netdev - Re: [PATCH v6 net-next 14/15] net: bonding: ensure .ndo_get

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Date:   Tue, 12 Jan 2021 12:10:38 -0800
From:   Saeed Mahameed <saeed@...nel.org>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Eric Dumazet <edumazet@...gle.com>,
        George McCollister <george.mccollister@...il.com>,
        Oleksij Rempel <o.rempel@...gutronix.de>,
        Jay Vosburgh <j.vosburgh@...il.com>,
        Veaceslav Falico <vfalico@...il.com>,
        Andy Gospodarek <andy@...yhouse.net>,
        Arnd Bergmann <arnd@...db.de>, Taehee Yoo <ap420073@...il.com>,
        Jiri Pirko <jiri@...nulli.us>, Florian Westphal <fw@...len.de>,
        Nikolay Aleksandrov <nikolay@...dia.com>,
        Pravin B Shelar <pshelar@....org>,
        Sridhar Samudrala <sridhar.samudrala@...el.com>
Subject: Re: [PATCH v6 net-next 14/15] net: bonding: ensure .ndo_get_stats64
 can sleep

On Tue, 2021-01-12 at 16:37 +0200, Vladimir Oltean wrote:
> On Mon, Jan 11, 2021 at 03:38:49PM -0800, Saeed Mahameed wrote:
> > GFP_ATOMIC is a little bit aggressive especially when user daemons
> > are
> > periodically reading stats. This can be avoided.
> > 
> > You can pre-allocate with GFP_KERNEL an array with an "approximate"
> > size.
> > then fill the array up with whatever slaves the the bond has at
> > that
> > moment, num_of_slaves  can be less, equal or more than the array
> > you
> > just allocated but we shouldn't care ..
> > 
> > something like:
> > rcu_read_lock()
> > nslaves = bond_get_num_slaves();
> > rcu_read_unlock()
> > sarray = kcalloc(nslaves, sizeof(struct bonding_slave_dev),
> > GFP_KERNEL);
> > rcu_read_lock();
> > bond_fill_slaves_array(bond, sarray); // also do: dev_hold()
> > rcu_read_unlock();
> > 
> > 
> > bond_get_slaves_array_stats(sarray);
> > 
> > bond_put_slaves_array(sarray);
> 
> I don't know what to say about acquiring RCU read lock twice and
> traversing the list of interfaces three or four times.

You can optimize this by tracking #num_slaves.

> On the other hand, what's the worst that can happen if the GFP_ATOMIC
> memory allocation fails. It's not like there is any data loss.
> User space will retry when there is less memory pressure.

Anyway Up to you, i just don't like it when we use GFP_ATOMIC when it
can be avoided, especially for periodic jobs, like stats polling..