Re: [PATCH v2 2/4] sched/fair: add util_est on top of PELT

[Peter Zijlstra]

On Fri, Dec 15, 2017 at 02:02:18PM +0000, Patrick Bellasi wrote:
> On 13-Dec 17:05, Peter Zijlstra wrote:
> > On Tue, Dec 05, 2017 at 05:10:16PM +0000, Patrick Bellasi wrote:
> > > +	if (cfs_rq->nr_running > 0) {
> > > +		util_est  = cfs_rq->util_est_runnable;
> > > +		util_est -= task_util_est(p);
> > > +		if (util_est < 0)
> > > +			util_est = 0;
> > > +		cfs_rq->util_est_runnable = util_est;
> > > +	} else {
> > 
> > I'm thinking that's an explicit load-store to avoid intermediate values
> > landing in cfs_rq->util_esp_runnable, right?
> 
> Was mainly to have an unsigned util_est for the following "sub"...
> 
> 
> > That would need READ_ONCE() / WRITE_ONCE() I think, without that the
> > compiler is free to munge the lot together.
> 
> ... do we still need the {READ,WRITE}_ONCE() in this case?
> I guess adding them however does not hurts.

I think the compiler is free to munge it into something like:

	cfs_rq->util_est_runnable -= task_util_est();
	if (cfs_rq->util_est_runnable < 0)
		cfs_rq->util_est_runnable = 0

and its a fairly simple optimization at that, it just needs to observe
that util_est is an alias for cfs_rq->util_est_runnable.

Using the volatile load/store completely destroys that optimization
though, so yes, I'd say its definitely needed.