Re: [PATCH?] Livelock in pick_next_task_fair() / idle_balance()

[Yuyang Du <yuyang.du@intel.com>]

On Fri, Jul 03, 2015 at 06:38:31PM +0200, Peter Zijlstra wrote:
> > I'm not against having a policy that sits somewhere in between, we just
> > have to agree it is the right policy and clean up the load-balance code
> > such that the implemented policy is clear.
> 
> Right, for balancing its a tricky question, but mixing them without
> intent is, as you say, a bit of a mess.
> 
> So clearly blocked load doesn't make sense for (new)idle balancing. OTOH
> it does make some sense for the regular periodic balancing, because
> there we really do care mostly about the averages, esp. so when we're
> overloaded -- but there are issues there too.
> 
> Now we can't track them both (or rather we could, but overhead).
> 
> I like Yuyang's load tracking rewrite, but it changes exactly this part,
> and I'm not sure I understand the full ramifications of that yet.
 
Thanks. It would be a pure average policy, which is non-perfect like now,
and certainly needs a mixing like now, but it is worth a starter, because
it is simple and reasaonble, and based on it, the other parts can be simple
and reasonable.

> One way out would be to split the load balancer into 3 distinct regions;
> 
>  1) get a task on every CPU, screw everything else.
>  2) get each CPU fully utilized, still ignoring 'load'
>  3) when everybody is fully utilized, consider load.
> 
> If we make find_busiest_foo() select one of these 3, and make
> calculate_imbalance() invariant to the metric passed in, and have things
> like cpu_load() and task_load() return different, but coherent, numbers
> depending on which region we're in, this almost sounds 'simple'.
> 
> The devil is in the details, and the balancer is a hairy nest of details
> which will make the above non-trivial.
> 
> But for 1) we could simply 'balance' on nr_running, for 2) we can
> 'balance' on runnable_avg and for 3) we'll 'balance' on load_avg (which
> will then include blocked load).
> 
> Let me go play outside for a bit so that it can sink in what kind of
> nonsense my heat addled brain has just sprouted :-)