Re: [ANNOUNCE] pahole and other DWARF2 utilities

Arnaldo Carvalho de Melo
2006-11-03T15:52:03Z

On Mon, Oct 30, 2006 at 08:33:34PM -0800, Andrew Morton wrote:
> On Mon, 30 Oct 2006 18:33:19 -0300
> Arnaldo Carvalho de Melo <acme@mandriva.com> wrote:
>
> > Hi,
> >
> > I've been working on some DWARF2 utilities and thought that it
> > is about time I announce it to the community, so that what is already
> > available can be used by people interested in reducing structure sizes
> > and otherwise taking advantage of the information available in the elf
> > sections of files compiled with 'gcc -g' or in the case of the kernel
> > with CONFIG_DEBUG_INFO enabled, so here it goes the description of said
> > tools:
> >
> > pahole: Poke-a-Hole is a tool to find out holes in structures, holes
> > being defined as the space between members of functions due to alignemnt
> > rules that could be used for new struct entries or to reorganize
> > existing structures to reduce its size, without more ado lets see what
> > that means:
> >
> > ...
> >
> > Further ideas on how to use the DWARF2 information include tools
> > that will show where inlines are being used, how much code is added by
> > inline functions,
>
> It would be quite useful to be able to identify inlined functions which are
> good candidates for uninlining.

Getting there, next step is to per CU (Compilation Unit, .o files)
inlining stats :-)

Ah, the sizes are different because sometimes just some parts of inline
functions are "sourced", as indicated by the DW_AT_ranges DWARF
attribute.

Repo continues at:

git://git.kernel.org/pub/scm/linux/kernel/git/acme/pahole.git

Another suggested was for a stack hole finding tool, similar to what
pahole does for structs :-)

Another example, this time for schedule():

http://oops.merseine.nu:81/acme/schedule.inlines.txt

Regards,

- Arnaldo

commit a42afe1acffc5e57ab504c008b8b75c124bf07de
Author: Arnaldo Carvalho de Melo <acme@mandriva.com>
Date: Fri Nov 3 12:41:19 2006 -0300

[CLASSES]: Add support for DW_TAG_inlined_subroutine

Output of pfunct using this information (all for a make allyesconfig build):

Top 5 functions by size of inlined functions in net/ipv4:

[acme@newtoy guinea_pig-2.6]$ pfunct -I net/ipv4/built-in.o | sort -k3 -nr | head -5
ip_route_input: 19 7086
tcp_ack: 33 6415
do_ip_vs_set_ctl: 23 4193
q931_help: 8 3822
ip_defrag: 19 3318
[acme@newtoy guinea_pig-2.6]$

And by number of inline expansions:

[acme@newtoy guinea_pig-2.6]$ pfunct -I net/ipv4/built-in.o | sort -k2 -nr | head -5
dump_packet: 35 905
tcp_v4_rcv: 34 1773
tcp_recvmsg: 34 928
tcp_ack: 33 6415
tcp_rcv_established: 31 1195
[acme@newtoy guinea_pig-2.6]$

And the list of expansions on a specific function:

[acme@newtoy guinea_pig-2.6]$ pfunct -i net/ipv4/built-in.o tcp_v4_rcv
/* net/ipv4/tcp_ipv4.c:1054 */
int tcp_v4_rcv(struct sk_buff * skb);
/* size: 2189, variables: 8, goto labels: 6, inline expansions: 34 (1773 bytes) */

/* inline expansions in tcp_v4_rcv:
current_thread_info: 8
pskb_may_pull: 36
pskb_may_pull: 29
tcp_v4_checksum_init: 139
__fswab32: 2
__fswab32: 2
inet_iif: 12
__inet_lookup: 292
__fswab16: 20
inet_ehashfn: 25
inet_ehash_bucket: 18
prefetch: 4
prefetch: 4
prefetch: 4
sock_hold: 4
xfrm4_policy_check: 59
nf_reset: 66
sk_filter: 135
__skb_trim: 20
get_softnet_dma: 68
tcp_prequeue: 257
sk_add_backlog: 40
sock_put: 27
xfrm4_policy_check: 46
tcp_checksum_complete: 29
current_thread_info: 8
sock_put: 20
xfrm4_policy_check: 50
tcp_checksum_complete: 29
current_thread_info: 8
current_thread_info: 8
sock_put: 20
xfrm4_policy_check: 50
tcp_checksum_complete: 29
current_thread_info: 8
inet_iif: 9
inet_lookup_listener: 36
inet_twsk_put: 114
tcp_v4_timewait_ack: 153
*/
[acme@newtoy guinea_pig-2.6]$

Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>