netdev - Re: [PATCH v6 net-next 2/5] net: implement support for low latency socket polling

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Message-ID: <1369834647.5109.15.camel@edumazet-glaptop>
Date:	Wed, 29 May 2013 06:37:27 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Eliezer Tamir <eliezer.tamir@...ux.intel.com>
Cc:	David Miller <davem@...emloft.net>, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org,
	Jesse Brandeburg <jesse.brandeburg@...el.com>,
	Don Skidmore <donald.c.skidmore@...el.com>,
	e1000-devel@...ts.sourceforge.net,
	Willem de Bruijn <willemb@...gle.com>,
	Andi Kleen <andi@...stfloor.org>, HPA <hpa@...or.com>,
	Eilon Greenstien <eilong@...adcom.com>,
	Or Gerlitz <or.gerlitz@...il.com>,
	Alex Rosenbaum <alexr@...lanox.com>,
	Eliezer Tamir <eliezer@...ir.org.il>
Subject: Re: [PATCH v6 net-next 2/5] net: implement support for low latency
 socket polling

On Wed, 2013-05-29 at 09:39 +0300, Eliezer Tamir wrote:

> +/* we don't mind a ~2.5% imprecision */
> +#define TSC_MHZ (tsc_khz >> 10)
> +
> +static inline unsigned long ll_end_time(void)
> +{
> +	return TSC_MHZ * ACCESS_ONCE(sysctl_net_ll_poll) + get_cycles();
> +}static inline unsigned long ll_end_time(void)
>+{
>+	return TSC_MHZ * ACCESS_ONCE(sysctl_net_ll_poll) + get_cycles();
>+}

This can overflow.

Multiply is giving 32bits, as tsc_khz is an int, and sysctl_net_ll_poll
is an int.

unsigned long sysctl_net_ll_poll ?

Also, if we want this to work on i386, the correct type to use for
ll_end_time(void) would be cycles_t



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