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

[Yury Norov <yury.norov@gmail.com>]

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.