From 0a30288da1aec914e158c2d7a3482a85f632750f Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Tue, 23 Sep 2014 15:24:32 -0400 Subject: [PATCH] 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 Reported-by: Christoph Hellwig Link: http://lkml.kernel.org/g/20140919113815.GA10791@lst.de Fixes: add703fda981 ("blk-mq: use percpu_ref for mq usage count") Cc: Kent Overstreet Cc: Jens Axboe Tested-by: Christoph Hellwig Signed-off-by: Jens Axboe --- block/blk-mq.c | 11 ++++++++++- include/linux/percpu-refcount.h | 1 + lib/percpu-refcount.c | 16 ++++++++++++++++ 3 files changed, 27 insertions(+), 1 deletion(-) 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)); diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h index 3dfbf237cd8f..ef5894ca8e50 100644 --- a/include/linux/percpu-refcount.h +++ b/include/linux/percpu-refcount.h @@ -71,6 +71,7 @@ void percpu_ref_reinit(struct percpu_ref *ref); void percpu_ref_exit(struct percpu_ref *ref); void percpu_ref_kill_and_confirm(struct percpu_ref *ref, percpu_ref_func_t *confirm_kill); +void __percpu_ref_kill_expedited(struct percpu_ref *ref); /** * percpu_ref_kill - drop the initial ref diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c index fe5a3342e960..a89cf09a8268 100644 --- a/lib/percpu-refcount.c +++ b/lib/percpu-refcount.c @@ -184,3 +184,19 @@ void percpu_ref_kill_and_confirm(struct percpu_ref *ref, call_rcu_sched(&ref->rcu, percpu_ref_kill_rcu); } EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm); + +/* + * XXX: Temporary kludge to work around SCSI blk-mq stall. Used only by + * block/blk-mq.c::blk_mq_freeze_queue(). Will be removed during v3.18 + * devel cycle. Do not use anywhere else. + */ +void __percpu_ref_kill_expedited(struct percpu_ref *ref) +{ + WARN_ONCE(ref->pcpu_count_ptr & PCPU_REF_DEAD, + "percpu_ref_kill() called more than once on %pf!", + ref->release); + + ref->pcpu_count_ptr |= PCPU_REF_DEAD; + synchronize_sched_expedited(); + percpu_ref_kill_rcu(&ref->rcu); +} -- 2.30.2