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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
(10ms by default) continuously without intervening sleeps, wq_worker_tick()
which is called from scheduler_tick() will detect the condition and
automatically mark it CPU_INTENSIVE.
The mechanism isn't foolproof:
* Detection depends on tick hitting the work item. Getting preempted at the
right timings may allow a violating work item to evade detection at least
temporarily.
* nohz_full CPUs may not be running ticks and thus can fail detection.
* Even when detection is working, the 10ms detection delays can add up if
many CPU-hogging work items are queued at the same time.
However, in vast majority of cases, this should be able to detect violations
reliably and provide reasonable protection with a small increase in code
complexity.
If some work items trigger this condition repeatedly, the bigger problem
likely is the CPU being saturated with such per-cpu work items and the
solution would be making them UNBOUND. The next patch will add a debug
mechanism to help spot such cases.
v4: Documentation for workqueue.cpu_intensive_thresh_us added to
kernel-parameters.txt.
v3: Switch to use wq_worker_tick() instead of hooking into preemptions as
suggested by Peter.
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>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
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| .. | ||
| irq | ||
| wrappers | ||
| asm-annotations.rst | ||
| assoc_array.rst | ||
| boot-time-mm.rst | ||
| cachetlb.rst | ||
| circular-buffers.rst | ||
| cpu_hotplug.rst | ||
| debug-objects.rst | ||
| debugging-via-ohci1394.rst | ||
| dma-api-howto.rst | ||
| dma-api.rst | ||
| dma-attributes.rst | ||
| dma-isa-lpc.rst | ||
| entry.rst | ||
| errseq.rst | ||
| genalloc.rst | ||
| generic-radix-tree.rst | ||
| genericirq.rst | ||
| gfp_mask-from-fs-io.rst | ||
| idr.rst | ||
| index.rst | ||
| kernel-api.rst | ||
| kobject.rst | ||
| kref.rst | ||
| librs.rst | ||
| local_ops.rst | ||
| maple_tree.rst | ||
| memory-allocation.rst | ||
| memory-hotplug.rst | ||
| mm-api.rst | ||
| netlink.rst | ||
| packing.rst | ||
| padata.rst | ||
| pin_user_pages.rst | ||
| printk-basics.rst | ||
| printk-formats.rst | ||
| printk-index.rst | ||
| protection-keys.rst | ||
| rbtree.rst | ||
| refcount-vs-atomic.rst | ||
| symbol-namespaces.rst | ||
| this_cpu_ops.rst | ||
| timekeeping.rst | ||
| tracepoint.rst | ||
| unaligned-memory-access.rst | ||
| watch_queue.rst | ||
| workqueue.rst | ||
| xarray.rst | ||