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

From: Paul E. McKenney
Date: Tue May 19 2009 - 08:33:30 EST


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.

Thanx, Paul
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