Messages in this thread | | | Date | Fri, 8 Jan 2021 12:46:36 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH -tip V3 0/8] workqueue: break affinity initiatively |
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On Sat, Dec 26, 2020 at 10:51:08AM +0800, Lai Jiangshan wrote: > From: Lai Jiangshan <laijs@linux.alibaba.com> > > 06249738a41a ("workqueue: Manually break affinity on hotplug") > said that scheduler will not force break affinity for us.
So I've been looking at this the past day or so, and the more I look, the more I think commit:
1cf12e08bc4d ("sched/hotplug: Consolidate task migration on CPU unplug")
is a real problem and we need to revert it (at least for now).
Let me attempt a brain dump:
- the assumption that per-cpu kernel threads are 'well behaved' on hot-plug has, I think, been proven incorrect, it's far worse than just bounded workqueue. Therefore, it makes sense to provide the old semantics.
- making the current code provide the old semantics (forcing affinity on per-cpu kernel threads) is tricky, but could probably be done:
* we need to disallow new per-cpu kthreads while going down * we need to force push more agressive; basically when rcuwait_active(rq->hotplug_wait) push everything except that task, irrespective of is_per_cpu_kthread() * we need to disallow wakeups of anything not the hotplug thread or stop-machine from happening from the rcuwait_wait_event()
and I have patches for most of that... except they're adding more complexity than 1cf12e08bc4d ever deleted.
However, even with all that, there's a further problem...
Fundamentally, waiting for !rq_has_pinned_tasks() so late in hot-un-plug, is wrong I think. It means that migrate_disable() code might encounter a mostly torn down CPU. This is OK-ish for per-cpu kernel threads [*], but is now exposed to any random odd kernel code that does migrate_disable().
[*] arguably running 'work' this late is similarly problematic.
Let me go do lunch and ponder this further..
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