Re: [PATCH 5/5] blk-mq: Replace tags->lock with SRCU for tag iterators

[Ming Lei <ming.lei@redhat.com>]

On Wed, Aug 06, 2025 at 05:21:32PM +0800, Yu Kuai wrote:
> Hi,
> 
> 在 2025/08/01 19:44, Ming Lei 写道:
> > Replace the spinlock in blk_mq_find_and_get_req() with an SRCU read lock
> > around the tag iterators.
> > 
> > This is done by:
> > 
> > - Holding the SRCU read lock in blk_mq_queue_tag_busy_iter(),
> > blk_mq_tagset_busy_iter(), and blk_mq_hctx_has_requests().
> > 
> > - Removing the now-redundant tags->lock from blk_mq_find_and_get_req().
> > 
> > This change improves performance by replacing a spinlock with a more
> > scalable SRCU lock, and fixes lockup issue in scsi_host_busy() in case of
> > shost->host_blocked.
> > 
> > Meantime it becomes possible to use blk_mq_in_driver_rw() for io
> > accounting.
> > 
> > Signed-off-by: Ming Lei<ming.lei@redhat.com>
> > ---
> >   block/blk-mq-tag.c | 12 ++++++++----
> >   block/blk-mq.c     | 24 ++++--------------------
> >   2 files changed, 12 insertions(+), 24 deletions(-)
> 
> I think it's not good to use blk_mq_in_driver_rw() for io accounting, we
> start io accounting from blk_account_io_start(), where such io is not in
> driver yet.

In blk_account_io_start(), the current IO is _not_ taken into account in
update_io_ticks() yet.

Also please look at 'man iostat':

    %util  Percentage  of  elapsed time during which I/O requests were issued to the device (bandwidth utilization for the device). Device
           saturation occurs when this value is close to 100% for devices serving requests serially.  But for devices serving requests  in
           parallel, such as RAID arrays and modern SSDs, this number does not reflect their performance limits.

which shows %util in device level, not from queuing IO to complete io from device.

That said the current approach for counting inflight IO via percpu counter
from blk_account_io_start() is not correct from %util viewpoint from
request based driver.


Thanks, 
Ming