On Thu, May 28, 2009 at 12:57:05AM +0200, Ingo Molnar wrote:

* Paul E. McKenney [off-list ref] wrote:

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On Wed, May 20, 2009 at 10:09:24AM +0200, Ingo Molnar wrote:

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* Paul E. McKenney [off-list ref] wrote:

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On Tue, May 19, 2009 at 02:44:36PM +0200, Ingo Molnar wrote:

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* Paul E. McKenney [off-list ref] wrote:

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On Tue, May 19, 2009 at 10:58:25AM +0200, Ingo Molnar wrote:

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* Paul E. McKenney [off-list ref] wrote:

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On Mon, May 18, 2009 at 05:42:41PM +0200, Ingo Molnar wrote:

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* Paul E. McKenney [off-list ref] wrote:

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i might be missing something fundamental here, but why not just 
have per CPU helper threads, all on the same waitqueue, and wake 
them up via a single wake_up() call? That would remove the SMP 
cross call (wakeups do immediate cross-calls already).

My concern with this is that the cache misses accessing all the 
processes on this single waitqueue would be serialized, slowing 
things down. In contrast, the bitmask that smp_call_function() 
traverses delivers on the order of a thousand CPUs' worth of bits 
per cache miss.  I will give it a try, though.

At least if you go via the migration threads, you can queue up 
requests to them locally. But there's going to be cachemisses 
_anyway_, since you have to access them all from a single CPU, 
and then they have to fetch details about what to do, and then 
have to notify the originator about completion.

Ah, so you are suggesting that I use smp_call_function() to run 
code on each CPU that wakes up that CPU's migration thread?  I 
will take a look at this.

My suggestion was to queue up a dummy 'struct migration_req' up with 
it (change migration_req::task == NULL to mean 'nothing') and simply 
wake it up using wake_up_process().

OK.  I was thinking of just using wake_up_process() without the
migration_req structure, and unconditionally setting a per-CPU
variable from within migration_thread() just before the list_empty()
check.  In your approach we would need a NULL-pointer check just
before the call to __migrate_task().

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That will force a quiescent state, without the need for any extra 
information, right?

Yep!

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This is what the scheduler code does, roughly:

                wake_up_process(rq->migration_thread);
                wait_for_completion(&req.done);

and this will always have to perform well. The 'req' could be put 
into PER_CPU, and a loop could be done like this:

	for_each_online_cpu(cpu)
                wake_up_process(cpu_rq(cpu)->migration_thread);

	for_each_online_cpu(cpu)
                wait_for_completion(&per_cpu(req, cpu).done);

hm?

My concern is the linear slowdown for large systems, but this 
should be OK for modest systems (a few 10s of CPUs).  However, I 
will try it out -- it does not need to be a long-term solution, 
after all.

I think there is going to be a linear slowdown no matter what - 
because sending that many IPIs is going to be linear. (there are 
no 'broadcast to all' IPIs anymore - on x86 we only have them if 
all physical APIC IDs are 7 or smaller.)

With the current code, agreed.  One could imagine making an IPI 
tree, so that a given CPU IPIs (say) eight subordinates.  Making 
this work nice with CPU hotplug would be entertaining, to say the 
least.

Certainly! :-)

As a general note, unrelated to your patches: i think our 
CPU-hotplug related complexity seems to be a bit too much. This is 
really just a gut feeling - from having seen many patches that also 
have hotplug notifiers.

I'm wondering whether this is because it's structured in a 
suboptimal way, or because i'm (intuitively) under-estimating the 
complexity of what it takes to express what happens when a CPU is 
offlined and then onlined?

I suppose that I could take this as a cue to reminisce about the 
old days in a past life with a different implementation of CPU 
online/offline, but life is just too short for that sort of thing.  
Not that guys my age let that stop them.  ;-)

And in that past life, exercising CPU online/offline usually 
exposed painful bugs in new code, so I cannot claim that the 
old-life approach to CPU hotplug was perfect.  Interestingly 
enough, running uniprocessor also exposed painful bugs more often 
than not.  Of course, the only way to run uniprocessor was to 
offline all but one of the CPUs, so you would hit the 
online/offline bugs before hitting the uniprocessor-only bugs.

The thing that worries me most about CPU hotplug in Linux is that 
there is no clear hierarchy of CPU function in the offline 
process, given that the offlining process invokes notifiers in the 
same order as does the onlining process.  Whether this is a real 
defect in the CPU hotplug design or is instead simply a symptom of 
my not yet being fully comfortable with the two-phase CPU-removal 
process is an interesting question to which I do not have an 
answer.

I strongly believe it's the former.

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Either way, the thought process is different.  In my old life, 
CPUs shed roles in the opposite order that they acquired them.  

Yeah, that looks a whole lot more logical to do.

Hmmm...  Making the transition work nicely would require some thought.
It might be good to retain the two-phase nature, even when reversing
the order of offline notifications.  This would address one disadvantage
of the past-life version, which was unnecessary migration of processes
off of the CPU in question, only to find that a later notifier aborted
the offlining.

So only the first phase is permitted to abort the offlining of the CPU,
and this first phase must also set whatever state is necessary to prevent
some later operation from making it impossible to offline the CPU.
The second phase would unconditionally take the CPU out of service.
In theory, this approach would allow incremental conversion of the
notifiers, waiting to remove the stop_machine stuff until all notifiers
had been converted.

If this actually works out, the sequence of changes would be as follows:

1.	Reverse the order of the offline notifications, fixing any
	bugs induced/exposed by this change.

2.	Incrementally convert notifiers to the new mechanism.  This
	will require more thought.

3.	Get rid of the stop_machine and the CPU_DEAD once all are
	converted.

Or we might find that simply reversing the order (#1 above) suffices.

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This meant that a given CPU was naturally guaranteed to be 
correctly taking interrupts for the entire time that it was 
capable of running user-level processes. Later in the offlining 
process, it would still take interrupts, but would be unable to 
run user processes.  Still later, it would no longer be taking 
interrupts, and would stop participating in RCU and in the global 
TLB-flush algorithm.  There was no need to stop the whole machine 
to make a given CPU go offline, in fact, most of the work was done 
by the CPU in question.

In the case of RCU, this meant that there was no need for 
double-checking for offlined CPUs, because CPUs could reliably 
indicate a quiescent state on their way out.

On the other hand, there was no equivalent of dynticks in the old 
days. And it is dynticks that is responsible for most of the 
complexity present in force_quiescent_state(), not CPU hotplug.

So I cannot hold up RCU as something that would be greatly 
simplified by changing the CPU hotplug design, much as I might 
like to.  ;-)

We could probably remove a fair bit of dynticks complexity by 
removing non-dynticks and removing non-hrtimer. People could still 
force a 'periodic' interrupting mode (if they want, or if their hw 
forces that), but that would be a plain periodic hrtimer firing off 
all the time.

Hmmm...  That would not simplify RCU much, but on the other hand (1) the
rcutree.c dynticks approach is already quite a bit simpler than the
rcupreempt.c approach and (2) doing this could potentially simplify
other things.

							Thanx, Paul