netdev - Re: [net 02/11] net/mlx5e: Fix multicast counter not up-to-date in "ip -s"

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Message-ID: <20200702212511.1fcbbb26@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Thu, 2 Jul 2020 21:25:11 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Saeed Mahameed <saeedm@...lanox.com>
Cc:     Tariq Toukan <tariqt@...lanox.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Ron Diskin <rondi@...lanox.com>
Subject: Re: [net 02/11] net/mlx5e: Fix multicast counter not up-to-date in
 "ip -s"

On Fri, 3 Jul 2020 03:45:45 +0000 Saeed Mahameed wrote:
> On Thu, 2020-07-02 at 18:47 -0700, Jakub Kicinski wrote:
> > On Thu,  2 Jul 2020 15:19:14 -0700 Saeed Mahameed wrote:  
> > > From: Ron Diskin <rondi@...lanox.com>
> > > 
> > > Currently the FW does not generate events for counters other than
> > > error
> > > counters. Unlike ".get_ethtool_stats", ".ndo_get_stats64" (which ip
> > > -s
> > > uses) might run in atomic context, while the FW interface is non
> > > atomic.
> > > Thus, 'ip' is not allowed to issue fw commands, so it will only
> > > display
> > > cached counters in the driver.
> > > 
> > > Add a SW counter (mcast_packets) in the driver to count rx
> > > multicast
> > > packets. The counter also counts broadcast packets, as we consider
> > > it a
> > > special case of multicast.
> > > Use the counter value when calling "ip -s"/"ifconfig".  Display the
> > > new
> > > counter when calling "ethtool -S", and add a matching counter
> > > (mcast_bytes) for completeness.  
> > 
> > What is the problem that is being solved here exactly?
> > 
> > Device counts mcast wrong / unsuitably?
> >   
> 
> To read mcast counter we need to execute FW command which is blocking,
> we can't block in atomic context .ndo_get_stats64 :( .. we have to
> count in SW. 
> 
> the previous approach wasn't accurate as we read the mcast counter in a
> background thread triggered by the previous read.. so we were off by
> the interval between two reads.

And that's bad enough to cause trouble? What's the worst case time
delta you're seeing?

> > > Fixes: f62b8bb8f2d3 ("net/mlx5: Extend mlx5_core to support
> > > ConnectX-4 Ethernet functionality")
> > > Signed-off-by: Ron Diskin <rondi@...lanox.com>
> > > Reviewed-by: Tariq Toukan <tariqt@...lanox.com>
> > > Signed-off-by: Saeed Mahameed <saeedm@...lanox.com>