On Tue, Nov 02, 2021 at 07:27:35PM +0800, John Garry wrote: > Kashyap reports high CPU usage in blk_mq_queue_tag_busy_iter() and callees > using megaraid SAS RAID card since moving to shared tags [0]. > > Previously, when shared tags was shared sbitmap, this function was less > than optimum since we would iter through all tags for all hctx's, > yet only ever match upto tagset depth number of rqs. > > Since the change to shared tags, things are even less efficient if we have > parallel callers of blk_mq_queue_tag_busy_iter(). This is because in > bt_iter() -> blk_mq_find_and_get_req() there would be more contention on > accessing each request ref and tags->lock since they are now shared among > all HW queues. > > Optimise by having separate calls to bt_for_each() for when we're using > shared tags. In this case no longer pass a hctx, as it is no longer > relevant, and teach bt_iter() about this. > > Ming suggested something along the lines of this change, apart from a > different implementation. > > [0] https://lore.kernel.org/linux-block/e4e92abbe9d52bcba6b8cc6c91c442cc@xxxxxxxxxxxxxx/ > > Signed-off-by: John Garry <john.garry@xxxxxxxxxx> Looks fine, Reviewed-by: Ming Lei <ming.lei@xxxxxxxxxx> Thanks, Ming
