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b66b036c95drm_pagemap_dev_unhold_work() processes an unbatched llist of pagemaps
on the system workqueue. This creates potential latency traps during
heavy teardown cycles due to two compounding factors:
1. The loop contains a per-item drm_dbg() trace. If dynamic debug is
enabled on a slow serial console, synchronous print latency scales
linearly with the batch size and can block worker execution.
2. The llist can accumulate large batches during intensive unmap
operations, monopolizing worker resources without yielding.
This causes below log warning:
"workqueue: drm_pagemap_dev_unhold_work [drm_gpusvm_helper] hogged CPU
for >10000us 19 times, consider switching to WQ_UNBOUND"
Fix this by implementing a latency mitigation strategy:
- Move the teardown work to system_unbound_wq to avoid tying up
per-CPU bound worker pools.
- Add cond_resched() inside the teardown loop to yield the CPU during
large batch teardowns, ensuring system responsiveness.
- Convert the drm_dbg() trace to drm_dbg_ratelimited() to mitigate
serial console bottlenecks while preserving debuggability.
v2:
- Changed workqueue from deprecated system_unbound_wq to system_dfl_wq.
(Arvind Yadav)
Fixes:
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