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Date: Fri, 6 Jan 2012 11:20:15 +0000
From: Ian Campbell <Ian.Campbell@...rix.com>
To: Eric Dumazet <eric.dumazet@...il.com>
CC: "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
David Miller <davem@...emloft.net>
Subject: Re: [PATCH 2/6] net: pad skb data and shinfo as a whole rather than
individually
On Thu, 2012-01-05 at 20:22 +0000, Eric Dumazet wrote:
> Take a look at kfree_skb() and please list all skb_shared_info needed
> fields, for a typical skb with nr_frags= 0 or 1.
>
> If all fit in a single cache line, its ok.
It doesn't fit in a single cache line today.
Before I started touching any of this stuff (e.g. in v3.1) kfree_skb
would touch 3x32 byte caches lines (with boundaries between
frag_list/hwtstamps and destructor_arg/frags[0]) or 2x64 byte cache
lines (boundary between between frag_list/hwtstamps).
After the v2 version of these patches the situation is much the same,
although the boundaries are different and which field is in which line
has shifted a bit. The boundaries are now gso_type/frag_list and
dataref/destructor_arg for 32 byte cache lines and gso_type/frag_list
for 64 byte cache lines.
With the proposed reordering to put destructor_arg first it actually
shrinks to just 2 32 byte cache lines (boundary between hwtstamps and
ip6_frag_id) or a single 64 byte cache line, since the other 32&64
boundary is between destructor_arg and nr_frags.
Even if we don't consider destructor_arg to be rarely used / cold things
are no worse than they were before (3x32 or 2x64 byte cache lines
touched).
So I think if anything I think these patches improve things when
nr_frags = 0 or 1 or at at least no worse than the current situation.
> If not, this adds a cache line miss in a critical path.
I think it does not, but please do check my working (see below).
> Also in transmit path, when an skb is queued by cpu X on qdisc, and
> dequeued by another cpu Y to be delivered to device.
You mean the path starting at dev_queue_xmit -> q->enqueue is true ->
__dev_xmit_skb ?
I didn't follow it properly but it doesn't seem like there is much use
of the shinfo at all in this area qdisc, the only thing I spotted was
the use of the gso_{size,segs} fields in qdisc_bstats_update and those
are always on the same cache line. Did I miss something?
Anyway, by similar logic to the kfree_skb case I think the number of
cachelines touched will always be <= the current number.
Ian.
My working:
Used in kfree_skb?
unsigned short nr_frags; YES (if dataref -> 0 etc)
unsigned short gso_size;
unsigned short gso_segs;
unsigned short gso_type;
__be32 ip6_frag_id;
__u8 tx_flags; YES (if dataref -> 0 etc)
struct sk_buff *frag_list; YES (if dataref -> 0 etc)
struct skb_shared_hwtstamps hwtstamps;
atomic_t dataref; YES
void * destructor_arg; YES if skb->destructor || tx_flags & ZEROCOPY
-> I count this as unused by
kfree_skb for the purposes here
skb_frag_t frags[MAX_SKB_FRAGS]; YES (<nr_frags, if dataref->0 etc)
LEGEND:
*<-used in kfree_skb
N<-offset from start of shinfo offset from end of shinfo-> -N
v3.1:
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= 32 & 64 byte cache line =-=-=-=-=
-40 -384
--------------------------------------------- 32 byte cache line --------------
-8 -352
struct skb_shared_info {
*0 unsigned short nr_frags; -344
2 unsigned short gso_size; -342
/* Warning: this field is not always filled in (UFO)! */
4 unsigned short gso_segs; -340
6 unsigned short gso_type; -338
8 __be32 ip6_frag_id; -336
*12 __u8 tx_flags; -332
*16 struct sk_buff *frag_list; -328
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= 32 & 64 byte cache line =-=-=-=-=
24 struct skb_shared_hwtstamps hwtstamps; -320
/*
* Warning : all fields before dataref are cleared in __alloc_skb()
*/
*40 atomic_t dataref; -304
/* Intermediate layers must ensure that destructor_arg
* remains valid until skb destructor */
48 void * destructor_arg; -296
/* must be last field, see pskb_expand_head() */
--------------------------------------------- 32 byte cache line --------------
*56 skb_frag_t frags[MAX_SKB_FRAGS]; -16x18
= -288
= 9x32
or 4.5x64
or 2.25x128
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= 32 & 64 & 128 byte cache line =-=
344 = 10.75x32 or 5.375x64 or 2.6875x128 -0
};
skb patches v2:
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= 32 & 64 byte cache line =-=-=-=-=
-56 -512
--------------------------------------------- 32 byte cache line --------------
-24 -480
struct skb_shared_info {
*0 unsigned char nr_frags; -456
*1 __u8 tx_flags; -455
2 unsigned short gso_size; -454
/* Warning: this field is not always filled in (UFO)! */
4 unsigned short gso_segs; -452
6 unsigned short gso_type; -450
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= 32 & 64 byte cache line =-=-=-=-=
*8 struct sk_buff *frag_list; -448
16 struct skb_shared_hwtstamps hwtstamps; -440
32 __be32 ip6_frag_id; -424
/*
* Warning : all fields before dataref are cleared in __alloc_skb()
*/
*36 atomic_t dataref; -420
/* Intermediate layers must ensure that destructor_arg
* remains valid until skb destructor */
--------------------------------------------- 32 byte cache line --------------
40 void * destructor_arg; -416
/* must be last field, see pskb_expand_head() */
*48 skb_frag_t frags[MAX_SKB_FRAGS]; -24*17
= -408
= 12.75x32
or 6.375x64
or 3.1875x128
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= 32 & 64 & 128 byte cache line =-=
456 = 14.25x32 or 7.125x64 or 3.5625x128 -0
};
skb patches + put destructor_arg first
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= 32 & 64 byte cache line =-=-=-=-=
-56 -512
--------------------------------------------- 32 byte cache line
-24 -480
struct skb_shared_info {
/* Intermediate layers must ensure that destructor_arg
* remains valid until skb destructor */
0 void * destructor_arg; -456
/* Warning all fields from here until dataref are cleared in __alloc_skb() */
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= 32 & 64 byte cache line =-=-=-=-=
*8 unsigned char nr_frags; -448
*9 __u8 tx_flags; -447
10 unsigned short gso_size; -446
/* Warning: this field is not always filled in (UFO)! */
12 unsigned short gso_segs; -444
14 unsigned short gso_type; -442
*16 struct sk_buff *frag_list; -440
24 struct skb_shared_hwtstamps hwtstamps; -432
--------------------------------------------- 32 byte cache line --------------
40 __be32 ip6_frag_id; -416
/*
* Warning : all fields before dataref are cleared in __alloc_skb()
*/
*44 atomic_t dataref; -412
/* must be last field, see pskb_expand_head() */
*48 skb_frag_t frags[MAX_SKB_FRAGS]; -24*17
= -408
= 12.75x32
or 6.375x64
or 3.1875x128
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= 32 & 64 & 128 byte cache line =-=
}; 456 = 14.25x32 or 7.125x64 or 3.5625x128
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