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

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Date:   Mon, 20 Mar 2017 09:18:55 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
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, 2017-03-20 at 07:59 -0700, Eric Dumazet wrote:
> On Mon, 2017-03-20 at 07:51 -0700, Eric Dumazet wrote:
> 
> > atomic_cmpxchg() on PowerPC is horribly more expensive because of the
> > added two SYNC instructions.
> 
> Although I just saw that refcount was using atomic_cmpxchg_relaxed()
> 
> Time to find some documentation (probably missing) or get some specs for
> this thing.

Interesting.

UDP ipv4 xmit path gets a ~25 % improvement on PPC with this patch.

( 20 concurrent netperf -t UDP_STREAM  : 2.45 Mpps -> 3.07 Mpps )

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index 8471dd116771462d149e1da2807e446b69b74bcc..9f14aebf0ae1f5f366cfff0fbf58c48603916bc7 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -497,14 +497,14 @@ u32 ip_idents_reserve(u32 hash, int segs)
 	u32 now = (u32)jiffies;
 	u32 new, delta = 0;
 
-	if (old != now && cmpxchg(p_tstamp, old, now) == old)
+	if (old != now && cmpxchg_relaxed(p_tstamp, old, now) == old)
 		delta = prandom_u32_max(now - old);
 
 	/* Do not use atomic_add_return() as it makes UBSAN unhappy */
 	do {
 		old = (u32)atomic_read(p_id);
 		new = old + delta + segs;
-	} while (atomic_cmpxchg(p_id, old, new) != old);
+	} while (atomic_cmpxchg_relaxed(p_id, old, new) != old);
 
 	return new - segs;
 }