Message-ID: <4CE683E1.6010500@kernel.org>
Date: Fri, 19 Nov 2010 15:04:17 +0100
From: Tejun Heo <tj@kernel.org>
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To: Serge Hallyn <serge.hallyn@canonical.com>
CC: Kapil Arya <kapil@ccs.neu.edu>, Gene Cooperman <gene@ccs.neu.edu>,
        linux-kernel@vger.kernel.org, xemul@sw.ru,
        Linux Containers <containers@lists.osdl.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>
Subject: Re: [Ksummit-2010-discuss] checkpoint-restart: naked patch
References: <20101104164401.GC10656@sundance.ccs.neu.edu> <4CD3CE29.2010105@kernel.org> <20101106053204.GB12449@count0.beaverton.ibm.com> <20101106204008.GA31077@sundance.ccs.neu.edu> <4CD5D99A.8000402@cs.columbia.edu> <20101107184927.GF31077@sundance.ccs.neu.edu> <4CD72150.9070705@cs.columbia.edu> <4CE3C334.9080401@kernel.org> <20101117153902.GA1155@hallyn.com> <4CE3F8D1.10003@kernel.org> <20101119041045.GC24031@hallyn.com>
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On 11/19/2010 05:10 AM, Serge Hallyn wrote:
> Hey Tejun  :)

Hey, :-)

>> and in light of already working userland alternative and the
> 
> Here's where we disagree.  If you are right about a viable userland
> alternative ('already working' isn't even a preqeq in my opinion,
> so long as it is really viable), then I'm with you, but I'm not buying
> it at this point.
> 
> Seriously.  Truly.  Honestly.  I am *not* looking for any extra kernel
> work at this moment, if we can help it in any way.

What's so wrong with Gene's work?  Sure, it has some hacky aspects but
let's fix those up.  To me, it sure looks like much saner and
manageable approach than in-kernel CR.  We can add nested ptrace,
CLONE_SET_PID (or whatever) in pidns, integrate it with various ns
supports, add an ability to adjust brk, export inotify state via
fdinfo and so on.

The thing is already working, the codebase of core part is fairly
small and condor is contemplating integrating it, so at least some
people in HPC segment think it's already viable.  Maybe the HPC
cluster I'm currently sitting near is special case but people here
really don't run very fancy stuff.  In most cases, they're fairly
simple (from system POV) C programs reading/writing data and burning a
_LOT_ of CPU cycles inbetween and admins here seem to think dmtcp
integrated with condor would work well enough for them.

Sure, in-kernel CR has better or more reliable coverage now but by how
much?  The basic things are already there in userland.  The tradeoff
simply doesn't make any sense.  If it were a well separated self
sustained feature, it probably would be able to get in, but it's all
over the place and requires a completely new concept - the
quasi-ABI'ish binary blob which would probably be portable across
different kernel versions with some massaging.  I personally think the
idea is fundamentally flawed (just go through the usual ABI!) but even
if it were not it would require _MUCH_ stronger rationale than it
currently has to be even considered for mainline inclusion.

Maybe it's just me but most of the arguments for in-kernel CR look
very weak.  They're either about remote toy use cases or along the
line that userland CR currently doesn't do everything kernel CR does
(yet).  Even if it weren't for me, I frankly can't see how it would be
included in mainline.

I think it would be best for everyone to improve userland CR.  A lot
of knowdledge and experience gained through kernel CR would be
applicable and won't go wasted.  Strong resistance against direction
change certainly is understandable but IMHO pushing the current
direction would only increase loss.  I of course could be completely
wrong and might end up getting mails filled up with megabytes of "told
you so" later, but, well, at this point, in-kernel CR already looks
half dead to me.

Thank you.

-- 
tejun
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