• Ming Lei's avatar
    blk-mq: remove synchronize_rcu() from blk_mq_del_queue_tag_set() · 97889f9a
    Ming Lei authored
    We have to remove synchronize_rcu() from blk_queue_cleanup(),
    otherwise long delay can be caused during lun probe. For removing
    it, we have to avoid to iterate the set->tag_list in IO path, eg,
    blk_mq_sched_restart().
    
    This patch reverts 5b79413946d (Revert "blk-mq: don't handle
    TAG_SHARED in restart"). Given we have fixed enough IO hang issue,
    and there isn't any reason to restart all queues in one tags any more,
    see the following reasons:
    
    1) blk-mq core can deal with shared-tags case well via blk_mq_get_driver_tag(),
    which can wake up queues waiting for driver tag.
    
    2) SCSI is a bit special because it may return BLK_STS_RESOURCE if queue,
    target or host is ready, but SCSI built-in restart can cover all these well,
    see scsi_end_request(), queue will be rerun after any request initiated from
    this host/target is completed.
    
    In my test on scsi_debug(8 luns), this patch may improve IOPS by 20% ~ 30%
    when running I/O on these 8 luns concurrently.
    
    Fixes: 705cda97 ("blk-mq: Make it safe to use RCU to iterate over blk_mq_tag_set.tag_list")
    Cc: Omar Sandoval <osandov@fb.com>
    Cc: Bart Van Assche <bart.vanassche@wdc.com>
    Cc: Christoph Hellwig <hch@lst.de>
    Cc: Martin K. Petersen <martin.petersen@oracle.com>
    Cc: linux-scsi@vger.kernel.org
    Reported-by: default avatarAndrew Jones <drjones@redhat.com>
    Tested-by: default avatarAndrew Jones <drjones@redhat.com>
    Signed-off-by: default avatarMing Lei <ming.lei@redhat.com>
    Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
    97889f9a
blk-mq.c 74 KB