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From:   "Michael Kelley (LINUX)" <mikelley@microsoft.com>
To:     vkuznets <vkuznets@redhat.com>, Yury Norov <yury.norov@gmail.com>
CC:     Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Andrew Morton <akpm@linux-foundation.org>,
        =?iso-8859-2?Q?Micha=B3_Miros=B3aw?= <mirq-linux@rere.qmqm.pl>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        David Laight <David.Laight@aculab.com>,
        Joe Perches <joe@perches.com>, Dennis Zhou <dennis@kernel.org>,
        Emil Renner Berthing <kernel@esmil.dk>,
        Nicholas Piggin <npiggin@gmail.com>,
        Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>,
        Alexey Klimov <aklimov@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        KY Srinivasan <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Wei Liu <wei.liu@kernel.org>, Dexuan Cui <decui@microsoft.com>,
        "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>
Subject: RE: [PATCH 43/54] drivers/hv: replace cpumask_weight with
 cpumask_weight_eq
Thread-Topic: [PATCH 43/54] drivers/hv: replace cpumask_weight with
 cpumask_weight_eq
Thread-Index: AQHYEIkT9Q6vYA1gsUeLLWD0pXcM+qxx5hyAgAUUIpA=
Date:   Thu, 27 Jan 2022 15:02:05 +0000
Message-ID: <MWHPR21MB1593C8511E18E4539CECAEC5D7219@MWHPR21MB1593.namprd21.prod.outlook.com>
References: <20220123183925.1052919-1-yury.norov@gmail.com>
 <20220123183925.1052919-44-yury.norov@gmail.com> <87sftdij76.fsf@redhat.com>
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From: Vitaly Kuznetsov <vkuznets@redhat.com> Sent: Monday, January 24, 2022=
 1:20 AM
>=20
> Yury Norov <yury.norov@gmail.com> writes:
>=20
> > init_vp_index() calls cpumask_weight() to compare the weights of cpumas=
ks
> > We can do it more efficiently with cpumask_weight_eq because conditiona=
l
> > cpumask_weight may stop traversing the cpumask earlier (at least one), =
as
> > soon as condition is met.
>=20
> 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.
>=20
> >
> > 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 *cha=
nnel)
> >  		}
> >  		alloced_mask =3D &hv_context.hv_numa_map[numa_node];
> >
> > -		if (cpumask_weight(alloced_mask) =3D=3D
> > -		    cpumask_weight(cpumask_of_node(numa_node))) {
> > +		if (cpumask_weight_eq(alloced_mask,
> > +			    cpumask_weight(cpumask_of_node(numa_node)))) {
>=20
> This code is not performace critical and I prefer the old version:
>=20
>  	cpumask_weight() =3D=3D cpumask_weight()
>=20
>  looks better than
>=20
> 	cpumask_weight_eq(..., cpumask_weight())
>=20
> (let alone the inner cpumask_of_node()) to me.
>=20
> >  			/*
> >  			 * We have cycled through all the CPUs in the node;
> >  			 * reset the alloced map.
>=20
> --
> 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 w=
ay
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