netdev - Re: [PATCH v3] hv_netvsc: Add per-cpu ethtool stats for netvsc

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Message-ID: <87k1pkrj84.fsf@vitty.brq.redhat.com>
Date:   Tue, 24 Jul 2018 13:00:59 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Yidong Ren <Yidong.Ren@...rosoft.com>
Cc:     KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "David S. Miller" <davem@...emloft.net>,
        "devel\@linuxdriverproject.org" <devel@...uxdriverproject.org>,
        "netdev\@vger.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        Madhan Sivakumar <madhans@...rosoft.com>
Subject: Re: [PATCH v3] hv_netvsc: Add per-cpu ethtool stats for netvsc

Yidong Ren <Yidong.Ren@...rosoft.com> writes:

>> From: Yidong Ren <yidren@...uxonhyperv.com>
>> Sent: Monday, July 23, 2018 6:26 PM
>> +	pcpu_sum = kvmalloc(sizeof(struct netvsc_ethtool_pcpu_stats) *
>> +			num_present_cpus(), GFP_KERNEL);
>
> Since there is no plan for CPU hotplug in Hyper-V in short term, it is fine 
> to use num_present_cpus for now. We can move to debugfs later if necessary.

While you do for_each_present_cpu() in netvsc_get_ethtool_stats(),
netvsc_get_pcpu_stats() does for_each_possible_cpu(). This looks
inconsistent.

The allocation you're doing here is short-lived so I would suggest you
use possible_cpus everywhere. Even knowing there's no CPU hotplug on
Hyper-V at this moment, it can appear later and we'll get a hard-to-find
issue. Moreover, we may consider using netvsc driver on e.g. KVM with
Hyper-V enlightenments and KVM has CPU hotplug already.

-- 
  Vitaly