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

* Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:

> On Wed, May 20, 2009 at 10:09:24AM +0200, Ingo Molnar wrote:
> >
> > * Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> >
> > > On Tue, May 19, 2009 at 02:44:36PM +0200, Ingo Molnar wrote:
> > > >
> > > > * Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> > > >
> > > > > On Tue, May 19, 2009 at 10:58:25AM +0200, Ingo Molnar wrote:
> > > > > >
> > > > > > * Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> > > > > >
> > > > > > > On Mon, May 18, 2009 at 05:42:41PM +0200, Ingo Molnar wrote:
> > > > > > > >
> > > > > > > > * Paul E. McKenney <paulmck@linux.vnet.ibm.com> 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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