Re: How to notify app of changed cpu/mem/io node configuration?

[Dave Hansen <haveblue@us.ibm.com>]

On Tue, 2004-06-29 at 18:22, Paul Jackson wrote:
> Dave wrote:
> > (in response to my query of whether you meant "sync" or "async"):
> > Some kind of notification that the kernel sends where it can be sure
> > that the app received it.
> 
> Huh - I'm confused as to what you think is essential here.  I have this
> sense that you are saying we need reliably acknowledged notification.
> But this seems impossible to me - one cannot have the kernel depending
> on some random bit of user code sending a timely acknowledgement, much
> less on such code doing something competent with the notice.

I was thinking we could just make sure that it handled the signal.  But,
that doesn't seem horribly feasible at this point.,

> > Rudely being migrated to CPU 0 would break the app,
> > and could (in theory) cause data corruption.  
> 
> Then I am surpised that you accept the following bit of code, now
> in Linus' 2.6.7:
> 
>   kernel/sched.c:migrate_all_tasks()
> 
>                 if (dest_cpu = NR_CPUS) {
>                         cpus_setall(tsk->cpus_allowed);
>                         dest_cpu = any_online_cpu(tsk->cpus_allowed);

I don't accept a lot of the cpu hotplug code as it is :).

But, it at least avoids having to really worry about processes handling
the notification, and won't kill them.  I guess slow is better than
dead.

> The usual affect of this is to force cpus_allowed to all possible CPUs,
> and to set "dest_cpu = 0".  It happens in the case of a migration, when
> none of the remaining CPUs allowed to a task are online.
> 
> Hmmm ... thinking out loud ... why not set cpus_allowed to the single
> cpu which is the lowest numbered online cpu, rather than all cpus?
> I see no need to give orphans the run of the entire city!

The only problem that might cause would be scheduler imbalances.  It
would sure beat the living daylights out of CPU0 if a lot of tasks were
sent over there.  This way, the scheduler at least has a fighting chance
to balance them out.  

> > > ...  before it told the kernel "all clear - remove old Node from service"
> > 
> > I just worry about the complexity of feeding all of this information
> > back to the kernel. 
> 
> Huh - what's to tell the kernel at this step?  "Remove CPU or Memory
> node X from service, with ruthless abandon to any residual user tasks or
> memory left there" -- that's all I see.

I was thinking about how to have a userspace /sbin/hotplug handler tell
the kernel which tasks had been migrated or handled correctly.  But,
that doesn't make _any_ sense as I think back.  I think I was in a
meeting when I sent that and my brain was turned off.

-- Dave



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