netdev - Re: [RFC PATCH] net: frag limit checks need to use percpu_counter

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Date:   Thu, 31 Aug 2017 08:58:02 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     liujian56@...wei.com, netdev@...r.kernel.org,
        Florian Westphal <fw@...len.de>
Subject: Re: [RFC PATCH] net: frag limit checks need to use
 percpu_counter_compare

On Thu, 31 Aug 2017 12:20:19 +0200
Jesper Dangaard Brouer <brouer@...hat.com> wrote:

> +static inline bool frag_mem_over_limit(struct netns_frags *nf, int thresh)
>  {
> -	return percpu_counter_read(&nf->mem);
> +	/* When reading counter here, __percpu_counter_compare() call
> +	 * will invoke __percpu_counter_sum() when needed.  Which
> +	 * depend on num_online_cpus()*batch size, as each CPU can
> +	 * potentential can hold a batch count.
> +	 *
> +	 * With many CPUs this heavier sum operation will
> +	 * unfortunately always occur.
> +	 */
> +	if (__percpu_counter_compare(&nf->mem, thresh,
> +				     frag_percpu_counter_batch) > 0)
> +		return true;
> +	else
> +		return false;

You don't need an if() here.

	return __percpu_counter_compare(&nf->mem, thresh,
				     frag_percpu_counter_batch) > 0;