[Devel] Re: [PATCH] ia64 sn xpc: Convert to use kthread API.

[Eric W. Biederman]

Dean Nelson <dcn at sgi.com> writes:

> On Fri, Apr 27, 2007 at 12:34:02PM -0600, Eric W. Biederman wrote:
>> Dean Nelson <dcn at sgi.com> writes:
>> >
>> > Christoph is correct in that XPC has a single thread that exists throughout
>> > its lifetime, another set of threads that exist for the time that active
>> > contact with other XPCs running on other SGI system partitions exists, and
>> > finally there is a pool of threads that exist on an as needed basis once
>> > a channel connection has been established between two partitions.
>> >
>> > In principle I approve of the kthread API and its use as opposed to what
>> > XPC currently does (calls kernel_thread(), daemonize(),
> wait_for_completion(),
>> > and complete()). So Christoph's patch that changes the single long-lived
>> > thread to use kthread_stop() and kthread_should_stop() is appreciated.
>> >
>> > But the fact that another thread, started at the xpc_init() time, that does
>> > discovery of other SGI system partitions wasn't converted points out a
>> > weekness in either my thinking or the kthread API. This discovery thread
>> > does its job and then exits. Should XPC be rmmod'd while the discovery
>> > thread is still running we would need to do a kthread_stop() against it. 
>> > But kthread_stop() isn't set up to deal with a task that has already exited.
>> > And if what once was the task structure of this exited task has been
>> > reallocated to another new task, we'd end up stopping it should it be
>> > operating under the kthread API, or possibly waiting a very long time
>> > for it to exit if it is not.
>> 
>> Patches are currently under development to allow kthreads to exit
>> before kthread_stop is called.  The big thing is that once we allow
>> kernel threads that exited by themselves to be reaped by kthread_stop
>> we have some significant work to do.
>> 
>> > I'm also a little uneasy that kthread_stop() has an "only one thread can
>> > stop another thread at a time" design. It's a potential bottleneck on
>> > very large systems where threads are blocked and unable to respond to a
>> > kthread_should_stop() for some period of time.
>> 
>> There are already patches out there to fix this issue.
>> 
>> > XPC is in need of threads that can block indefinitely, which is why XPC
>> > is in the business of maintaining a pool of threads. Currently there is
>> > no such capability (that I know of) that is provided by linux. Workqueues
>> > can't block indefinitely.
>> 
>> I'm not certain I understand this requirement.  Do you mean block indefinitely
>> unless requested to stop?
>
> These threads can block waiting for a hardware DMA engine, which has a 28
> second timeout setpoint.

Ok.  So this is an interruptible sleep?
Do you have any problems being woken up out of that interruptible sleep
by kthread_stop?

I am in the process of modifying kthread_stop to wake up thread in an
interruptible sleep and set signal_pending, so they will break out.

>> > And for performance reasons these threads need to be able to be created
>> > quickly. These threads are involved in delivering messages to XPC's users
>> > (like XPNET) and we had latency issues that led us to use kernel_thread()
>> > directly instead of the kthread API. Additionally, XPC may need to have
>> > hundreds of these threads active at any given time.
>> 
>> Ugh.  Can you tell me a little more about the latency issues?
>
> After placing a message in a local message queue, one SGI system partition
> will interrupt another to retrieve the message. We need to minimize the
> time from entering XPC's interrupt handler to the time that the message
> can be DMA transferred and delivered to the consumer (like XPNET) to
> whom it was sent.
>
>> Is having a non-halting kthread_create enough to fix this?
>> So you don't have to context switch several times to get the
>> thread running?
>> 
>> Or do you need more severe latency reductions?
>> 
>> The more severe fix would require some significant changes to copy_process
>> and every architecture would need to be touched to fix up copy_thread.
>> It is possible, it is a lot of work, and the reward is far from obvious.
>
> I think a non-halting kthread_create() should be sufficient. It is in
> effect what XPC has now in calling kernel_thread() directly.

A little different but pretty close.

We call kthread_create() it prepares everything and places it on
a queue and wakes up kthreadd.

kthreadd then wakes up and forks the thread.

After the thread has finishing setting up it will call complete on
a completion so kthread_create can continue on it's merry way
but it should not need to go to sleep waiting for someone to
call kthread_bind.

But if you can live with what I have just described that will
be easy to code up.

It is a little slower then kernel_thread but hopefully not much.

> Taking it one step further, if you added the notion of a thread pool,
> where upon exit, a thread isn't destroyed but rather is queued ready to
> handle the next kthread_create_quick() request.

That might happen.  So far I am avoiding the notion of a thread pool for
as long as I can.  There is some sense in it, especially in generalizing
the svc thread pool code from nfs.  But if I don't have to go there I would
prefer it.

>> > I think it would be great if the kthread API (or underlying implementation)
>> > could be changed to handle these issues. I'd love for XPC to not have to
>> > maintain this sort of thing itself.
>> 
>> Currently daemonize is a serious maintenance problem.
>> 
>> Using daemonize and kernel_thread to create kernel threads is a blocker
>> in implementing the pid namespace because of their use of pid_t.
>> 
>> So I am motivated to get this fixed.
>
> This would also address the problems we see with huge pid spaces for
> kernel threads on our largest machines.  In the example from last week,
> we had 10 threads each on 4096 cpus.  If we reworked work_queues to use
> the kthread_create_nonblocking() thread pool, we could probably collapse
> the need for having all of those per-task, per-cpu work queues.

Patches have already been sent (and I don't think found problems with)
that make kthreadd pid == 2, and they also modify daemonize to reparent
to kthreadd instead of init.

Eric
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