* 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().
That will force a quiescent state, without the need for any extra
information, right?
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?
Ingo
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