netdev - RE: [PATCH 07/17] net: convert sock.sk_refcnt from atomic_t to refcount

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Date:   Mon, 20 Mar 2017 14:10:24 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Herbert Xu' <herbert@...dor.apana.org.au>,
        Peter Zijlstra <peterz@...radead.org>
CC:     David Miller <davem@...emloft.net>,
        "eric.dumazet@...il.com" <eric.dumazet@...il.com>,
        "elena.reshetova@...el.com" <elena.reshetova@...el.com>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bridge@...ts.linux-foundation.org" 
        <bridge@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kuznet@....inr.ac.ru" <kuznet@....inr.ac.ru>,
        "jmorris@...ei.org" <jmorris@...ei.org>,
        "kaber@...sh.net" <kaber@...sh.net>,
        "stephen@...workplumber.org" <stephen@...workplumber.org>,
        "ishkamiel@...il.com" <ishkamiel@...il.com>,
        "dwindsor@...il.com" <dwindsor@...il.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: RE: [PATCH 07/17] net: convert sock.sk_refcnt from atomic_t to
 refcount_t

From: Herbert Xu
> Sent: 20 March 2017 13:16
> On Mon, Mar 20, 2017 at 11:39:37AM +0100, Peter Zijlstra wrote:
> >
> > Can we at least give a benchmark and have someone run numbers? We should
> > be able to quantify these things.
> 
> Do you realise how many times this thing gets hit at 10Gb/s or
> higher? Anyway, since you're proposing this change you should
> demonstrate that it does not cause a performance regression.

What checks does refcnt_t actually do?

An extra decrement is hard to detect since the item gets freed early.
I guess making the main 'allocate/free' code hold (say) 64k references
would give some leeway for extra decrements.

An extra increment will be detected when the count eventually wraps.
Unless the error is in a very common path that won't happen for a long time.

On x86 the cpu flags from the 'lock inc/dec' could be used to reasonably
cheaply detect errors - provided you actually generate a forwards branch.

Otherwise having a common, but not every packet, code path verify that the
reference count is 'sane' would give reasonable coverage.

	David