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SubjectRe: [PATCH] time : SMP friendly alignment of struct clocksource
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On Tue, 2007-03-20 at 11:09 +0100, Eric Dumazet wrote:
> struct clocksource is a critical data structure.
>
> Most of its fields are read only, some of them are heavily modified at each timer interrupt.
>
> It makes sense to separate those fields and make sure they all share one cache line, or at least the minimum for machines with small cache lines.
>
> Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>


Sounds fine to me. Can you actually observe a difference?

Acked-by: John Stultz <johnstul@us.ibm.com>



> --- linux-2.6.21-rc4-mm1/include/linux/clocksource.h
> +++ linux-2.6.21-rc4-mm1-ed/include/linux/clocksource.h
> @@ -49,25 +49,35 @@ struct clocksource;
> * @flags: flags describing special properties
> * @vread: vsyscall based read
> * @cycle_interval: Used internally by timekeeping core, please ignore.
> * @xtime_interval: Used internally by timekeeping core, please ignore.
> */
> struct clocksource {
> + /*
> + * First part of structure is read mostly
> + */
> char *name;
> struct list_head list;
> int rating;
> cycle_t (*read)(void);
> cycle_t mask;
> u32 mult;
> u32 shift;
> unsigned long flags;
> cycle_t (*vread)(void);
>
> /* timekeeping specific data, ignore */
> - cycle_t cycle_last, cycle_interval;
> - u64 xtime_nsec, xtime_interval;
> + cycle_t cycle_interval;
> + u64 xtime_interval;
> + /*
> + * Second part is written at each timer interrupt
> + * Keep it in a different cache line to dirty no
> + * more than one cache line.
> + */
> + cycle_t cycle_last ____cacheline_aligned_in_smp;
> + u64 xtime_nsec;
> s64 error;
>
> #ifdef CONFIG_CLOCKSOURCE_WATCHDOG
> /* Watchdog related data, used by the framework */
> struct list_head wd_list;
> cycle_t wd_last;

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