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

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Message-ID: <20170320145938.cmj5m33uxunladli@hirez.programming.kicks-ass.net>
Date:   Mon, 20 Mar 2017 15:59:38 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        David Miller <davem@...emloft.net>, elena.reshetova@...el.com,
        keescook@...omium.org, netdev@...r.kernel.org,
        bridge@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        kuznet@....inr.ac.ru, jmorris@...ei.org, kaber@...sh.net,
        stephen@...workplumber.org, ishkamiel@...il.com,
        dwindsor@...il.com, akpm@...ux-foundation.org
Subject: Re: [PATCH 07/17] net: convert sock.sk_refcnt from atomic_t to
 refcount_t

On Mon, Mar 20, 2017 at 07:51:01AM -0700, Eric Dumazet wrote:
> PowerPC has no efficient atomic_inc() and this definitely shows on
> network intensive workloads involving concurrent cores/threads.

Correct, PPC LL/SC are dreadfully expensive.

> atomic_cmpxchg() on PowerPC is horribly more expensive because of the
> added two SYNC instructions.

Note that refcount_t uses atomic_cmpxchg_release() and
atomic_cmpxchg_relaxed() which avoid most of the painful barriers.