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

From: Ingo Molnar <[email-masked]>
Date: Tue, 19 May 2009 10:58:25 +0200

* Paul E. McKenney <[email-masked]> wrote:

> On Mon, May 18, 2009 at 05:42:41PM +0200, Ingo Molnar wrote:
> > 
> > * Paul E. McKenney <[email-masked]> 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