In the Linux kernel, the following vulnerability has been resolved:
idpf: convert workqueues to unbound
When a workqueue is created with WQ_UNBOUND, its work items are
served by special worker-pools, whose host workers are not bound to
any specific CPU. In the default configuration (i.e. when
queue_delayed_work and friends do not specify which CPU to run the
work item on), WQ_UNBOUND allows the work item to be executed on any
CPU in the same node of the CPU it was enqueued on. While this
solution potentially sacrifices locality, it avoids contention with
other processes that might dominate the CPU time of the processor the
work item was scheduled on.
This is not just a theoretical problem: in a particular scenario misconfigured process was hogging most of the time from CPU0, leaving less than 0.5% of its CPU time to the kworker. The IDPF workqueues that were using the kworker on CPU0 suffered large completion delays as a result, causing performance degradation, timeouts and eventual system crash.
* I have also run a manual test to gauge the performance
improvement. The test consists of an antagonist process
(./stress --cpu 2) consuming as much of CPU 0 as possible. This
process is run under taskset 01 to bind it to CPU0, and its
priority is changed with chrt -pQ 9900 10000 ${pid} and
renice -n -20 ${pid} after start.
Then, the IDPF driver is forced to prefer CPU0 by editing all calls
to queue_delayed_work, mod_delayed_work, etc... to use CPU 0.
Finally, ktraces for the workqueue events are collected.
Without the current patch, the antagonist process can force
arbitrary delays between workqueue_queue_work and
workqueue_execute_start, that in my tests were as high as
30ms. With the current patch applied, the workqueue can be
migrated to another unloaded CPU in the same node, and, keeping
everything else equal, the maximum delay I could see was 6us.