linux-kernel - Re: [PATCH 2.6.24-rc8-mm1 09/15] (RFC) IPC: new kernel API to change an ID

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Date:	Mon, 04 Feb 2008 16:41:26 +0300
From:	Kirill Korotaev <dev@...ru>
To:	Cedric Le Goater <clg@...ibm.com>
CC:	Pierre Peiffer <pierre.peiffer@...l.net>,
	containers@...ts.linux-foundation.org,
	Alexey Dobriyan <adobriyan@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2.6.24-rc8-mm1 09/15] (RFC) IPC: new kernel API to change
 an ID

Cedric Le Goater wrote:
> Hello Kirill !
> 
> Kirill Korotaev wrote:
>> Pierre,
>>
>> my point is that after you've added interface "set IPCID", you'll need
>> more and more for checkpointing:
>> - "create/setup conntrack" (otherwise connections get dropped),
>> - "set task start time" (needed for Oracle checkpointing BTW),
>> - "set some statistics counters (e.g. networking or taskstats)"
>> - "restore inotify"
>> and so on and so forth.
> 
> right. we know that we will have to handle a lot of these
> and more and we will need an API for it :) so how should we handle it ?
> through a dedicated syscall that would be able to checkpoint and/or
> restart a process, an ipc object, an ipc namespace, a full container ?
> will it take a fd or a big binary blob ?  
> I personally really liked Pavel idea's of filesystem. but we dropped the
> thread.

Imho having a file system interface means having all its problems.
Imagine you have some information about tasks exported with a file system interface.
Obviously to collect the information you have to hold some spinlock like tasklist_lock or similar.
Obviously, you have to drop the lock between sys_read() syscalls.
So interface gets much more complicated - you have to rescan the objects and somehow find the place where
you stopped previous read. Or you have to to force reader to read everything at once.

> that's for the user API but we will need also kernel services to expose
> (checkpoint) states and restore them. If it's too
> early to talk about the user API, we could try first to refactor
> the kernel internals to expose correctly what we need.

That's what I would start with.

> That's what Pierre's patchset is trying to do.

Not exactly. For checkpointing/restoring we actually need only one new API call for each
subsystem - create some object with given ID (and maybe parameters, if they are not dynamically changeable by user).
While Pierre's patchset adds different API call - change object ID.

Thanks,
Kirill
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