[SRU][Bionic][PATCH 0/2] Fix IO hang regression

[Seth Forshee]

BugLink: http://bugs.launchpad.net/bugs/1765232

Impact: Since 4.15.0-15 some machines have been failing to boot due to
IO hangs. This is caused by patches applied for LP #1759723, which
assigned managed interrupt vectors and reply queues for all possible
CPUs, not just present CPUs. Some drivers were not prepared to cope with
this and end up selecting reply queues not mapped to an online CPU,
causing IO hangs during boot.

Fix: There are driver fixes available upstream, but there are 8-ish
patches in total and we're extremely close to release, so the safer bet
it to just revert the patches for LP #1759723. We can consider
reintroducing them with required fixes at a later time.

Regression Potential: This is obviously going to reintroduce the problem
the patches were intended to fix. These are less serious than the
problems which the patches introduced, and IBM has given their okay to
revert them as well.

Test Case: Verified to fix affected hardware on LP #1765232.

Thanks,
Seth


Seth Forshee (2):
  Revert "blk-mq: simplify queue mapping & schedule with each possisble
    CPU"
  Revert "genirq/affinity: assign vectors to all possible CPUs"

 block/blk-mq.c        | 19 +++++++++++--------
 kernel/irq/affinity.c | 30 +++++++++++++++---------------
 2 files changed, 26 insertions(+), 23 deletions(-)