netdev - Re: [PATCH] ppp: add 64 bit stats

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Message-ID: <CABF+-6WHqzXhvv9etdAqsamZirbWxWJEsnU-CQ1H8tvynCTOZA@mail.gmail.com>
Date:	Mon, 23 Jul 2012 11:25:53 -0400
From:	Kevin Groeneveld <kgroeneveld@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH] ppp: add 64 bit stats

Hi Eric,

On Mon, Jul 23, 2012 at 1:16 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> Hmm. This patches adds races, but also adds a good amount of memory on
> these servers with thousand of ppp devices, and 64 cpus.

I am curious if you can elaborate on what is racy about the patch, I
am still trying to learn.  I assumed (possibly incorrectly) that
because I was using percpu variables that the stats updates didn't
need any extra synchronization as any concurrent updates would be on
different cpus.

> I really doubt ppp is performance sensitive, it so doesnt need percpu
> counter.
>
> If you really want 64bits stats on ppp, use proper synchronization
> around u64 counters (but shared ones)

I will work on an updated patch without the percpu variables.  I
didn't really think about servers with many cpus and many ppp sessions
when I created the patch, I was mainly thinking about my Linksys
router and other simple clients.  Many of the other network drivers
use percpu variables for their stats so I just followed along.

Would proper synchronization in this case just be wrapping the updates
in a spin_lock/spin_unlock?

Kevin
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