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SubjectRE: [PATCH 43/54] drivers/hv: replace cpumask_weight with cpumask_weight_eq
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From: Vitaly Kuznetsov <vkuznets@redhat.com> Sent: Monday, January 24, 2022 1:20 AM
>
> Yury Norov <yury.norov@gmail.com> writes:
>
> > init_vp_index() calls cpumask_weight() to compare the weights of cpumasks
> > We can do it more efficiently with cpumask_weight_eq because conditional
> > cpumask_weight may stop traversing the cpumask earlier (at least one), as
> > soon as condition is met.
>
> Same comment as for "PATCH 41/54": cpumask_weight_eq() can only stop
> earlier if the condition is not met, to prove the equality all bits need
> always have to be examined.
>
> >
> > Signed-off-by: Yury Norov <yury.norov@gmail.com>
> > ---
> > drivers/hv/channel_mgmt.c | 4 ++--
> > 1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c
> > index 60375879612f..7420a5fd47b5 100644
> > --- a/drivers/hv/channel_mgmt.c
> > +++ b/drivers/hv/channel_mgmt.c
> > @@ -762,8 +762,8 @@ static void init_vp_index(struct vmbus_channel *channel)
> > }
> > alloced_mask = &hv_context.hv_numa_map[numa_node];
> >
> > - if (cpumask_weight(alloced_mask) ==
> > - cpumask_weight(cpumask_of_node(numa_node))) {
> > + if (cpumask_weight_eq(alloced_mask,
> > + cpumask_weight(cpumask_of_node(numa_node)))) {
>
> This code is not performace critical and I prefer the old version:
>
> cpumask_weight() == cpumask_weight()
>
> looks better than
>
> cpumask_weight_eq(..., cpumask_weight())
>
> (let alone the inner cpumask_of_node()) to me.
>
> > /*
> > * We have cycled through all the CPUs in the node;
> > * reset the alloced map.
>
> --
> Vitaly

I agree with Vitaly in preferring the old version, and indeed performance
here is a shrug. But actually, I think the old version is a poorly coded way
to determine if the two cpumasks are equal. The following would correctly
capture the intent:

if (cpumask_equal(alloced_mask, cpumask_of_node(numa_node))

Michael



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