Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     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>,
        =?utf-8?Q?Micha=C5=82_Miros=C5=82aw?= <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,
        "K. Y. 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
Subject: Re: [PATCH 43/54] drivers/hv: replace cpumask_weight with
 cpumask_weight_eq
In-Reply-To: <20220123183925.1052919-44-yury.norov@gmail.com>
References: <20220123183925.1052919-1-yury.norov@gmail.com>
 <20220123183925.1052919-44-yury.norov@gmail.com>
Date:   Mon, 24 Jan 2022 10:20:13 +0100
Message-ID: <87sftdij76.fsf@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

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