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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     "Michael Kelley (LINUX)" <mikelley@microsoft.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" <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>,
        Yury Norov <yury.norov@gmail.com>
Subject: RE: [PATCH 43/54] drivers/hv: replace cpumask_weight with
 cpumask_weight_eq
In-Reply-To: <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>
 <MWHPR21MB1593C8511E18E4539CECAEC5D7219@MWHPR21MB1593.namprd21.prod.outlook.com>
Date:   Fri, 28 Jan 2022 10:31:52 +0100
Message-ID: <8735l8gq9j.fsf@redhat.com>
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"Michael Kelley (LINUX)" <mikelley@microsoft.com> writes:

> From: Vitaly Kuznetsov <vkuznets@redhat.com> Sent: Monday, January 24, 2022 1:20 AM
>> 
>> Yury Norov <yury.norov@gmail.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