Date: Mon, 20 Mar 2017 15:59:38 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>,
        David Miller <davem@davemloft.net>, elena.reshetova@intel.com,
        keescook@chromium.org, netdev@vger.kernel.org,
        bridge@lists.linux-foundation.org, linux-kernel@vger.kernel.org,
        kuznet@ms2.inr.ac.ru, jmorris@namei.org, kaber@trash.net,
        stephen@networkplumber.org, ishkamiel@gmail.com, dwindsor@gmail.com,
        akpm@linux-foundation.org
Subject: Re: [PATCH 07/17] net: convert sock.sk_refcnt from atomic_t to
 refcount_t
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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.