linux-kernel - Re: tbench regression in 2.6.25-rc1

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Date:	Mon, 18 Feb 2008 11:11:01 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
Cc:	David Miller <davem@...emloft.net>, herbert@...dor.apana.org.au,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: tbench regression in 2.6.25-rc1

On Mon, 18 Feb 2008 16:12:38 +0800
"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com> wrote:

> On Fri, 2008-02-15 at 15:22 -0800, David Miller wrote:
> > From: Eric Dumazet <dada1@...mosbay.com>
> > Date: Fri, 15 Feb 2008 15:21:48 +0100
> > 
> > > On linux-2.6.25-rc1 x86_64 :
> > > 
> > > offsetof(struct dst_entry, lastuse)=0xb0
> > > offsetof(struct dst_entry, __refcnt)=0xb8
> > > offsetof(struct dst_entry, __use)=0xbc
> > > offsetof(struct dst_entry, next)=0xc0
> > > 
> > > So it should be optimal... I dont know why tbench prefers __refcnt being 
> > > on 0xc0, since in this case lastuse will be on a different cache line...
> > > 
> > > Each incoming IP packet will need to change lastuse, __refcnt and __use, 
> > > so keeping them in the same cache line is a win.
> > > 
> > > I suspect then that even this patch could help tbench, since it avoids 
> > > writing lastuse...
> > 
> > I think your suspicions are right, and even moreso
> > it helps to keep __refcnt out of the same cache line
> > as input/output/ops which are read-almost-entirely :-
> I think you are right. The issue is these three variables sharing the same cache line
> with input/output/ops.
> 
> > )
> > 
> > I haven't done an exhaustive analysis, but it seems that
> > the write traffic to lastuse and __refcnt are about the
> > same.  However if we find that __refcnt gets hit more
> > than lastuse in this workload, it explains the regression.
> I also think __refcnt is the key. I did a new testing by adding 2 unsigned long
> pading before lastuse, so the 3 members are moved to next cache line. The performance is
> recovered.
> 
> How about below patch? Almost all performance is recovered with the new patch.
> 
> Signed-off-by: Zhang Yanmin <yanmin.zhang@...el.com>
> 
> ---
> 
> --- 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	2008-02-21 14:36:22.000000000 +0800
> @@ -52,11 +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 */
> +
> +	struct dst_entry	*path;
>  
>  #ifdef CONFIG_NET_CLS_ROUTE
>  	__u32			tclassid;
> @@ -70,10 +69,12 @@ struct dst_entry
>  	int			(*output)(struct sk_buff*);
>  
>  	struct  dst_ops	        *ops;
> -		
> -	unsigned long		lastuse;
> +
> +	u32			metrics[RTAX_MAX];
> +
>  	atomic_t		__refcnt;	/* client references	*/
>  	int			__use;
> +	unsigned long		lastuse;
>  	union {
>  		struct dst_entry *next;
>  		struct rtable    *rt_next;
> 
> 

Well, after this patch, we grow dst_entry by 8 bytes :

sizeof(struct dst_entry)=0xd0
offsetof(struct dst_entry, input)=0x68
offsetof(struct dst_entry, output)=0x70
offsetof(struct dst_entry, __refcnt)=0xb4
offsetof(struct dst_entry, lastuse)=0xc0
offsetof(struct dst_entry, __use)=0xb8
sizeof(struct rtable)=0x140


So we dirty two cache lines instead of one, unless your cpu have 128 bytes cache lines ?

I am quite suprised that my patch to not change lastuse if already set to jiffies changes nothing...

If you have some time, could you also test this (unrelated) patch ?

We can avoid dirty all the time a cache line of loopback device.

diff --git a/drivers/net/loopback.c b/drivers/net/loopback.c
index f2a6e71..0a4186a 100644
--- a/drivers/net/loopback.c
+++ b/drivers/net/loopback.c
@@ -150,7 +150,10 @@ static int loopback_xmit(struct sk_buff *skb, struct net_device *dev)
                return 0;
        }
 #endif
-       dev->last_rx = jiffies;
+#ifdef CONFIG_SMP
+       if (dev->last_rx != jiffies)
+#endif
+               dev->last_rx = jiffies;
 
        /* it's OK to use per_cpu_ptr() because BHs are off */
        pcpu_lstats = netdev_priv(dev);

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