[LU-8307] Add cond_resched between work items in ldlm_bl_thread_main

• Type: Bug
• Resolution: Fixed
• Priority: Minor
• Fix Version/s: Lustre 2.10.0
• Severity: 3
• Assignee: WC Triage
• Reporter: Patrick Farrell

Description

When clearing all of the ldlm LRUs (as Cray does at the end of
a job), a ldlm_bl_work_item is generated for each namespace
and then they are placed on a list for the ldlm_bl threads to
iterate over.

If the number of namespaces greatly exceeds the number of
ldlm_bl threads, a given thread will iterate over many
namespaces without sleeping looking for work. This can go
on for an extremely long time and result in an RCU stall.

This patch adds a cond_resched() between completing one
work item and looking for the next. This is a fairly cheap
operation, as it will only schedule if there is an
interrupt waiting, and it will not be called too much -
Even the largest file systems have < 100 namespaces per
ldlm_bl_thread currently.