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SubjectRe: [PATCH v3] sched/fair: update scale invariance of PELT
On Fri, May 18, 2018 at 11:17:38AM +0100, Patrick Bellasi wrote:
> On 18-May 11:36, Peter Zijlstra wrote:
> >
> > Replying to the latest version available; given the current interest I
> > figure I'd re-read some of the old threads and look at this stuff again.
> >
> > On Fri, Apr 28, 2017 at 04:23:55PM +0200, Vincent Guittot wrote:
> >
> > > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > > index 0978fb7..f8dde36 100644
> > > --- a/include/linux/sched.h
> > > +++ b/include/linux/sched.h
> > > @@ -313,6 +313,7 @@ struct load_weight {
> > > */
> > > struct sched_avg {
> > > u64 last_update_time;
> > > + u64 stolen_idle_time;
> > > u64 load_sum;
> > > u32 util_sum;
> > > u32 period_contrib;
> >
> > Right, so sadly Patrick stole that space with the util_est bits.
>
> Sorry :(
>
> However, I remember we already talked about the idea to
> update load_avg and runnable_load_avg to use u32:
>
> https://marc.info/?l=linux-kernel&m=151334269426419&w=2
>

Sadly not I fear, (runnable_)load for entities is obviously limited in
range, but we use the same struct sched_avg for the cfs_rq aggregates,
and there is no real limit on the total weight of a cfs_rq.

However, I think we can shrink util_avg unconditionally, which would
give us exactly the 4 byte hole we need.

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 959a8588e365..670b53b10a68 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -402,7 +402,8 @@ struct sched_avg {
u32 period_contrib;
unsigned long load_avg;
unsigned long runnable_load_avg;
- unsigned long util_avg;
+ u32 util_avg;
+ u32 stolen_idle_time;
struct util_est util_est;
} ____cacheline_aligned;

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