netdev - Re: [PATCH net v6 1/2] net: sched: tbf: fix the calculation of max

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Date:	Mon, 9 Dec 2013 11:26:02 +0800
From:	Yang Yingliang <yangyingliang@...wei.com>
To:	David Laight <David.Laight@...LAB.COM>, <davem@...emloft.net>,
	<netdev@...r.kernel.org>
CC:	<eric.dumazet@...il.com>, <brouer@...hat.com>, <jpirko@...hat.com>,
	<jbrouer@...hat.com>
Subject: Re: [PATCH net v6 1/2] net: sched: tbf: fix the calculation of max_size

On 2013/12/6 18:56, David Laight wrote:
>> From: Yang Yingliang
> ...
>>
>> +/* Time to Length, convert time in ns to length in bytes
>> + * to determinate how many bytes can be sent in given time.
>> + */
>> +static u64 psched_ns_t2l(const struct psched_ratecfg *r,
>> +			 u64 time_in_ns)
>> +{
>> +	/* The formula is :
>> +	 * len = (time_in_ns * r->rate_bytes_ps) / NSEC_PER_SEC
>> +	 */
>> +	u64 len = time_in_ns * r->rate_bytes_ps;
>> +
>> +	do_div(len, NSEC_PER_SEC);
> 
> You are multiplying two values then dividing by 10**9
> I'd guess that the intermediate value might exceed 2**64.
> 
>> +	if (unlikely(r->linklayer == TC_LINKLAYER_ATM))
>> +		len = (len / 53) * 48;
> 
> You probably want to do the multiply first.
> But why not scale rate_bytes_ps instead.
> 

When the linklayer is ATM, the formula to calculate tokens is:

((u64)(DIV_ROUND_UP(len,48)*53) * r->mult) >> r->shift

So, I scale len here.

Regards,
Yang


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