Re: [PATCH v3 2/3] sched/fair: use util_est in LB and WU paths

[Patrick Bellasi <patrick.bellasi@arm.com>]

On 25-Jan 20:03, Pavan Kondeti wrote:
> On Wed, Jan 24, 2018 at 07:31:38PM +0000, Patrick Bellasi wrote:
> > 
> > > > +       /*
> > > > +        * These are the main cases covered:
> > > > +        * - if *p is the only task sleeping on this CPU, then:
> > > > +        *      cpu_util (== task_util) > util_est (== 0)
> > > > +        *   and thus we return:
> > > > +        *      cpu_util_wake = (cpu_util - task_util) = 0
> > > > +        *
> > > > +        * - if other tasks are SLEEPING on the same CPU, which is just waking
> > > > +        *   up, then:
> > > > +        *      cpu_util >= task_util
> > > > +        *      cpu_util > util_est (== 0)
> > > > +        *   and thus we discount *p's blocked utilization to return:
> > > > +        *      cpu_util_wake = (cpu_util - task_util) >= 0
> > > > +        *
> > > > +        * - if other tasks are RUNNABLE on that CPU and
> > > > +        *      util_est > cpu_util
> > > > +        *   then we use util_est since it returns a more restrictive
> > > > +        *   estimation of the spare capacity on that CPU, by just considering
> > > > +        *   the expected utilization of tasks already runnable on that CPU.
> > > > +        */
> > > > +       util_est = cpu_rq(cpu)->cfs.util_est_runnable;
> > > > +       util = max(util, util_est);
> > > > +
> > > > +       return util;
> > 
> > I should instead clamp util before returning it! ;-)
> > 
> > > May be a separate patch to remove  the clamping part?
> > 
> > No, I think we should keep cpu_util_wake clamped to not affect the existing
> > call sites. I just need to remove it where not needed (done) and add it where
> > needed (will do on the next iteration).
> 
> cpu_util_wake() is called only from capacity_spare_wake(). There are no other
> callsites.

True...

> The capacity_spare_wake() is clamping the return value of
> cpu_util_wake() to CPU capacity.

... actually it's clamping negative numbers with:

   max_t(long, capacity_of(cpu) - cpu_util_wake(cpu, p), 0);

thus, by having cpu_util_wake returning potentially a value which is
bigger then capacity_of or capacity_orig_of we should not have issues
from a capacity_spare_wake() usage standpoint.

> The clamping is not needed, I think.

However, we can still argue that the cpu_util_wake() should never
return something bigger then the maximum possible capacity of a CPU.
At least that's the feature so fare.

Thus, even just for the sake of consistency, with previous returns
paths (e.g. when we bail out returning cpu_util), I would say that
it's worth to maintain this semantics.

With a clamping, all these functions:
 - cpu_util
 - cpu_util_est
 - cpu_util_wake
will always return a signal which is never bigger then the maximum
possible CPU capacity.

-- 
#include <best/regards.h>

Patrick Bellasi