tree 23347ffe48402d0fc8a31b201e1dc8949716ae4c
parent c7fd2e6d537153339880de29ed22ed5b0537edc6
author Tejun Heo <tj@kernel.org> 1683687427 -1000
committer Tejun Heo <tj@kernel.org> 1683687427 -1000

workqueue: Automatically mark CPU-hogging work items CPU_INTENSIVE

If a per-cpu work item hogs the CPU, it can prevent other work items from
starting through concurrency management. A per-cpu workqueue which intends
to host such CPU-hogging work items can choose to not participate in
concurrency management by setting %WQ_CPU_INTENSIVE; however, this can be
error-prone and difficult to debug when missed.

This patch adds an automatic CPU usage based detection. If a
concurrency-managed work item consumes more CPU time than the threshold (5ms
by default), it's marked CPU_INTENSIVE automatically on schedule-out.

The mechanism isn't foolproof in that the 5ms detection delays can add up if
many CPU-hogging work items are queued at the same time. However, in such
situations, the bigger problem likely is the CPU being saturated with
per-cpu work items and the solution would be making them UNBOUND.

For occasional CPU hogging, the new automatic mechanism should provide
reasonable protection with minimal increase in code complexity.

v2: Lai pointed out that wq_worker_stopping() also needs to be called from
    preemption and rtlock paths and an earlier patch was updated
    accordingly. This patch adds a comment describing the risk of infinte
    recursions and how they're avoided.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Acked-by: Hillf Danton <hdanton@sina.com>