[Devel] Re: Checkpoint/restart (was Re: [PATCH 0/4] - v2 - Object creation with a specified id)

[Oren Laadan]

Eric W. Biederman wrote:
> Oren Laadan <orenl at cs.columbia.edu> writes:
> 
>>> Consider more intimate kernel states like:
>>> a. task statistics
>>> b. task start time
>>> c. load average
>>> d. skb state and it's data.
>>> e. mount tree.
>>>
>>> If you think over, e.g. (b) is a bad thing. It was used to be accounted in
>> jiffies, then in timespec.
>>> (a) is another example of dataset which we can't predict. task statistics
>> change over a time.
>>> Why bother with such intimate data in user-space at all?
>>> Why the hell user-space should know about it and be ABLE to modify it?
>> Agreed.
> 
> Almost agreed.  The reason we care is that the data is visible to user
> space in some form.  So we need to save it, but hopefully not in it's internal
> kernel representation.  If we don't care at all we should not save it.

yes, we probably prefer an intermediate representation to some extent, mainly
because we (a) don't want garbage data that we don't use and (b) to make the
task of converting from old to new kernel version easier.

But only for these two reasons. I see zero value in avoiding representation
used internally by the kernel for the sake of avoiding it. In fact, if it's
that much easier to stick to that representation in a specific case -- so be
it. There shall be userland utilities that will be able to inspect the contents
(like those that can inspect the contents of internal file system data).

>>> My personal vision is that:
>>> 1. user space must initialize checkpointing/restore state via some system
>> call,
>>>    supply file descriptor from where data can be read/written to.
>>> 2. must call the syscall asking kernel to restore/save different subsytems one
>> by one.
>>> 3. finalize cpt/restore state via the syscall
>>> But user-space MUST NOT bother about data content. At least not about the data
>> supplied by the kernel.
>>> It can add additional sections if needed, e.g. about iptables state.
>> I mostly agree with the vision that checkpoint/restart is probably best
>> implemented as a black box:
> 
> I would claim an atomic unit.  Roughly like a coredump is today.  We know
> what a core dump does and we don't care how it does it.
> 
>> * First, much of the work required to restore the state of a process
>> as well as the state of its resources, requires kernel interfaces that
>> are lower than the ones available to user-space. Working in user-space
>> will require that we design new complex interfaces for this purpose only.
> 
> Yes.
> 
>> * Second, much of the state that needs to be saved was not, is not, and
>> should probably never be exported to user-space (e.g. interval socket
>> buffers, t->did_exec and many more). It is only accessible to kernel
>> code, so an in-kernel module (for checkpoint/restart) makes sense. It is
>> that sort of internals that may (and will) change as the kernel evolves
>> - precisely because it is not visible to user-space and not bound to it.
> 
> No.  If the state can be inferred from user space it is visible to user
> space.  However there is state visible to user space like did_exec
> that is not directly manipulatable by user space.

Not all state can be inferred; and the point is that I don't want to need
to chase the kernel devs and add such interfaces every time some state is
added.

> 
> In the worst case today we can restore a checkpoint by replaying all of
> the user space actions that took us to get there.  That is a tedious
> and slow approach.

ugh ... not only tedious - but not entirely correct: deterministic replay is
a very complicated problem (but thrilling) !  and you'll need to log/record
everything during the execution :(

> 
>> That said, we should still attempt to reuse existing kernel interfaces
>> and mechanisms as much as possible to save - and restore - the state of
>> processes, and prefer that over handcrafting special code. This is
>> especially true for restart: in checkpoint one has to _capture_ the
>> state by probing it in a passive manner; in contrast, in restart one
>> has to actively _construct_ new resources and ensure that their state
>> matches the saved state.
> 
>> For instance, it is possible to create the process tree in a container
>> during restart from user-space reusing clone() (I'd argue that it's
>> even better to do so from user-space). Likewise, it is possible to redo
>> an open file by opening the file then using dup2() syscall to adjust
>> the target fd if necessary. The two differ in that the latter allows
>> to adjust the (so called) resources identifier to the desired value
>> (because it is privately visible), while the former does not - it gives
>> a globally visible identifier. And this is precisely why this thread
>> had started: how to determine the resource identifier when requesting
>> to allocate a resource (in this example, the pid).
> 
> The limiting factor to me appears to be live migration.

limiting in what sense ?

> As I understand the concept.   Live migration is where you take you
> first take a snapshot of a running system, and restore that snapshot
> on another system and don't start it.
> 
> Then you repeat with an incremental snapshot and you do an incremental
> restore.
> 
> Until ideally the changes are small and you can afford to have the
> system paused for the amount of time it takes to transfer and restore
> the last incremental snapshot.

yes;  in practice, you mostly care about memory contents that have
changed in the interim, as saving/transfering memory is by far the
most time consuming component of a checkpoint/migration.

So you must track which memory pages were modified between successive
attempts to complete the migration. As for the rest - you can either
checkpoint all the rest during the final snapshot, or save all of them
at the beginning and track changes to them too.

> 
> I expect a design that allows for multiple cpus to work on the
> checkpoint/restore paths and that allows for incremental and
> thus live migration are going to be the limiting factors in a good
> interface design.

These two are important factors in my reasoning to use in kernel
implementation. Such implementation should allow concurrency (e.g.
using kernel threads), as well as the required atomicity to track
the changes between successive iterations.

> Pushing the work into the kernel with an atomic syscall style approach
> so that the normal people maintaining the kernel can have visibility
> of the checkpoint/restore code paths and can do some of the work
> seems healthy.

So now it seems that we agree on that a kernel approach is suitable.

> 
>>> Having all this functionality in a signle syscall we specifically CLAIM a
>> black box,
>>> and that no one can use this interfaces for something different from
>> checkpoint/restore.
>>
>> True, except for what can be done (and is easier to actually that way)
>> in user space; the most obvious example being clone() and setsid() -
>> which are a pain to adjust after the fact. In particular, everything
>> that is private in the kernel now (un-exported) should remain that way,
>> unless there is an (other) compelling reason to expose it.
>> (see http://www.ncl.cs.columbia.edu/publications/usenix2007_fordist.pdf)
> 
> Yes.  A very good example of something that is user visible but should
> not be user settable (except during restore) is the start time for a
> monotonic clock.  If you allow someone to set it arbitrarily it is no
> longer a monotonic clock and it looses all value.
> 
> So here is one suggestion:
> sys_processtree_isolate(container_init_pid);
> sys_checkpoint(old_dir_fd, new_dir_fd, container_init_pid);
> sys_processtree_unisolate(container_init_pid);
> 
> container_init_pid = sys_restore(dir_fd, old_container_init_pid);
> sys_processtree_unisolate(container_init_pid);

what do you mean by "isolate" and "unisolate" ?

> 
> Then save the different components in different files.  And in the non-trivial cases
> have tagged data in the files.  The tags allow us to have different data types
> easily. 
> 

why different files ?  why not a single file, streamed - beneficial
for live migration, for example, or pass the data through filters
(encryption, signature, conversion between kernel versions, etc).

> For data that already have or should have an interface for
> persistence.  Filesystems being the primary example we don't handle
> this way.  Instead we use the already existing techniques to
> backup/snapshot the data and then to restore the data.
> 
> Isolation should be used instead of freezing if we can, because
> isolation is a much cheaper concept, and it allows for multi-machine
> synchronized checkpoints.  On the big scale if I unplug a machines

if you let the processes in a container to proceed execution (even
if isolated) during a checkpoint, you will be unable to capture a
state that is consistent among all processes in that container. So
you must freeze them somehow.

> network cable (isolating it) then I don't care when between the time I
> isolate the machine and the time I plug the cable back in.  It all
> looks the same to the outside world.  But just the comunications of
> the machine were frozen.  Likewise for processes.  SIGSTOP is good

I am not aware of any simple way that can prevent any sort of interaction
between processes (by simple, I mean like unplugging a cable). For
example, how would you prevent shared memory alterations ?  signals that
are raised due to async IO completion ? and many more examples.

> but I suspect it may be excessive, so denies us optimization
> opportunities (at least in the interface).
> 


> So in summary we want an interface that allows for.
> - Existing persistent kernel state to using the preexisting mechanisms.
> - The persistence code to be looked after by the people working on the
>   kernel subsystem.
> - Migration between kernel versions, (possibly with user space intervention).
> - Live migration 
> - Minimal intervention of the processes being checkpointed.
> - Denying persistence to applications that need it.
> - Checkpoints coordinated between multiple containers or real
>   machines.

you mean distributed checkpoint/restart ?  not an easy job either, and
should not, IMHO, be done at kernel level. Perhaps you'll find this
paper interesting:
http://www.ncl.cs.columbia.edu/publications/cluster2005_fordist.pdf

> - Documented in terms of isolation rather then freezing, as it is the
>   isolation that is the important property. 

can you please define "isolation" in this context ?  "freezing" is very
simple: the processes cannot do anything while they are checkpointed.
(in the case of live migration, they aren't frozen except for very short
time - aka downtime).

> - Restricted access to user space visible but immutable state.

> - Ideally sufficiently general that debuggers can take advantage
>   of the checkpoints.

This last point is interesting and important, but not high priority.
It is always possible to take a checkpoint image and - in user space -
convert it to a format that is readable by debuggers. So it should not
be a guiding factor in designing the checkpoint/restart mechanism.

Oren.

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