Re: [PATCH RFC] v5 expedited "big hammer" RCU grace periods


* Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote:

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

* Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote:

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

* Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote:

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

* Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote:

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

* Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote:

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().

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

Yep!

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.

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.

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.

Ingo
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