Date: Mon, 17 Dec 2007 09:03:37 -0800
From: Arjan van de Ven <arjan@infradead.org>
To: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org,
       Ingo Molnar <mingo@elte.hu>
Subject: Re: [PATCH] Reduce overhead of CONFIG_TIMER_STATS
Message-ID: <20071217090337.0b1af816@laptopd505.fenrus.org>
In-Reply-To: <20071216212944.GA5128@ghostprotocols.net>
References: <20071216212944.GA5128@ghostprotocols.net>
Organization: Intel
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On Sun, 16 Dec 2007 19:29:44 -0200
Arnaldo Carvalho de Melo <acme@redhat.com> 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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