Messages in this thread | | | Subject | Re: [PATCH v2 3/3] blk-mq: Optimise blk_mq_queue_tag_busy_iter() for shared tags | From | John Garry <> | Date | Mon, 6 Dec 2021 20:10:22 +0000 |
| |
On 06/12/2021 12:49, 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@mail.gmail.com/ >
Fixes: e155b0c238b2 ("blk-mq: Use shared tags for shared sbitmap support")
> Signed-off-by: John Garry<john.garry@huawei.com> > Reviewed-by: Hannes Reinecke<hare@suse.de> > Reviewed-by: Ming Lei<ming.lei@redhat.com> > Reported-and-tested-by: Kashyap Desai<kashyap.desai@broadcom.com>
| |