Re: [PATCH 1/4] blk-mq: introduce BLK_MQ_F_SCHED_USE_HW_TAG

On Thu, May 4, 2017 at 12:46 AM, Omar Sandoval [off-list ref] wrote:

On Fri, Apr 28, 2017 at 11:15:36PM +0800, Ming Lei wrote:

quoted

When blk-mq I/O scheduler is used, we need two tags for
submitting one request. One is called scheduler tag for
allocating request and scheduling I/O, another one is called
driver tag, which is used for dispatching IO to hardware/driver.
This way introduces one extra per-queue allocation for both tags
and request pool, and may not be as efficient as case of none
scheduler.

Also currently we put a default per-hctx limit on schedulable
requests, and this limit may be a bottleneck for some devices,
especialy when these devices have a quite big tag space.

This patch introduces BLK_MQ_F_SCHED_USE_HW_TAG so that we can
allow to use hardware/driver tags directly for IO scheduling if
devices's hardware tag space is big enough. Then we can avoid
the extra resource allocation and make IO submission more
efficient.

Signed-off-by: Ming Lei <redacted>
---
 block/blk-mq-sched.c   | 10 +++++++++-
 block/blk-mq.c         | 35 +++++++++++++++++++++++++++++------
 include/linux/blk-mq.h |  1 +
 3 files changed, 39 insertions(+), 7 deletions(-)

One more note on this: if we're using the hardware tags directly, then
we are no longer limited to q->nr_requests requests in-flight. Instead,
we're limited to the hw queue depth. We probably want to maintain the
original behavior,

That need further investigation, and generally scheduler should be happy with
more requests which can be scheduled.

We can make it as one follow-up.

so I think we need to resize the hw tags in blk_mq_init_sched() if we're using hardware tags.

That might not be good since hw tags are used by both scheduler and dispatching.


Thanks,
Ming Lei