Re: [PATCH bpf-next 2/7] lib: reciprocal_div: implement the improved algorithm on the paper mentioned

From: Daniel Borkmann <daniel@iogearbox.net>
To: Jakub Kicinski <jakub.kicinski@netronome.com>,
	Song Liu <liu.song.a23@gmail.com>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	oss-drivers@netronome.com, Networking <netdev@vger.kernel.org>,
	Jiong Wang <jiong.wang@netronome.com>
Subject: Re: [PATCH bpf-next 2/7] lib: reciprocal_div: implement the improved algorithm on the paper mentioned
Date: Wed, 27 Jun 2018 10:54:29 +0200	[thread overview]
Message-ID: <1a9f5bc2-2c56-ad73-6d01-3455142e477b@iogearbox.net> (raw)
In-Reply-To: <20180626135258.7e80bead@cakuba.netronome.com>

On 06/26/2018 10:52 PM, Jakub Kicinski wrote:
> On Mon, 25 Jun 2018 23:21:10 -0700, Song Liu wrote:
>>> +struct reciprocal_value_adv reciprocal_value_adv(u32 d, u8 prec)
>>> +{
>>> +       struct reciprocal_value_adv R;
>>> +       u32 l, post_shift;
>>> +       u64 mhigh, mlow;
>>> +
>>> +       l = fls(d - 1);
>>> +       post_shift = l;
>>> +       /* NOTE: mlow/mhigh could overflow u64 when l == 32 which means d has
>>> +        * MSB set. This case needs to be handled before calling
>>> +        * "reciprocal_value_adv", please see the comment at
>>> +        * include/linux/reciprocal_div.h.
>>> +        */  
>>
>> Shall we handle l == 32 case better? I guess the concern here is extra
>> handling may slow down the fast path? If that's the case, we should
>> at least add a WARNING on the slow path.
> 
> Agreed, I think Jiong is travelling, hence no response.  We'll respin.

Ok, since there's going to be a respin, I've tossed the current series from
patchwork in that case.

Thanks,
Daniel