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

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Message-ID: <20180613151854.25be8b33@xeon-e3>
Date:   Wed, 13 Jun 2018 15:18:54 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Yidong Ren <Yidong.Ren@...rosoft.com>
Cc:     Yidong Ren <yidren@...uxonhyperv.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH v2] hv_netvsc: Add per-cpu ethtool stats for netvsc

On Wed, 13 Jun 2018 22:03:34 +0000
Yidong Ren <Yidong.Ren@...rosoft.com> wrote:

> > From: devel <driverdev-devel-bounces@...uxdriverproject.org> On Behalf
> > Of Stephen Hemminger  
> > > +/* statistics per queue (rx/tx packets/bytes) */ #define
> > > +NETVSC_PCPU_STATS_LEN (num_present_cpus() *  
> > ARRAY_SIZE(pcpu_stats))
> > 
> > Even though Hyper-V/Azure does not support hot plug cpu's it might be
> > better to num_cpu_possible to avoid any possible future surprises.  
> 
> That will create a very long output (num_cpu_possible = 128 on my machine) for ethtool,
> While doesn't provide additional info.
> num_present_cpus() would cause problem only if someone removed cpu 
> between netvsc_get_sset_count() and netvsc_get_strings() and netvsc_get_ethtool_stats(). 
> 
> An alternative way could be: Check all stats, and only output if not zero. 
> This need to be done in two pass. First pass to get the correct count, second pass to print the number.
> Is there an elegant way to do this? 

Ok, but there is a race between getting names and getting statistics.
If a cpu was added/removed then statistics would not match.