On Sun, 16 Dec 2007 19:29:44 -0200
Arnaldo Carvalho de Melo <[email protected]> wrote:
> Hi,
> While looking at the pahole output for struct timer_list on
> recent kernels I noticed that there is a 4 bytes padding on struct
> timer_list that gets propagated to many structs on 64 bits
> architectures:
>
> [acme@doppio linux-2.6]$ pahole -C timer_list /tmp/tcp.o.before
> struct timer_list {
> struct list_head entry; /* 0 16 */
> long unsigned int expires; /* 16 8 */
> void (*function)(long unsigned int); /*
> 24 8 */ long unsigned int data; /* 32 8 */
> struct tvec_t_base_s *base; /* 40 8 */
> void * start_site; /* 48 8 */
> char start_comm[16]; /* 56 16 */
> /* --- cacheline 1 boundary (64 bytes) was 8 bytes ago --- */
> int start_pid; /* 72 4 */
>
> /* size: 80, cachelines: 2 */
> /* padding: 4 */
> /* last cacheline: 16 bytes */
> };
> [acme@doppio linux-2.6]$
>
> So the attached patch reduces the 4 bytes hole overhead of
> CONFIG_TIMER_STATS on 64bit architectures by shrinking the field for
> the process name by 4 bytes.
>
> Statistically this doesn't affects that many process names as
> most are less than 12 bytes. As CONFIG_TIMER_STATS is enabled at least
> on fedora kernels I think that we can, with this patch, still reap the
> benefits of powertopping.
I'm still worried that this means that PowerTOP will end up displaying shortened names..
it's already sometimes tricky to know who's guilty at 16... at 12 I fear it just gets worse.
Isn't there some other reorder that you can do to still get rid of the hole?
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