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    SubjectRe: tbench regression in 2.6.25-rc1
    On Mon, 18 Feb 2008 16:12:38 +0800
    "Zhang, Yanmin" <yanmin_zhang@linux.intel.com> wrote:

    > On Fri, 2008-02-15 at 15:22 -0800, David Miller wrote:
    > > From: Eric Dumazet <dada1@cosmosbay.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@intel.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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