Re: [PATCH] sched/fair: Fix cfs_rq avg tracking underflow

[Yuyang Du]

On Fri, Jun 17, 2016 at 11:19:48AM +0200, Peter Zijlstra wrote:
> +/*
> + * Unsigned subtract and clamp on underflow.
> + *
> + * Explicitly do a load-store to ensure the intermediate value never hits
> + * memory. This allows lockless observations without ever seeing the negative
> + * values.
> + */
> +#define sub_positive(_ptr, _val) do {				\
> +	typeof(_ptr) ptr = (_ptr);				\
> +	typeof(*ptr) val = (_val);				\
> +	typeof(*ptr) res, var = READ_ONCE(*ptr);		\
> +	res = var - val;					\
> +	if (res > var)						\
> +		res = 0;					\
> +	WRITE_ONCE(*ptr, res);					\
> +} while (0)
> +

maybe sub_nonnegative() or sub_til_zero() ...

I wonder whether it is the if statement finally allows GCC to 'registerize'
load_avg, and I almost tried it...