b82d4b197c ("blkcg: make request_queue bypassing on allocation") made
request_queues bypassed on allocation to avoid switching on and off
bypass mode on a queue being initialized. Some drivers allocate and
then destroy a lot of queues without fully initializing them and
incurring bypass latency overhead on each of them could add upto
significant overhead.
Unfortunately, blk_init_allocated_queue() is never used by queues of
bio-based drivers, which means that all bio-based driver queues are in
bypass mode even after initialization and registration complete
successfully.
Due to the limited way request_queues are used by bio drivers, this
problem is hidden pretty well but it shows up when blk-throttle is
used in combination with a bio-based driver. Trying to configure
(echoing to cgroupfs file) blk-throttle for a bio-based driver hangs
indefinitely in blkg_conf_prep() waiting for bypass mode to end.
This patch moves the initial blk_queue_bypass_end() call from
blk_init_allocated_queue() to blk_register_queue() which is called for
any userland-visible queues regardless of its type.
I believe this is correct because I don't think there is any block
driver which needs or wants working elevator and blk-cgroup on a queue
which isn't visible to userland. If there are such users, we need a
different solution.
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Reported-by: Joseph Glanville <joseph.glanville@xxxxxxxxxxxxxx>
Cc: Vivek Goyal <vgoyal@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
Jens, while these are fixes, I think it isn't extremely urgent and
routing these through 3.7-rc1 should be enough.
Thanks.
block/blk-core.c | 7 ++-----
block/blk-sysfs.c | 6 ++++++
2 files changed, 8 insertions(+), 5 deletions(-)
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -608,8 +608,8 @@ struct request_queue *blk_alloc_queue_no
/*
* A queue starts its life with bypass turned on to avoid
* unnecessary bypass on/off overhead and nasty surprises during
- * init. The initial bypass will be finished at the end of
- * blk_init_allocated_queue().
+ * init. The initial bypass will be finished when the queue is
+ * registered by blk_register_queue().
*/
q->bypass_depth = 1;
__set_bit(QUEUE_FLAG_BYPASS, &q->queue_flags);
@@ -714,9 +714,6 @@ blk_init_allocated_queue(struct request_
return NULL;
blk_queue_congestion_threshold(q);
-
- /* all done, end the initial bypass */
- blk_queue_bypass_end(q);
return q;
}
EXPORT_SYMBOL(blk_init_allocated_queue);
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -527,6 +527,12 @@ int blk_register_queue(struct gendisk *d
if (WARN_ON(!q))
return -ENXIO;
+ /*
+ * Initialization must be complete by now. Finish the initial
+ * bypass from queue allocation.
+ */
+ blk_queue_bypass_end(q);
+
ret = blk_trace_init_sysfs(dev);
if (ret)
return ret;
--
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