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SubjectRe: [PATCH 2/3] work_on_cpu: Use our own workqueue.

* Andrew Morton <> wrote:

> > > Yet another kernel thread for each CPU. All because of some dung
> > > way down in arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.c.
> > >
> > > Is there no other way?
> >
> > Perhaps, but this works. Trying to be clever got me into this mess in
> > the first place.
> >
> > We could stop using workqueues and change work_on_cpu to create a
> > thread every time, which would give it a new failure mode so I don't
> > know that everyone could use it any more. Or we could keep a single
> > thread around to do all the cpus, and duplicate much of the workqueue
> > code.
> >
> > None of these options are appealing...
> Can we try harder please? 10 screenfuls of kernel threads in the ps
> output is just irritating.
> How about banning the use of work_on_cpu() from schedule_work() handlers
> and then fixing that driver somehow?

Yes, but that's fundamentally fragile: anyone who happens to stick the
wrong thing into keventd (and it's dead easy because schedule_work() is
easy to use) will lock up work_on_cpu() users.

work_on_cpu() is an important (and lowlevel enough) facility to be
isolated from casual interaction like that.

> What _is_ the bug anyway? The only description we were given was
> Impact: remove potential clashes with generic kevent workqueue
> Annoyingly, some places we want to use work_on_cpu are already in
> workqueues. As per Ingo's suggestion, we create a different
> workqueue for work_on_cpu.
> which didn't bother telling anyone squat.
> When was this bug added? Was it added into that driver or was it due to
> infrastructural changes?

This fixes lockups during bootup caused by the cpumask changes/cleanups
which changed set_cpus_allowed()+on-kernel-stack-cpumask_t to

Which was fine except it didnt take into account the interaction with the
kevents workqueue and the very wide cross section for worklet dependencies
that this brings with itself. work_on_cpu() was rarely used before so this
didnt show up.


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