linux-kernel - Re: [PATCH v2] hv_netvsc: Add per-cpu ethtool stats for netvsc

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Date:   Wed, 13 Jun 2018 13:57:33 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Yidong Ren <yidren@...uxonhyperv.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "David S. Miller" <davem@...emloft.net>,
        devel@...uxdriverproject.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Yidong Ren <yidren@...rosoft.com>
Subject: Re: [PATCH v2] hv_netvsc: Add per-cpu ethtool stats for netvsc



On 06/13/2018 12:36 PM, Yidong Ren wrote:
> From: Yidong Ren <yidren@...rosoft.com>
> 
> This patch implements following ethtool stats fields for netvsc:
> cpu<n>_tx/rx_packets/bytes
> cpu<n>_vf_tx/rx_packets/bytes

...
>  
> +	pcpu_sum = alloc_percpu(struct netvsc_ethtool_pcpu_stats);
> +	netvsc_get_pcpu_stats(dev, pcpu_sum);
> +	for_each_present_cpu(cpu) {
> +		struct netvsc_ethtool_pcpu_stats *this_sum =
> +			per_cpu_ptr(pcpu_sum, cpu);
> +		for (j = 0; j < ARRAY_SIZE(pcpu_stats); j++)
> +			data[i++] = *(u64 *)((void *)this_sum
> +					     + pcpu_stats[j].offset);
> +	}
> +	free_percpu(pcpu_sum);
>


Using alloc_percpu() / free_percpu() for a short section of code makes no sense.

You actually want to allocate memory local to this cpu, possibly in one chunk,
not spread all over the places.

kvmalloc(nr_cpu_ids * sizeof(struct netvsc_ethtool_pcpu_stats))  should be really better,
since it would most of the time be satisfied by a single kmalloc()