Re: [PATCH 1/4] bitops: Add single_bit_set()

["Vaittinen, Matti" <Matti.Vaittinen@fi.rohmeurope.com>]

Morning <please apply local timezone> Yuru, & all,

On 11/22/21 19:54, Yury Norov wrote:
> On Mon, Nov 22, 2021 at 02:57:27PM +0200, Andy Shevchenko wrote:
>> On Mon, Nov 22, 2021 at 12:42:21PM +0000, Vaittinen, Matti wrote:
>>> On 11/22/21 13:28, Andy Shevchenko wrote:
>>>> On Mon, Nov 22, 2021 at 01:03:25PM +0200, Matti Vaittinen wrote:
>>>>> There are cases when it is useful to check a bit-mask has only one bit
>>>>> set. Add a generic helper for it instead of baking own one for each
>>>>> user.
>>
>>>> So, you decided to reinvent hamming weight...
>>>> Please, drop this patch and use corresponding hweight() call.
>>
>>> Thanks Andy.
>>>
>>> There are few differences to hamming weight here. We scan only given
>>> amount of bits - and we will end scanning immediately when we hit second
>>> set bit. Oh, and obviously we only return information whether there is
>>> exactly one bit set. So no, this is not hamming weight().
>>
>> What do you mean by this?
>>
>> hweight() will return you the number of the non-zero elements in the set.
>> In application to boolean based arrays it means the number of bits that
>> are set. Obviously, the condition `hweight() == 1` is what you are looking
>> for.
> 
> Hi Andy,
> 
> I think, Matti means earlier return when part of bitmap counts set
> bits to a greater nubmer, and we can skip the rest. Right, Matti?

Yes.

> But in general, it might be useful for long bitmaps.
> 
> The more complete way of doing this would be adding a new set of
> functions: bitmap_weight_{eq,neq,gt,le}
> 
> I'm looking at how bitmap_weight is used in the kernel and see
> quite a lot of places where this optimization may take place. For
> example otx2_remove_flow() in drivers/net/ethernet/marvell/octeontx2/nic/otx2_flows.c:
> 
>          if (bitmap_weight(&flow_cfg->dmacflt_bmap,
>                            flow_cfg->dmacflt_max_flows) == 1)
>                  otx2_update_rem_pfmac(pfvf, DMAC_ADDR_DEL);
> 
> may be replaced with:
> 
>          if (bitmap_weight_eq(&flow_cfg->dmacflt_bmap, flow_cfg->dmacflt_max_flows, 1)
>                  otx2_update_rem_pfmac(pfvf, DMAC_ADDR_DEL);
> 
> Most of that places are in drivers however, and the length of bitmaps
> there is typically small, so that there's no chance to get any
> measurable performance improvement.
> 
> There is always a chance that we have opencoded bitmap_weight_eq()
> et all. If we add these API, it might help people wright better code.
> 
> What do you think?

My uneducated opinion (for what it matters :]) is thet the cost of 
adding such functions is negligible so I am all for adding them if there 
are even few users who can benefit from those.

Best Regards
	--Matti Vaittinen

-- 
The Linux Kernel guy at ROHM Semiconductors

Matti Vaittinen, Linux device drivers
ROHM Semiconductors, Finland SWDC
Kiviharjunlenkki 1E
90220 OULU
FINLAND

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