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Date:   Tue, 12 Nov 2019 15:18:19 +0800
From:   Tony Lu <tonylu@linux.alibaba.com>
To:     Stephen Hemminger <stephen@networkplumber.org>
Cc:     davem@davemloft.net, shemminger@osdl.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] net: remove static inline from dev_put/dev_hold
Message-ID: <20191112071819.GB67139@TonyMac-Alibaba>
Reply-To: Tony Lu <tonylu@linux.alibaba.com>
References: <20191111140502.17541-1-tonylu@linux.alibaba.com>
 <20191111085632.24d88706@hermes.lan>
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On Mon, Nov 11, 2019 at 08:56:32AM -0800, Stephen Hemminger wrote:
> On Mon, 11 Nov 2019 22:05:03 +0800
> Tony Lu <tonylu@linux.alibaba.com> wrote:
> 
> > This patch removes static inline from dev_put/dev_hold in order to help
> > trace the pcpu_refcnt leak of net_device.
> > 
> > We have sufferred this kind of issue for several times during
> > manipulating NIC between different net namespaces. It prints this
> > log in dmesg:
> > 
> >   unregister_netdevice: waiting for eth0 to become free. Usage count = 1
> > 
> > However, it is hard to find out who called and leaked refcnt in time. It
> > only left the crime scene but few evidence. Once leaked, it is not
> > safe to fix it up on the running host. We can't trace dev_put/dev_hold
> > directly, for the functions are inlined and used wildly amoung modules.
> > And this issue is common, there are tens of patches fix net_device
> > refcnt leak for various causes.
> > 
> > To trace the refcnt manipulating, this patch removes static inline from
> > dev_put/dev_hold. We can use handy tools, such as eBPF with kprobe, to
> > find out who holds but forgets to put refcnt. This will not be called
> > frequently, so the overhead is limited.
> > 
> > Signed-off-by: Tony Lu <tonylu@linux.alibaba.com>
> 
> In the past dev_hold/dev_put was in the hot path for several
> operations. What is the performance implication of doing this?

From code analysis, there should be a little performance backwards.
I don't have the benchmark data for now. I will make a kernel module to
take a series of benchmarks for dev_put/dev_hold. Actually there is a plan
to take a whole solution for this issue. The benchmarks will be done
after this.

Cheers
Tony Lu