Subject: Re: tbench regression in 2.6.25-rc1

From: "Zhang, Yanmin" <[email-masked]>
Date: Wed, 20 Feb 2008 16:41:04 +0800

Comparing with kernel 2.6.24, tbench result has regression with
2.6.25-rc1.
1) On 2 quad-core processor stoakley: 4%.
2) On 4 quad-core processor tigerton: more than 30%.

bisect located below patch.

b4ce92775c2e7ff9cf79cca4e0a19c8c5fd6287b is first bad commit
commit b4ce92775c2e7ff9cf79cca4e0a19c8c5fd6287b
Author: Herbert Xu <[email-masked]>
Date:   Tue Nov 13 21:33:32 2007 -0800

    [IPV6]: Move nfheader_len into rt6_info
    
    The dst member nfheader_len is only used by IPv6.  It's also currently
    creating a rather ugly alignment hole in struct dst.  Therefore this patch
    moves it from there into struct rt6_info.

Above patch changes the cache line alignment, especially member __refcnt. I did a 
testing by adding 2 unsigned long pading before lastuse, so the 3 members,
lastuse/__refcnt/__use, are moved to next cache line. The performance is recovered.

I created a patch to rearrange the members in struct dst_entry.

With Eric and Valdis Kletnieks's suggestion, I made finer arrangement.
1) Move tclassid under ops in case CONFIG_NET_CLS_ROUTE=y. So sizeof(dst_entry)=200
no matter if CONFIG_NET_CLS_ROUTE=y/n. I tested many patches on my 16-core tigerton by
moving tclassid to different place. It looks like tclassid could also have impact on
performance.
If moving tclassid before metrics, or just don't move tclassid, the performance isn't
good. So I move it behind metrics.
2) Add comments before __refcnt.

On 16-core tigerton:
If CONFIG_NET_CLS_ROUTE=y, the result with below patch is about 18% better than
the one without the patch;
If CONFIG_NET_CLS_ROUTE=n, the result with below patch is about 30% better than
the one without the patch.

With 32bit 2.6.25-rc1 on 8-core stoakley, the new patch doesn't introduce regression.

Thank Eric, Valdis, and David!

Signed-off-by: Zhang Yanmin <[email-masked]>
Acked-by: Eric Dumazet <[email-masked]>

---

--- linux-2.6.25-rc1/include/net/dst.h	2008-02-21 14:33:43.000000000 +0800
+++ linux-2.6.25-rc1_work/include/net/dst.h	2008-02-22 12:52:19.000000000 +0800
@@ -52,15 +52,10 @@ struct dst_entry
 	unsigned short		header_len;	/* more space at head required */
 	unsigned short		trailer_len;	/* space to reserve at tail */
 
-	u32			metrics[RTAX_MAX];
-	struct dst_entry	*path;
-
-	unsigned long		rate_last;	/* rate limiting for ICMP */
 	unsigned int		rate_tokens;
+	unsigned long		rate_last;	/* rate limiting for ICMP */
 
-#ifdef CONFIG_NET_CLS_ROUTE
-	__u32			tclassid;
-#endif
+	struct dst_entry	*path;
 
 	struct neighbour	*neighbour;
 	struct hh_cache		*hh;
@@ -70,10 +65,20 @@ struct dst_entry
 	int			(*output)(struct sk_buff*);
 
 	struct  dst_ops	        *ops;
-		
-	unsigned long		lastuse;
+
+	u32			metrics[RTAX_MAX];
+
+#ifdef CONFIG_NET_CLS_ROUTE
+	__u32			tclassid;
+#endif
+
+	/*
+	 * __refcnt wants to be on a different cache line from
+	 * input/output/ops or performance tanks badly
+	 */
 	atomic_t		__refcnt;	/* client references	*/
 	int			__use;
+	unsigned long		lastuse;
 	union {
 		struct dst_entry *next;
 		struct rtable    *rt_next;