linux-kernel - Re: [PATCH 43/54] drivers/hv: replace cpumask_weight with cpumask_weight

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Message-ID: <87sftdij76.fsf@redhat.com>
Date:   Mon, 24 Jan 2022 10:20:13 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Yury Norov <yury.norov@...il.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michał Mirosław <mirq-linux@...e.qmqm.pl>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        David Laight <David.Laight@...lab.com>,
        Joe Perches <joe@...ches.com>, Dennis Zhou <dennis@...nel.org>,
        Emil Renner Berthing <kernel@...il.dk>,
        Nicholas Piggin <npiggin@...il.com>,
        Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
        Alexey Klimov <aklimov@...hat.com>,
        linux-kernel@...r.kernel.org,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
        linux-hyperv@...r.kernel.org
Subject: Re: [PATCH 43/54] drivers/hv: replace cpumask_weight with
 cpumask_weight_eq

Yury Norov <yury.norov@...il.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@...il.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