Re: [PATCH v6 04/16] sched/core: uclamp: Add CPU's clamp buckets refcounting

Patrick Bellasi
2019-01-21T15:54:07Z

On 21-Jan 16:17, Peter Zijlstra wrote:
> On Tue, Jan 15, 2019 at 10:15:01AM +0000, Patrick Bellasi wrote:
> > +#ifdef CONFIG_UCLAMP_TASK
>
> > +struct uclamp_bucket {
> > + unsigned long value : bits_per(SCHED_CAPACITY_SCALE);
> > + unsigned long tasks : BITS_PER_LONG - bits_per(SCHED_CAPACITY_SCALE);
> > +};
>
> > +struct uclamp_cpu {
> > + unsigned int value;
>
> /* 4 byte hole */
>
> > + struct uclamp_bucket bucket[UCLAMP_BUCKETS];
> > +};
>
> With the default of 5, this UCLAMP_BUCKETS := 6, so struct uclamp_cpu
> ends up being 7 'unsigned long's, or 56 bytes on 64bit (with a 4 byte
> hole).

Yes, that's dimensioned and configured to fit into a single cache line
for all the possible 5 (by default) clamp values of a clamp index
(i.e. min or max util).

>
> > +#endif /* CONFIG_UCLAMP_TASK */
> > +
> > /*
> > * This is the main, per-CPU runqueue data structure.
> > *
> > @@ -835,6 +879,11 @@ struct rq {
> > unsigned long nr_load_updates;
> > u64 nr_switches;
> >
> > +#ifdef CONFIG_UCLAMP_TASK
> > + /* Utilization clamp values based on CPU's RUNNABLE tasks */
> > + struct uclamp_cpu uclamp[UCLAMP_CNT] ____cacheline_aligned;
>
> Which makes this 112 bytes with 8 bytes in 2 holes, which is short of 2
> 64 byte cachelines.

Right, we have 2 cache lines where:
- the first $L tracks 5 different util_min values
- the second $L tracks 5 different util_max values

> Is that the best layout?

It changed few times and that's what I found more reasonable for both
for fitting the default configuration and also for code readability.
Notice that we access RQ and SE clamp values with the same patter,
for example:

{rq|p}->uclamp[clamp_idx].value

Are you worried about the holes or something else specific ?

> > +#endif
> > +
> > struct cfs_rq cfs;
> > struct rt_rq rt;
> > struct dl_rq dl;
> > --
> > 2.19.2
> >

--
#include <best/regards.h>

Patrick Bellasi