initialization basically includes the following 3 steps:
> 1. create ioctl, allocates queue and call add_disk()
> 2. table load ioctl, determines device type and initialize queue
> if request-based
> 3. resume ioctl, device becomes functional
>
> So it is better to have dm's queue stay in bypass mode until
> the initialization completes in table load ioctl.
>
> The effect of additional blk_queue_bypass_start():
>
> 3.7-rc2 (plain)
> # time for n in $(seq 1000); do dmsetup create --noudevsync --notable a; \
> dmsetup remove a; done
>
> real 0m15.434s
> user 0m0.423s
> sys 0m7.052s
>
> 3.7-rc2 (with this patch)
> # time for n in $(seq 1000); do dmsetup create --noudevsync --notable a; \
> dmsetup remove a; done
> real 0m19.766s
> user 0m0.442s
> sys 0m6.861s
>
> If this additional cost is not negligible, we need a variant of add_disk()
> that does not end bypassing.
Or call blk_queue_bypass_start() before add_disk():
diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index ad02761..d14639b 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -1868,9 +1868,9 @@ static struct mapped_device *alloc_dev(int minor)
md->disk->queue = md->queue;
md->disk->private_data = md;
sprintf(md->disk->disk_name, "dm-%d", minor);
- add_disk(md->disk);
/* Until md type is determined, put the queue in bypass mode */
blk_queue_bypass_start(md->queue);
+ add_disk(md->disk);
format_dev_t(md->name, MKDEV(_major, minor));
md->wq = alloc_workqueue("kdmflush",
---
If the patch is modified like above, we could fix the issue
without incurring additional cost on dm device creation.
# time for n in $(seq 1000); do dmsetup create --noudevsync --notable a; \
dmsetup remove a; done
real 0m15.684s
user 0m0.404s
sys 0m7.181s
--
Jun'ichi Nomura, NEC Corporation
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