On Wed, Jun 10, 2026 at 11:44 AM Christoph Hellwig [off-list ref]
wrote:

Just dropping the fairness was rejected before and there is no
explanation here on why any of that has changed.

I missed the fact that this patch had been previously rejected.

The motivation for this change stems from performance issue we
encountered due to false sharing of the 'nr_active_requests_shared_tags'
counter
on certain CPU architectures. I initially submitted a patch to move that
counter to
its own cache line to avoid conflicts with 'nr_requests' and other hot
fields
(see:
https://patchwork.kernel.org/project/linux-scsi/patch/20260402074637.92417-3-sumit.saxena@broadcom.com/
).

During the review, Bart shared his work, which eliminates the
counter entirely by removing the fairness throttling. My testing confirmed
that
this approach resolved the performance issues and improved IOPS.
This patch is part of a larger set, and I have reported the cumulative
performance
improvements in the cover letter.

Thanks,
Sumit

On Tue, Jun 09, 2026 at 05:48:02PM +0530, Sumit Saxena wrote:

quoted

From: Bart Van Assche <bvanassche@acm.org>

Original patch [1] by Bart Van Assche; this version is rebased onto the
current tree.  In testing it improves IOPS by roughly 16-18% by removing
the fair-sharing throttle on shared tag queues.

This patch removes the following code and structure members:
- The function hctx_may_queue().
- blk_mq_hw_ctx.nr_active and

request_queue.nr_active_requests_shared_tags

quoted

  and also all the code that modifies these two member variables.

.. and besides that, this commit message is still entirely useless
as it doesn't explain any of the thoughts of why this change is safe
and desirable.  While the mechanics above are totally obvious from
the code change itself.