Re: sched: Improve load balancing in the presence of idle CPUs

[Srikar Dronamraju <srikar@linux.vnet.ibm.com>]

* Jason Low <jason.low2@hp.com> [2015-04-08 19:39:15]:

> On Wed, 2015-04-08 at 16:42 +0530, Srikar Dronamraju wrote:
> > * Jason Low <jason.low2@hp.com> [2015-04-07 17:07:46]:
> > > @@ -7687,7 +7700,7 @@ static inline bool nohz_kick_needed(struct rq *rq)
> > >  	int nr_busy, cpu = rq->cpu;
> > >  	bool kick = false;
> > > 
> > > -	if (unlikely(rq->idle_balance))
> > > +	if (unlikely(idle_cpu(cpu)))
> > >  		return false;
> > 
> > 
> > The only other place that we use idle_balance is
> > run_rebalance_domains(). Would it make sense to just use idle_cpu() in
> > run_rebalance_domains() and remove rq->idle_balance?
> 
> Hi Srikar,
> 
> So the idle_balance is used for storing the idle state of the CPU before
> calling the softirq, for load balancing decisions. In that case, we may
> need to keep this extra variable in order to store that information.
> 


I am not sure if you got what I wanted to convey.

rq->idle_balance gets updated at every scheduler_tick() but the only user of
rq->idle_balance (after your change) seems to be run_rebalance_domains().
Now can we remove rq->idle_balance. This would mean we would have to
call idle_cpu() instead of using rq->idle_balance in
run_rebalance_domains(). (similar to what your above change)

That way we can reduce the rq struct size and we might end up calling
idle_cpu() lesser number of times.

-- 
Thanks and Regards
Srikar Dronamraju