netdev - Re: [PATCH net-next 2/2] net: sched: do not acquire qdisc spinlock in qdisc/class stats dump

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Date:	Mon, 06 Jun 2016 16:53:19 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Cong Wang <xiyou.wangcong@...il.com>
Cc:	Eric Dumazet <edumazet@...gle.com>,
	"David S . Miller" <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	John Fastabend <john.fastabend@...il.com>,
	Kevin Athey <kda@...gle.com>,
	Xiaotian Pei <xiaotian@...gle.com>
Subject: Re: [PATCH net-next 2/2] net: sched: do not acquire qdisc spinlock
 in qdisc/class stats dump

On Mon, 2016-06-06 at 16:15 -0700, Cong Wang wrote:
> On Mon, Jun 6, 2016 at 9:37 AM, Eric Dumazet <edumazet@...gle.com> wrote:
> >  void
> > -__gnet_stats_copy_basic(struct gnet_stats_basic_packed *bstats,
> > +__gnet_stats_copy_basic(const seqcount_t *running,
> > +                       struct gnet_stats_basic_packed *bstats,
> >                         struct gnet_stats_basic_cpu __percpu *cpu,
> >                         struct gnet_stats_basic_packed *b)
> >  {
> > +       unsigned int seq;
> > +
> >         if (cpu) {
> >                 __gnet_stats_copy_basic_cpu(bstats, cpu);
> > -       } else {
> > +               return;
> > +       }
> > +       do {
> > +               if (running)
> > +                       seq = read_seqcount_begin(running);
> >                 bstats->bytes = b->bytes;
> >                 bstats->packets = b->packets;
> > -       }
> > +       } while (running && read_seqcount_retry(running, seq));
> >  }
> 
> Why only these basic stats need to get read seqlock?

(seqcount)

> Queue stats (gnet_stats_copy_queue()) too, right?

All these values are 32bit values, right ?

struct gnet_stats_queue {
        __u32   qlen;
        __u32   backlog;
        __u32   drops;
        __u32   requeues;
        __u32   overlimits;
};

Really sounds overkill to care about these, as probably no one needs to
get a 'consistent view of all these counters in a snapshot'.

Even as of today, the qlen/backlog pair is wrong. No one ever used these
values in an SNMP agent.

Note that qlen/backlog is changed both by enqueue/dequeue, so the
seqcount protection would not work.

With the percpu stats thing, stats can not be fetched in a 'consistent'
way.