Messages in this thread | | | Date | Mon, 22 Nov 2021 23:33:46 -0800 | From | Yury Norov <> | Subject | Re: [PATCH 1/4] bitops: Add single_bit_set() |
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On Tue, Nov 23, 2021 at 05:26:56AM +0000, Vaittinen, Matti wrote: > 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.
I think I changed my opinion. We have enough examples of opencoded bitmap_weight_{eq,...} in core code which will definitely benefit from this optimization. For example, sched_cpu_activate:
if (cpumask_weight(cpu_smt_mask(cpu)) == 2) static_branch_inc_cpuslocked(&sched_smt_present);
Considering computers with thousands of CPUs, early return would save a lot.
I'll take a look on it at this weekend.
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