Messages in this thread | | | Date | Mon, 22 Nov 2021 21:56:46 +0200 | From | Andy Shevchenko <> | Subject | Re: [PATCH 1/4] bitops: Add single_bit_set() |
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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
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