linux-kernel - Re: [PATCH] sched/fair: Fix cfs

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Message-ID: <20160617020109.GQ8105@intel.com>
Date:	Fri, 17 Jun 2016 10:01:09 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Chris Wilson <chris@...is-wilson.co.uk>,
	Andrey Ryabinin <aryabinin@...tuozzo.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Mike Galbraith <efault@....de>,
	Thomas Gleixner <tglx@...utronix.de>, bsegall@...gle.com,
	morten.rasmussen@....com, pjt@...gle.com, steve.muckle@...aro.org,
	linux-kernel@...r.kernel.org, kernel@...p.com
Subject: Re: [PATCH] sched/fair: Fix cfs_rq avg tracking underflow

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...