On 6/16/21 9:40 AM, Jan Kara wrote: > On Thu 03-06-21 12:47:21, Jan Kara wrote: >> Provided the device driver does not implement dispatch budget accounting >> (which only SCSI does) the loop in __blk_mq_do_dispatch_sched() pulls >> requests from the IO scheduler as long as it is willing to give out any. >> That defeats scheduling heuristics inside the scheduler by creating >> false impression that the device can take more IO when it in fact >> cannot. >> >> For example with BFQ IO scheduler on top of virtio-blk device setting >> blkio cgroup weight has barely any impact on observed throughput of >> async IO because __blk_mq_do_dispatch_sched() always sucks out all the >> IO queued in BFQ. BFQ first submits IO from higher weight cgroups but >> when that is all dispatched, it will give out IO of lower weight cgroups >> as well. And then we have to wait for all this IO to be dispatched to >> the disk (which means lot of it actually has to complete) before the >> IO scheduler is queried again for dispatching more requests. This >> completely destroys any service differentiation. >> >> So grab request tag for a request pulled out of the IO scheduler already >> in __blk_mq_do_dispatch_sched() and do not pull any more requests if we >> cannot get it because we are unlikely to be able to dispatch it. That >> way only single request is going to wait in the dispatch list for some >> tag to free. >> >> Reviewed-by: Ming Lei <ming.lei@xxxxxxxxxx> >> Signed-off-by: Jan Kara <jack@xxxxxxx> >> --- >> block/blk-mq-sched.c | 12 +++++++++++- >> block/blk-mq.c | 2 +- >> block/blk-mq.h | 2 ++ >> 3 files changed, 14 insertions(+), 2 deletions(-) >> >> Jens, can you please merge the patch? Thanks! > > Ping Jens? Didn't I reply? It's merged for a while: https://git.kernel.dk/cgit/linux-block/commit/?h=for-5.14/block&id=613471549f366cdf4170b81ce0f99f3867ec4d16 -- Jens Axboe
