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authorTejun Heo <tj@kernel.org>2014-09-24 13:31:50 -0400
committerTejun Heo <tj@kernel.org>2014-09-24 13:37:21 -0400
commit17497acbdce9506fd6a75115dee4ab80c3cc5ee5 (patch)
treefc56c3784250dd79761f34c30e1a2218974eaf76 /block/blk-sysfs.c
parent1cae13e75b7a7848c03138636d4eb8d8a5054dd5 (diff)
blk-mq, percpu_ref: start q->mq_usage_counter in atomic mode
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 above ten seconds when scsi-mq is used. [ 0.949892] scsi host0: Virtio SCSI HBA [ 1.007864] scsi 0:0:0:0: Direct-Access QEMU QEMU HARDDISK 1.1. PQ: 0 ANSI: 5 [ 1.021299] scsi 0:0:1:0: Direct-Access QEMU QEMU HARDDISK 1.1. PQ: 0 ANSI: 5 [ 1.520356] tsc: Refined TSC clocksource calibration: 2491.910 MHz <stall> [ 16.186549] sd 0:0:0:0: Attached scsi generic sg0 type 0 [ 16.190478] sd 0:0:1:0: Attached scsi generic sg1 type 0 [ 16.194099] osd: LOADED open-osd 0.2.1 [ 16.203202] sd 0:0:0:0: [sda] 31457280 512-byte logical blocks: (16.1 GB/15.0 GiB) [ 16.208478] sd 0:0:0:0: [sda] Write Protect is off [ 16.211439] sd 0:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA [ 16.218771] sd 0:0:1:0: [sdb] 31457280 512-byte logical blocks: (16.1 GB/15.0 GiB) [ 16.223264] sd 0:0:1:0: [sdb] Write Protect is off [ 16.225682] sd 0:0:1:0: [sdb] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA This is also the reason why request_queues start in bypass mode which is ended on blk_register_queue() as shutting down a fully functional queue also involves a RCU grace period and the queues for non-existent SCSI devices never reach registration. blk-mq basically needs to do the same thing - start the mq in a degraded mode which is faster to shut down and then make it fully functional only after the queue reaches registration. percpu_ref recently grew facilities to force atomic operation until explicitly switched to percpu mode, which can be used for this purpose. This patch makes blk-mq initialize q->mq_usage_counter in atomic mode and switch it to percpu mode only once blk_register_queue() is reached. Note that this issue was previously worked around by 0a30288da1ae ("blk-mq, percpu_ref: implement a kludge for SCSI blk-mq stall during probe") for v3.17. The temp fix was reverted in preparation of adding persistent atomic mode to percpu_ref by 9eca80461a45 ("Revert "blk-mq, percpu_ref: implement a kludge for SCSI blk-mq stall during probe""). This patch and the prerequisite percpu_ref changes will be merged during v3.18 devel cycle. 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") Reviewed-by: Kent Overstreet <kmo@daterainc.com> Cc: Jens Axboe <axboe@kernel.dk> Cc: Johannes Weiner <hannes@cmpxchg.org>
Diffstat (limited to 'block/blk-sysfs.c')
-rw-r--r--block/blk-sysfs.c11
1 files changed, 9 insertions, 2 deletions
diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 17f5c84ce7bf..521ae9089c50 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -551,12 +551,19 @@ int blk_register_queue(struct gendisk *disk)
return -ENXIO;
/*
- * Initialization must be complete by now. Finish the initial
- * bypass from queue allocation.
+ * SCSI probing may synchronously create and destroy a lot of
+ * request_queues for non-existent devices. Shutting down a fully
+ * functional queue takes measureable wallclock time as RCU grace
+ * periods are involved. To avoid excessive latency in these
+ * cases, a request_queue starts out in a degraded mode which is
+ * faster to shut down and is made fully functional here as
+ * request_queues for non-existent devices never get registered.
*/
if (!blk_queue_init_done(q)) {
queue_flag_set_unlocked(QUEUE_FLAG_INIT_DONE, q);
blk_queue_bypass_end(q);
+ if (q->mq_ops)
+ blk_mq_finish_init(q);
}
ret = blk_trace_init_sysfs(dev);