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authorTejun Heo <tj@kernel.org>2014-09-23 15:24:32 -0400
committerJens Axboe <axboe@fb.com>2014-09-24 08:29:36 -0600
commit0a30288da1aec914e158c2d7a3482a85f632750f (patch)
tree748c41c772a551dd7c94d46770aadd8a06ec8013 /fs/ext3/super.c
parentMerge tag 'rdma-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/roland/infiniband (diff)
downloadlinux-dev-0a30288da1aec914e158c2d7a3482a85f632750f.tar.xz
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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>
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