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

[Eric W. Biederman]

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?

> 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?

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 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.

Eric
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