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

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Date:   Fri, 28 Jan 2022 10:31:52 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     "Michael Kelley (LINUX)" <mikelley@...rosoft.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" <linux-kernel@...r.kernel.org>,
        KY 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" <linux-hyperv@...r.kernel.org>,
        Yury Norov <yury.norov@...il.com>
Subject: RE: [PATCH 43/54] drivers/hv: replace cpumask_weight with
 cpumask_weight_eq

"Michael Kelley (LINUX)" <mikelley@...rosoft.com> writes:

> From: Vitaly Kuznetsov <vkuznets@...hat.com> Sent: Monday, January 24, 2022 1:20 AM
>> 
>> Yury Norov <yury.norov@...il.com> writes:
>> 
...
>> >
>> > 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.
>> 
> 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))
>

Indeed. While it seems that only CPUs from 'cpumask_of_node(numa_node)'
can be set in 'alloced_mask' (and thus the comparison is valid), there's
no real need to weigh anything. I'll send a patch.

-- 
Vitaly