On Tue, Jun 09, 2015 at 03:14:46PM +0900, Tejun Heo wrote:
> Hey, Peter.
>
> On Fri, Jun 05, 2015 at 06:22:16PM +0200, Peter Zijlstra wrote:
> > There's a lot more problems with workqueues:
> >
> > - they're not regular tasks and all the task controls don't work on
> > them. This means all things scheduler, like cpu-affinity, nice, and
> > RT/deadline scheduling policies. Instead there is some half baked
> > secondary interface for some of these.
>
> Because there's a pool of them and the workers come and go
> dynamically. There's no way around it. The attributes just have to
> be per-pool.
Sure, but there's a few possible ways to still make that work with the
regular syscall interfaces.
1) propagate the change to any one worker to all workers of the same
pool
2) have a common ancestor task for each pool, and allow changing that.
You can combine that with either the propagation like above, or a
rule that workers kill themselves if they observe their parent
changed (eg. check a attribute sequence count after each work).
> > But this also very much includes things like cgroups, which brings me
> > to the second point.
> >
> > - its oblivious to cgroups (as it is to RT priority for example) both
> > leading to priority inversion. A work enqueued from a deep/limited
> > cgroup does not inherit the task's cgroup. Instead this work is ran
> > from the root cgroup.
> >
> > This breaks cgroup isolation, more significantly so when a large part
> > of the actual work is done from workqueues (as some workloads end up
> > being). Instead of being able to control the work, it all ends up in
> > the root cgroup outside of control.
>
> cgroup support will surely be added but I'm not sure we can or should
> do inheritance automatically.
I think its a good default to inherit stuff from the task that queued
it.
> Using a different API doesn't solve the
> problem automatically either. A lot of kthreads are shared
> system-wide after all. We'll need an abstraction layer to deal with
> that no matter where we do it.
Yes, hardware threads are global, but so is the hardware. Those are not
a problem provided the thread map 1:1 with the actual devices and do not
service multiple devices from a single thread.
Once you start combining things you start to get all the above problems
all over again.
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