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

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Date:	Wed, 10 Jun 2015 12:40:57 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	Petr Mladek <pmladek@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	Richard Weinberger <richard@....at>,
	Steven Rostedt <rostedt@...dmis.org>,
	David Woodhouse <dwmw2@...radead.org>,
	linux-mtd@...ts.infradead.org,
	Trond Myklebust <trond.myklebust@...marydata.com>,
	Anna Schumaker <anna.schumaker@...app.com>,
	linux-nfs@...r.kernel.org, Chris Mason <clm@...com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jiri Kosina <jkosina@...e.cz>, Borislav Petkov <bp@...e.de>,
	Michal Hocko <mhocko@...e.cz>, live-patching@...r.kernel.org,
	linux-api@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 00/18] kthreads/signal: Safer kthread API and signal
 handling

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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