• Tejun Heo's avatar
    blk-mq, percpu_ref: implement a kludge for SCSI blk-mq stall during probe · 0a30288d
    Tejun Heo authored
    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: default avatarTejun Heo <tj@kernel.org>
    Reported-by: default avatarChristoph Hellwig <hch@infradead.org>
    Link: http://lkml.kernel.org/g/20140919113815.GA10791@lst.de
    Fixes: add703fd ("blk-mq: use percpu_ref for mq usage count")
    Cc: Kent Overstreet <kmo@daterainc.com>
    Cc: Jens Axboe <axboe@kernel.dk>
    Tested-by: default avatarChristoph Hellwig <hch@lst.de>
    Signed-off-by: default avatarJens Axboe <axboe@fb.com>
    0a30288d
percpu-refcount.c 6.76 KB