Mike has been running into an issue with BFQ, where things grind to a halt. This is because of how BFQ limits the shallow depth. If it ends up limiting it to something low that is smaller than the wake batch sizing for sbitmap, we can run into cases where we never wake up folks waiting for a tag. The end result is an idle system with no IO pending, but with tasks waiting for a tag with no one to wake them up. Kyber could run into the same issue, if the async depth is limited low enough.
This patchset adds a helper to inform sbitmap about shallow depth limiting, and handles this from blk-mq-sched. block/bfq-iosched.c | 13 ++++++++++--- block/blk-mq-sched.c | 26 ++++++++++++++++++++++++++ block/blk-mq-sched.h | 3 +++ block/blk-mq.c | 8 +------- block/kyber-iosched.c | 14 ++++++++++---- include/linux/elevator.h | 2 +- include/linux/sbitmap.h | 11 +++++++++++ lib/sbitmap.c | 17 ++++++++++++++++- 8 files changed, 78 insertions(+), 16 deletions(-) -- Jens Axboe
