netdev - Re: [RFC net-next 1/2] net: net-porcfs: Reduce rcu lock critical section

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Date:   Wed, 11 Apr 2018 18:59:06 +0000
From:   Saeed Mahameed <saeedm@...lanox.com>
To:     "eric.dumazet@...il.com" <eric.dumazet@...il.com>,
        "davem@...emloft.net" <davem@...emloft.net>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [RFC net-next 1/2] net: net-porcfs: Reduce rcu lock critical
 section

On Tue, 2018-04-10 at 13:35 -0700, Eric Dumazet wrote:
> 
> On 04/10/2018 10:16 AM, David Miller wrote:
> > 
> > The tradeoff here is that now you are doing two unnecessary atomic
> > operations per stats dump.
> > 
> > That is what the RCU lock allows us to avoid.
> > 
> 
> dev_hold() and dev_put() are actually per cpu increment and
> decrements,
> pretty cheap these days.
> 

Yes, i am adding only 2 cpu instructions here.
I think the trade-off here is too small and the price to finally have
get_stats64 called from non atomic context is really worth it.

It  looks really odd to me that the device chain locks are held for
such long periods, while we already have the means to avoid this, same
goes for rtnl_lock, same trick can work here for many use cases and
many ndos, we are just over protective for no reasons.

> Problem here is that any preemption of the process holding device
> reference
> might trigger warnings in device unregister.
> 

This is true for any other place dev_hold is used,
as explained in the commit message dev_hold is used for a very brief
moment before calling the stats ndo and released directly after.

looking at 

netdev_wait_allrefs(...)
[...]
	msleep(250);

	refcnt = netdev_refcnt_read(dev);

	if (time_after(jiffies, warning_time + 10 * HZ)) {
		pr_emerg("unregister_netdevice: waiting for %s to
become free. Usage count = %d\n",
			 dev->name, refcnt);
		warning_time = jiffies;
	}

The holder will get enough time to release the netdev way before the
warning is triggered.

The warning is triggered only if someone holds the dev for more than 10
seconds which is impossible for the stats ndo to take more than this,
in fact i just did a quick measurement and it seems that in average
get_stats64 ndo takes 0.5us !