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

[Andy Shevchenko]

On Mon, Nov 22, 2021 at 09:54:14AM -0800, 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?
> 
> I agree that for Matti's usecase it's useless because 32-bit int is small,
> and hweight() would count set bits with a single machine instruction. (And
> it should be hweight32(), not bitmap_weight() in this case.)
> 
> 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?

Before answering this I would like to see how hweight() is currently being used
in the kernel against bitmaps. Like histogram collection

	comparison number	number of usages
	variadic			X
	1				Y
	2				Z
	...				...


-- 
With Best Regards,
Andy Shevchenko