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

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jan 21, 2019 at 03:54:07PM +0000, Patrick Bellasi wrote:
> 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).

And I suppose you picked 5 because 20% is a 'nice' number? whereas
16./666/% is a bit odd?

> > > +#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

Well, not quite so, if you want that you should put
____cacheline_aligned on struct uclamp_cpu. Such that the individual
array entries are each aligned, the above only alignes the whole array,
so the second uclamp_cpu is spread over both lines.

But I think this is actually better, since you have to scan both
min/max anyway, and allowing one the straddle a line you have to touch
anyway, allows for using less lines in total.

Consider for example the case where UCLAMP_BUCKETS=8, then each
uclamp_cpu would be 9 words or 72 bytes. If you force align the member,
then you end up with 4 lines, whereas now it would be 3.

> > 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 ?

Not sure; just mostly asking if this was by design or by accident.

One thing I did wonder though; since bucket[0] is counting the tasks
that are unconstrained and it's bucket value is basically fixed (0 /
1024), can't we abuse that value field to store uclamp_cpu::value ?

OTOH, doing that might make the code really ugly with all them:

  if (!bucket_id)

exceptions all over the place.