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

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Date:   Tue, 10 Apr 2018 13:16:49 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     saeedm@...lanox.com
Cc:     netdev@...r.kernel.org
Subject: Re: [RFC net-next 1/2] net: net-porcfs: Reduce rcu lock critical
 section

From: Saeed Mahameed <saeedm@...lanox.com>
Date: Tue, 10 Apr 2018 10:08:11 -0700

> The current net proc fs sequence file implementation to show current
> namespace netdevs list statistics and mc lists holds the rcu lock
> throughout the whole process, from dev seq start up to dev seq stop.
> 
> This is really greedy and demanding from device drivers since
> ndo_get_stats64 called from dev_seq_show while the rcu lock is held.
> 
> The rcu lock is needed to guarantee that device chain is not modified
> while the dev sequence file is walking through it and handling the
> netdev in the same time.
> 
> To minimize this critical section and drastically reduce the time rcu lock
> is being held, all we need is to grab the rcu lock only for the brief
> moment where we are looking for the next netdev to handle, if found,
> dev_hold it to guarantee it kept alive while accessed later in seq show
> callback and release the rcu lock immediately.
> 
> The current netdev being handled will be released "dev_put" when the seq next
> callback is called or dev seq stop is called.
> 
> Signed-off-by: Saeed Mahameed <saeedm@...lanox.com>

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.