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

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Message-ID: <20150609061446.GV21465@mtj.duckdns.org>
Date:	Tue, 9 Jun 2015 15:14:46 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Peter Zijlstra <peterz@...radead.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

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
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