Re: [PATCH] RCU: remove alignment padding from rcu_data on 64 bit builds

[Eric Dumazet <eric.dumazet@gmail.com>]

Le jeudi 05 mai 2011 à 16:50 +0100, Richard Kennedy a écrit :
> Reorder rcu_data to remove 8 bytes of alignment padding on 64 bit builds
> so saving a total of 16 bytes per cpu, (rcu_sched_data & rcu_bh_data).
>     
> When CONFIG_NO_HZ is set, it shrinks the size of this structure from 264
> to 256 bytes allowing it to fit into one fewer cache lines.
> 
> Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk>
> ---
> 
> patch against v2.6.39-rc6
> compiled & tested on x86_64
> 
> I've been running with this patch for some weeks and have not seen any
> issues.
> 
> regards
> Richard
> 
> 
> 
> diff --git a/kernel/rcutree.h b/kernel/rcutree.h
> index e8f057e..fcced0f 100644
> --- a/kernel/rcutree.h
> +++ b/kernel/rcutree.h
> @@ -176,6 +176,9 @@ struct rcu_data {
>  	bool		qs_pending;	/* Core waits for quiesc state. */
>  	bool		beenonline;	/* CPU online at least once. */
>  	bool		preemptable;	/* Preemptable RCU? */
> +
> +	int cpu;
> +
>  	struct rcu_node *mynode;	/* This CPU's leaf of hierarchy */
>  	unsigned long grpmask;		/* Mask to apply to leaf qsmask. */
>  
> @@ -238,8 +241,6 @@ struct rcu_data {
>  	unsigned long n_rp_gp_started;
>  	unsigned long n_rp_need_fqs;
>  	unsigned long n_rp_need_nothing;
> -
> -	int cpu;
>  };
>  

We could shrink this structure more, converting some 64bit fields to
32bit ones.

Rationale is the algo is working well on 32bit arches, no need to use
64bit fields.

candidates : completed, gpnum, passed_quiesc_completed, qlen,
qlen_last_fqs_check, blimit.

Counters might be converted too.