linux-kernel - Re: A Plumber’s Wish List for Linux

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Message-ID: <20111014153844.GP14968@somewhere>
Date:	Fri, 14 Oct 2011 17:38:47 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	Tejun Heo <tj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kay Sievers <kay.sievers@...y.org>,
	linux-kernel@...r.kernel.org, lennart@...ttering.net,
	harald@...hat.com, david@...ar.dk, greg@...ah.com,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Oleg Nesterov <oleg@...hat.com>,
	Paul Menage <paul@...lmenage.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Pavel Emelyanov <xemul@...allels.com>
Subject: Re: A Plumber’s Wish List for Linux

On Wed, Oct 12, 2011 at 10:16:41PM +0400, Cyrill Gorcunov wrote:
> On Wed, Oct 12, 2011 at 10:40:14AM -0700, Tejun Heo wrote:
> ...
> > 
> > In general, I think making freezer work nicely with the rest of the
> > system is a good idea and have been working towards that direction.
> > Allowing a frozen task to be killed is not only handy for use cases
> > like above but also makes solving freezer involved deadlocks much less
> > likely and easier to solve.  Another that I have in mind is allowing
> > ptrace from unfrozen task to a frozen task.  This can be helpful in
> > general debugging (currently attaching to multi-threaded, violently
> > cloning process is quite cumbersome) and userland checkpointing.
> 
> Yeah, being able to ptrace a frozen cgroup would be great for us.
> We stick with signals start/stop cycle at moment but the final target
> is the cgroups and freezer of course. (btw while were poking freezer
> code I noticed that there is no shortcut to move all tasks in cgroup
> into the root cgroup, so I guess say "echo -1 > tasks" might be a good
> addition to move all tasks from some particular cgroup to the root
> by single action).

Well, wouldn't it be better to pull that complexity to userspace?
After all, moving tasks from a cgroup to another is not a performance
critical operation so that probably doesn't need to be all handled by
the kernel.

If one worries about concurrent clone/fork while moving tasks, then
freezing the cgroup and moving its tasks away from userspace could
be enough?
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