Re: [RFC PATCH 00/18] kthreads/signal: Safer kthread API and signal handling

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.

   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.  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.

Thanks.

-- 
tejun