On Wed 16-06-21 09:43:06, Jens Axboe wrote: > 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: No, I didn't get email back and didn't notice the patch is already in your tree. Sorry for the noise. Honza > > https://git.kernel.dk/cgit/linux-block/commit/?h=for-5.14/block&id=613471549f366cdf4170b81ce0f99f3867ec4d16 > > -- > Jens Axboe > -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
