diff options
| author | Tejun Heo <tj@kernel.org> | 2014-09-23 15:24:32 -0400 |
|---|---|---|
| committer | Jens Axboe <axboe@fb.com> | 2014-09-24 08:29:36 -0600 |
| commit | 0a30288da1aec914e158c2d7a3482a85f632750f (patch) | |
| tree | 748c41c772a551dd7c94d46770aadd8a06ec8013 /block | |
| parent | 452b6361c4d9baf6940adb7b1316e0f386c39799 (diff) | |
blk-mq, percpu_ref: implement a kludge for SCSI blk-mq stall during probe
blk-mq uses percpu_ref for its usage counter which tracks the number
of in-flight commands and used to synchronously drain the queue on
freeze. percpu_ref shutdown takes measureable wallclock time as it
involves a sched RCU grace period. This means that draining a blk-mq
takes measureable wallclock time. One would think that this shouldn't
matter as queue shutdown should be a rare event which takes place
asynchronously w.r.t. userland.
Unfortunately, SCSI probing involves synchronously setting up and then
tearing down a lot of request_queues back-to-back for non-existent
LUNs. This means that SCSI probing may take more than ten seconds
when scsi-mq is used.
This will be properly fixed by implementing a mechanism to keep
q->mq_usage_counter in atomic mode till genhd registration; however,
that involves rather big updates to percpu_ref which is difficult to
apply late in the devel cycle (v3.17-rc6 at the moment). As a
stop-gap measure till the proper fix can be implemented in the next
cycle, this patch introduces __percpu_ref_kill_expedited() and makes
blk_mq_freeze_queue() use it. This is heavy-handed but should work
for testing the experimental SCSI blk-mq implementation.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Christoph Hellwig <hch@infradead.org>
Link: http://lkml.kernel.org/g/20140919113815.GA10791@lst.de
Fixes: add703fda981 ("blk-mq: use percpu_ref for mq usage count")
Cc: Kent Overstreet <kmo@daterainc.com>
Cc: Jens Axboe <axboe@kernel.dk>
Tested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>
Diffstat (limited to 'block')
| -rw-r--r-- | block/blk-mq.c | 11 |
1 files changed, 10 insertions, 1 deletions
diff --git a/block/blk-mq.c b/block/blk-mq.c index c88e6089746d..df8e1e09dd17 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -119,7 +119,16 @@ void blk_mq_freeze_queue(struct request_queue *q) spin_unlock_irq(q->queue_lock); if (freeze) { - percpu_ref_kill(&q->mq_usage_counter); + /* + * XXX: Temporary kludge to work around SCSI blk-mq stall. + * SCSI synchronously creates and destroys many queues + * back-to-back during probe leading to lengthy stalls. + * This will be fixed by keeping ->mq_usage_counter in + * atomic mode until genhd registration, but, for now, + * let's work around using expedited synchronization. + */ + __percpu_ref_kill_expedited(&q->mq_usage_counter); + blk_mq_run_queues(q, false); } wait_event(q->mq_freeze_wq, percpu_ref_is_zero(&q->mq_usage_counter)); |