[Bug 1840348] Re: Sharded OpWQ drops suicide_grace after waiting for work

[Dan Hill]

Attaching the proposed fix for 12.2.13 that I am testing.

** Patch added: "ceph_12.2.13-0ubuntu0.18.04.1+20200409sf00238701b1.debdiff"
   https://bugs.launchpad.net/ubuntu/+source/ceph/+bug/1840348/+attachment/5351517/+files/ceph_12.2.13-0ubuntu0.18.04.1+20200409sf00238701b1.debdiff

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Title:
  Sharded OpWQ drops suicide_grace after waiting for work

Status in ceph package in Ubuntu:
  Triaged

Bug description:
  [Impact]
  The Sharded OpWQ will opportunistically wait for more work when processing an empty queue. While waiting, the heartbeat timeout and suicide_grace values are modified. On Luminous, the `threadpool_default_timeout` grace is left applied and suicide_grace is left disabled. On later releases both the grace and suicide_grace are left disabled.

  After finding work, the original work queue grace/suicide_grace values
  are not re-applied. This can result in hung operations that do not
  trigger an OSD suicide recovery.

  The missing suicide recovery was observed on Luminous 12.2.11. The
  environment was consistently hitting a known authentication race
  condition (issue#37778 [0]) due to repeated OSD service restarts on a
  node exhibiting MCEs from a faulty DIMM.

  The auth race condition would stall pg operations. In some cases, the
  hung ops would persist for hours without suicide recovery.

  [Test Case]
  - In-Progress -
  Haven't landed on a reliable reproducer. Currently testing the fix by exercising I/O. Since the fix applies to all version of Ceph, the plan is to let this bake in the latest release before considering a back-port.

  [Regression Potential]
  This fix improves suicide_grace coverage of the Sharded OpWq.

  This change is made in a critical code path that drives client I/O. An
  OSD suicide will trigger a service restart and repeated restarts
  (flapping) will adversely impact cluster performance.

  The fix mitigates risk by keeping the applied suicide_grace value
  consistent with the value applied before entering
  `OSD::ShardedOpWQ::_process()`. The fix is also restricted to the
  empty queue edge-case that drops the suicide_grace timeout. The
  suicide_grace value is only re-applied when work is found after
  waiting on an empty queue.

  - In-Progress -
  The fix will bake upstream on later levels before back-port consideration.

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