linux-kernel - Re: [PATCH 07/17] net: convert sock.sk_refcnt from atomic_t to refcount

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Date:   Tue, 21 Mar 2017 16:51:13 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        David Miller <davem@...emloft.net>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        Network Development <netdev@...r.kernel.org>,
        bridge@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        James Morris <jmorris@...ei.org>,
        Patrick McHardy <kaber@...sh.net>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Hans Liljestrand <ishkamiel@...il.com>,
        David Windsor <dwindsor@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 07/17] net: convert sock.sk_refcnt from atomic_t to refcount_t

On Tue, Mar 21, 2017 at 2:23 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Tue, 2017-03-21 at 13:49 -0700, Kees Cook wrote:
>
>> Yeah, this is exactly what I'd like to find as well. Just comparing
>> cycles between refcount implementations, while interesting, doesn't
>> show us real-world performance changes, which is what we need to
>> measure.
>>
>> Is Eric's "20 concurrent 'netperf -t UDP_STREAM'" example (from
>> elsewhere in this email thread) real-world meaningful enough?
>
> Not at all ;)
>
> This was targeting the specific change I had in mind for
> ip_idents_reserve(), which is not used by TCP flows.

Okay, I just wanted to check. I didn't think so, but it was the only
example in the thread.

> Unfortunately there is no good test simulating real-world workloads,
> which are mostly using TCP flows.

Sure, but there has to be _something_ that can be used to test to
measure the effects. Without a meaningful test, it's weird to reject a
change for performance reasons.

> Most synthetic tools you can find are not using epoll(), and very often
> hit bottlenecks in other layers.
>
>
> It looks like our suggestion to get kernel builds with atomic_inc()
> being exactly an atomic_inc() is not even discussed or implemented.

So, FWIW, I originally tried to make this a CONFIG in the first couple
passes at getting a refcount defense. I would be fine with this, but I
was not able to convince Peter. :) However, things have evolved a lot
since then, so perhaps there are things do be done here.

> Coding this would require less time than running a typical Google kernel
> qualification (roughly one month, thousands of hosts..., days of SWE).

It wasn't the issue of coding time; just that it had been specifically
not wanted. :)

Am I understanding you correctly that you'd want something like:

refcount.h:
#ifdef UNPROTECTED_REFCOUNT
#define refcount_inc(x)   atomic_inc(x)
...
#else
void refcount_inc(...
...
#endif

some/net.c:
#define UNPROTECTED_REFCOUNT
#include <refcount.h>

or similar?

-Kees

-- 
Kees Cook
Pixel Security