Re: [PATCH v5 1/4] sched/fair: add util_est on top of PELT

[Peter Zijlstra]

On Thu, Feb 22, 2018 at 05:01:50PM +0000, Patrick Bellasi wrote:
> +struct util_est {
> +	unsigned int			enqueued;
> +	unsigned int			ewma;
> +#define UTIL_EST_WEIGHT_SHIFT		2
> +};

> +	ue = READ_ONCE(p->se.avg.util_est);

> +	WRITE_ONCE(p->se.avg.util_est, ue);

That is actually quite dodgy... and relies on the fact that we have the
8 byte case in __write_once_size() and __read_once_size()
unconditionally. It then further relies on the compiler DTRT for 32bit
platforms, which is generating 2 32bit loads/stores.

The advantage is of course that it will use single u64 loads/stores
where available.