[0/2,RFC] blk-mq: Improve performance of non-mq IO schedulers with multiple HW queues

Message

Jan Kara Dec. 18, 2020, 9:44 p.m. UTC

Hello!

This patch series aims to fix a regression we've noticed on our test grid when
support for multiple HW queues in megaraid_sas driver was added during the 5.10
cycle (103fbf8e4020 scsi: megaraid_sas: Added support for shared host tagset
for cpuhotplug). The commit was reverted in the end for other reasons but I
believe the fundamental problem still exists for any other similar setup. The
problem manifests when the storage card supports multiple hardware queues
however storage behind it is slow (single rotating disk in our case) and so
using IO scheduler such as BFQ is desirable. See the second patch for details.

FWIW I'm not 100% sold on this approach, in particular I'm not sure it cannot
cause some issues in higher-end setups but I want expect mq-deadline or BFQ
to be used there. Anyway that's why I'm sending this as an RFC.

								Honza